Having just discussed whether humans – as opposed to their machines – will one day make interstellar journeys, it’s a good time to ask where we could get today with near-term technologies. In other words, assuming reasonable progress in the next few decades, what would be the most likely outcome of a sustained effort to push our instruments into deep space? My assumption is that fusion engines will one day be available for spacecraft, but probably not soon, and antimatter, that quixotic ultimate power source for interstellar flight, is a long way from being harnessed for propulsion.

We’re left with conventional rocket propulsion with gravity assists, and sail technologies, which not coincidentally describes the two large interstellar missions currently being considered for the heliophysics decadal study. Both JHU/APL’s Interstellar Probe mission and JPL’s SGLF (Solar Gravity Lens Focal) mission aim at reaching well beyond our current distance holders, the now struggling Voyagers. The decadal choice will weigh the same question I ask above. What could we do in the near term to reach hundreds of AU from the Sun and get there in relatively timely fashion?

A paper from the JPL effort in Experimental Astronomy draws my attention because it pulls together where the SGLF concept is now, and the range of factors that are evolving to make it possible. I won’t go into detail on the overall design here because we’ve discussed it in the recent past (see for example Building Smallsat Capabilities for the Outer System and Self-Assembly: Reshaping Mission Design for starters). Instead, I want to dig into the new paper looking for points of interest for a mission that would move outward from the Sun’s gravitational lens and, beyond about 650 AU, begin imaging an exoplanet with a factor of 10¹¹ amplification.

Image: This is Figure 1 from the paper. Caption: The geometry of the solar gravity lens used to form an image of a distant object in the Einstein ring. Credit: Friedman et al.

Carrying a telescope in the meter-class, the spacecraft would reach its target distance after a cruise of about 25 years, which means moving at a speed well beyond anything humans have yet attained moving outward from the Sun. While Voyager 1 reached over 17 kilometers per second, we’re asking here for at least 90 km/sec. Remember that the focal line extends outward from close to 550 AU, and becomes usable for imaging around 650 AU. Our spacecraft can take advantage of it well beyond, perhaps out to 1500 AU.

So let’s clear up a common misconception. The idea is not to reach a specific distance from the Sun and maintain it. Rather, the SGLF would continue to move outward and maneuver within what can be considered an ‘image cylinder’ that extends from the focal region outward. This is a huge image. Working the math, the authors calculate that at 650 AU from the Sun, the light (seen as an ‘Einstein ring’ around the Sun) from an exoplanet 100 light years from our system would be compressed to a cylinder 1.3 kilometers in diameter. Remember, we have a meter-class telescope to work with.

Thus the idea is to position the spacecraft within the image cylinder, continuing to move along the focal line, but also moving within this huge image itself, collecting data pixel by pixel. This is not exactly a snapshot we’re trying to take. The SGLF craft must take brightness readings over a period that will last for years. Noise from the Sun’s corona is reduced as the spacecraft moves further and further from the Sun, but this is a lengthy process in terms of distance and time, with onboard propulsion necessary to make the necessary adjustments to collect the needed pixel data within the cylinder.

So we’re in continual motion within the image cylinder, and this gets further complicated by the range of motions of the objects we are studying. From the paper:

Even with the relatively small size of the image produced by the SGL, the spacecraft and telescope must be maneuvered over the distance of tens of kilometers to collect pixel-by-pixel all the data necessary to construct the image… This is needed as the image moves because of the multiple motions [that] are present, namely 1) the planet orbits its parent star, 2) the star moves with the respect to the Sun, and 3) the Sun itself is not static, but moves with respect to the solar system barycentric coordinates. To compensate for these motions, the spacecraft will need micro-thrusters and electric propulsion, the solar sail obviously being useless for propulsion so far from the Sun.

Bear in mind that, as the spacecraft continues to move outward from 650 AU, the diameter of the image becomes larger. We wind up with a blurring problem that has to be tackled by image processing algorithms. Get enough data, though, and the image can be deconvolved, allowing a sharp image of the exoplanet’s surface to emerge. As you would imagine, a coronagraph must be available to block out the Sun’s light.

What to do with the sail used to reach these distances? The mission plan is a close solar pass and sail deployment timed to produce maximum acceleration for the long cruise to destination. Solar sails are dead weight the further we get from the Sun, so you would assume the sail would be jettisoned, although it’s interesting to see that the team is working on ways to convert it into an antenna, or perhaps even a reflector for laser communications. As to power sources for electric propulsion within the image cylinder, the paper envisions using radioisotope thermoelectric generators, which are what will power up the craft’s communications, instruments and computing capabilities.

Image: This is Figure 4 from the paper. Caption: Trajectory of the mission design concept for a solar sailcraft to exit the solar system. Credit: Friedman et al./JPL.

Let’s clear up another misconception. If we deploy a sail at perihelion, we are relying on the solar photons delivering momentum to the sail (photons have no mass, but they do carry momentum). This is not the solar wind, which is a stream of particles moving at high velocity out from the Sun, and interesting in its own right in terms of various mission concepts that have been advanced in the literature. The problem with the solar wind, though, is that it is three orders of magnitude smaller than what we can collect from solar photons. What we need, then, is a photon sail of maximum size, and a payload of minimum mass, which is why the SGLF mission focuses on microsats. These may be networked or even undergo self-assembly during cruise to the gravity focus.

The size of a sail is always an interesting concept to play with. Ponder this: The sail mission to Halley’s Comet that Friedman worked on back in the mid-1970s would have demanded a sail that was 15 kilometers in diameter, in the form of a so-called heliogyro, whose blades would have been equivalent to a square sail half a mile to the side. That was a case of starting at the top, and as the paper makes clear, issues of packaging and deployment alone were enough to make the notion a non-starter.

Still, it was an audacious concept and it put solar sails directly into NASA’s sights for future development. The authors believe that based on our current experience with using sails in space, a sail of 100 X 100 square meters is about as large as we are able to work with, and it might require various methods of stiffening its structural booms. The beauty of the new SunVane concept is that it uses multiple sails, making it easier to package and more controllable in flight. This is the ‘Lightcraft’ design out of Xplore Inc., which may well represent the next step in sail evolution. If it functions as planned, this design could open up the outer system to microsat missions of all kinds.

Image: This is Figure 5 from the paper. Caption: Xplore’s Lightcraft TM advanced solar sail for rapid exploration of the solar system. Credit; Friedman et al./JPL.

Pushing out interstellar boundaries also means pushing materials science hard. After all, we’re contemplating getting as close to the Sun as we can with a sail that may be as thin as one micron, with a density less than 1 gram per square meter. The kind of sail contemplated here would weigh about 10 kg, with 40 kg for the spacecraft. The payload has to be protected from a solar flux that at 0.1 AU is 100 times what we receive on Earth, so the calculations play the need for shielding against the need to keep the craft as light as possible. An aluminized polymer film like Kapton doesn’t survive this close to the Sun, which is why so much interest has surfaced in materials that can withstand higher temperatures; we’ve looked at some of this work in these pages.

But the longer-term look is this:

Advanced technology may permit sails the size of a football field and spacecraft the size of modern CubeSats, and coming close to the Sun with exotic materials of high reflectivity and able to withstand the very high temperatures. That might permit going twice as fast, 40 AU/year or higher. If we can do that it will be worth waiting for. With long mission times, and with likely exoplanets in several different star systems being important targets of exploration we may want to develop a low cost, highly repeatable and flexible spacecraft architecture – one that might permit a series of small missions rather than one with a traditional large, complex spacecraft. The velocity might also be boosted with a hybrid approach, adding an electric propulsion to the solar sail.

It’s worth mentioning that we need electric propulsion on this craft anyway as the craft maneuvers to collect data near the gravitational focus. Testing all this out charts a developmental path through a technology demonstrator whose funding through a public-private partnership is currently being explored. This craft would make the solar flyby and develop the velocity needed for a fast exit out of the Solar System. A series of precursor missions could then test the needed technologies for deployment at the SGL We can envision Kuiper Belt exploration and, as the authors do, even a mission to a future interstellar object entering our system using these propulsion methods.

I recommend this new paper to anyone interested in keeping up with the JPL design for reaching the solar gravitational focus. As we’ve recently discussed, a vision emerges in which we combine solar sails with microsats that weigh in the range of 50 kilograms, with extensive networking capabilities and perhaps the ability to perform self-assembly during cruise. For the cost of a single space telescope, we could be sending multiple spacecraft to observe a number of different exoplanets before the end of this century, each with the capability to resolve features on the surface of these worlds. Resolution would be to the level of a few kilometers. We’re talking about continents, oceans, vegetation and, who knows, perhaps even signs of technology. And that would be on not one but thousands of potential targets within a ten light year radius from Earth.

The paper is Friedman et al., “A mission to nature’s telescope for high-resolution imaging of an exoplanet,” Experimental Astronomy 57 (2024), 1 (abstract).

How long before we can send humans to another star system? Ask people active in the interstellar community and you’ll get answers ranging from ‘at least a century’ to ‘never.’ I’m inclined toward a view nudging into the ‘never’ camp but not quite getting there. In other words, I think the advantages of highly intelligent instrumented payloads will always be apparent for missions of this duration, but I know human nature well enough to believe that somehow, sometime, a few hardy adventurers will find a way to make the journey. I do doubt that it will ever become commonplace.

You may well disagree, and I hope you’re right, as the scenarios open to humans with a galaxy stuffed with planets to experience are stunning. Having come into the field steeped in the papers and books of Robert Forward, I’ve always been partial to sail technologies and love the brazen, crazy extrapolation of Forward’s “Flight of the Dragonfly,” which appeared in Analog in 1982 and which would later be turned into the novel Rocheworld (Baen, 1990). This is the novel where Forward not only finds a bizarre way to keep a human crew sane through a multi-decade journey but also posits a segmented lightsail to get the crew home.

Image: The extraordinary Robert Forward, whose first edition of Flight of the Dragonfly was expanded a bit from the magazine serial and offered in book form in 1984. The book would later be revised and expanded further into the 1990 Baen title Rocheworld. The publishing history of this volume is almost as complex as the methods Forward used to get his crew back from Barnard’s Star!

Forward was a treasure. Like Freeman Dyson, his imagination was boundless. Whether we would ever choose to build the vast Fresnel lens he posited in the outer Solar System as a way of collimating a laser beam from near-Sol orbit, and whether we could ever use that beam to reflect off detached segments of the sail upon arrival to slow it down are matters that challenge all boundaries of engineering. I can hear Forward chuckling. Here’s the basic idea, as drawn from his original paper on the concept.

Image: Forward’s separable sail concept used for deceleration, from his paper “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” Journal of Spacecraft and Rockets 21 (1984), pp. 187-195. The ‘paralens’ in the image is a huge Fresnel lens made of concentric rings of lightweight, transparent material, with free space between the rings and spars to hold the vast structure together, all of this located between the orbits of Saturn and Uranus. Study the diagram and you’ll see that the sail has three ring segments, each of them separating to provide a separate source of braking or acceleration for the arrival, respectively, and departure of the crew. Imagine the laser targeting this would require. Credit: Robert Forward.

I tend to think that Les Johnson is right about sails as they fit into the interstellar picture. In a recent interview with a publication called The National, Johnson (NASA MSFC) made the case that we might well reach another star with a sail driven by a laser. Breakthrough Starshot, indeed, continues to study exactly that concept, using a robotic payload miniaturized for the journey and sent in swarms of relatively small sails driven by an Earth-based laser. But when it comes to human missions to even nearby stars, Johnson is more circumspect. Let me quote him on this from the article:

“As for humans, that’s a lot more complicated because it takes a lot of mass to keep a group of humans alive for a decade-to centuries-long space journey and that means a massive ship. For a human crewed ship, we will need fusion propulsion at a minimum and antimatter as the ideal. While we know these are physically possible, the technology level needed for interstellar travel seems very far away – perhaps 100 to 200 years in the future.”

Johnson’s background in sail technologies for both near and deep space at Marshall Space Flight Center is extensive. Indeed, there was a time when his business card described him as ‘Manager of Interstellar Propulsion Technology Research’ (he once told me it was “the coolest business card ever”). He has also authored (with Gregory Matloff and Giovanni Vulpetti), books like Solar Sails: A Novel Approach to Interstellar Travel (Springer, 2014) and A Traveler’s Guide to the Stars (Princeton University Press, 2022), as well as editing the recent Interstellar Travel: Purpose and Motivations (Elsevier, 2023). In addition to that, his science fiction novels have explored numerous deep space scenarios.

Image: NASA’s Les Johnson, a prolific author and specialist in sail technologies. Credit: NASA.

So there’s a much more optimistic take on the human interstellar guideline than the one I gave in my first paragraph, and of course I hope it’s on target. We’re probably not going to be going to what is sometimes called an “Earth 2.0,” in Johnson’s view, because he doubts there are any such reasonably close to us. That’s something we’ll be learning a great deal more about as future space instrumentation comes online, but we can bear in mind that the explorers who tackled the Pacific in the great era of sail didn’t set out thinking they were going to find another Europe, either. The point is to explore and to learn what you can, with all the unexpected benefits that brings.

Johnson’s early interest in sails, by the way, was fired not so much by Forward’s Rocheworld as by Larry Niven and Jerry Pournelle’s novel The Mote in God’s Eye (Simon & Schuster, 1974), where an incoming laser sail from another civilization is detected. The realization that unusual astronomical observations point to a technology, and a laser-beaming one at that, is an exciting part of the book. Here the authors’ human starship crew describes the detection of a strange light emanating from a smaller star (the Mote) in front of a much larger supergiant (the Eye):

“…I checked with Commander Sinclair. He says his grandfather told him the Mote was once brighter than Murcheson’s Eye, and bright green. And the way Gavin’s describing that holo – well, sir, stars don’t radiate all one color. So -”

“All the more reason to think the holo was retouched. But it is funny, with that intruder coming straight out of the Mote…”

“Light,” Potter said firmly.

“Light sail!” Rod shouted in sudden realization…”

For more on all this, see my Our View of a Decelerating Magsail in these pages. It’s not surprising that Niven and Pournelle ran their lightsail concept past Robert Forward at a time when the idea was just gaining traction. We all have career-changing literary experiences. I can remember how a childhood reading of Poul Anderson’s The Enemy Stars (J.B. Lippincott, 1959) utterly fired my imagination toward the idea of leaving the Solar System entirely. It was a finalist for the Hugo Award that year following serialization in Astounding, though I didn’t encounter it until later.

Johnson’s work at Marshall Space Flight Center takes in the deployment of a large solar sail quadrant for the Solar Cruiser mission that was first unfurled in 2022 to demonstrate TRL 5 capability, and has just been deployed at contractor Redwire Corp.’s facility in Longmont, Colorado to demonstrate TRL 6. In NASA’s terms, that means going from “Component or breadboard validation in relevant environment” (TRL 5) to “System or subsystem model or prototype demonstration in a relevant environment (ground or space).” In other words, this is progress. At TRL 6, a system is considered “a fully functional prototype or representational model.” Says Johnson in a recent email:

“25 years ago, when I first met Dr. Forward, he inspired me to plan a development program for solar sails that would eventually lead us to the stars. With Bob’s help, I laid out a milestone driven roadmap that began with the space flight of a 10 m² solar sail, which we did in 2010 with NanoSail-D.

“Next on the plan was the development of something an order of magnitude larger. This was achieved with the development and launch of the 86 m² Near Earth Asteroid Scout solar sail in 2022 and the soon to be launched ACS-3 sail. The Solar Cruiser sail is an order of magnitude larger still at 1653 Square meters. The next step is 10,000!”

Image: NASA and industry partners used two 100-foot lightweight composite booms to unfurl the 4,300-square-foot sail quadrant for the first time Oct. 13, 2022, at Marshall Space Flight Center, making it the largest solar sail quadrant ever deployed at the time. On Jan. 30, 2024, NASA cleared a key technology milestone, demonstrating TRL6 capability at Redwire’s new facility in Longmont, Colorado, with the successful deployment of one of four identical solar sail quadrants. Credit: NASA (although I’ve edited the caption slightly to reflect the TRL level reached).

Solar sails are becoming viable choices for space missions, and the Breakthrough Starshot investigations remind us that sails driven not just by sunlight but by lasers are within the bounds of physics. A key question that will be informed by our experience with solar sails is how laser-driven techniques scale. Theoretically, they seem to scale quite well. Are the huge structures Forward once wrote about remotely feasible (perhaps via nanotech construction methods), or is Johnson right that fusion and one day antimatter may be necessary for craft large enough (and fast enough) to carry human crews?

Some things just run in families. If you look into the life of Otto Struve, you’ll find that the Russian-born astronomer was the great grandson of Friedrich Georg Wilhelm von Struve, who was himself an astronomer known for his work on binary stars in the 19th Century. Otto’s father was an astronomer as well, as was his grandfather. That’s a lot of familial energy packed into the study of the stars, and the Struve of most recent fame (Otto died in 1963) drew on that energy to produce hundreds of scientific papers. Interestingly, the man who was director at Yerkes and the NRAO observatories was also an early SETI advocate who thought intelligence was rife in the Milky Way.

Of Baltic-German descent, Otto Struve might well have become the first person to discover an exoplanet, and therein hangs a tale. Poking around in the history of these matters, I ran into a paper that ran in 1952 in a publication called The Observatory titled “Proposal for a Project of High-Resolution Stellar Radial Velocity Work.” Then at UC Berkeley, Struve had written his PhD thesis on the spectroscopy of double star systems at the University of Chicago, so his paper might have carried more clout than it did. On the other hand, Struve was truly pushing the limits.

Image: Astronomer Otto Struve (1897-1963). Credit: Institute of Astronomy, Kharkiv National University.

For Struve was arguing that Doppler measurements – measuring the wavelength of light as a star moves toward and then away from the observer – might detect exoplanets, if they existed, a subject that was wildly speculative in that era. He was also saying that the kind of planet that could be detected this way would be as massive as Jupiter but in a tight orbit. I can’t call this a prediction of the existence of ‘hot Jupiters’ as much as a recognition that only that kind of planet would be available to the apparatus of the time. And in 1952, the idea of a Jupiter-class planet in that kind of orbit must have seemed like pure science fiction. And yet here was Struve:

…our hypothetical planet would have a velocity of roughly 200 km/sec. If the mass of this planet were equal to that of Jupiter, it would cause the observed radial velocity of the parent star to oscillate with a range of ± 0.2 km/sec—a quantity that might be just detectable with the most powerful Coudé spectrographs in existence. A planet ten times the mass of Jupiter would be very easy to detect, since it would cause the observed radial velocity of the star to oscillate with ± 2 km/sec. This is correct only for those orbits whose inclinations are 90°. But even for more moderate inclinations it should be possible, without much difficulty, to discover planets of 10 times the mass of Jupiter by the Doppler effect.

Struve suggested that binary stars would be a fertile hunting ground, for the radial velocity of the companion star would provide a “reliable standard of velocity.”

Imagine what would have happened if the discovery of 51 Pegasi (the work of Michel Mayor and Didier Queloz in 1995) had occurred in the early 1960s, when it was surely technically possible. Joshua Winn (Princeton University) speculates about this in his book The Little Book of Exoplanets (Princeton University Press, 2023). And if you start going down that road, you quickly run into another name that I only recently discovered, that of Kaj Aage Gunnar Strand (1907-2000). Working at Sproul Observatory (Swarthmore College) Strand announced that he had actually discovered a planet orbiting 61 Cygni in 1943. Struve considered this a confirmed exoplanet.

Now we’re getting deep into the weeds. Strand was using photometry, as reported in his paper “61 Cygni as a Triple System.” In other words, he was comparing the positions of the stars in the 61 Cygni binary system to demonstrate that they were changing over time in a cycle that showed the presence of an unseen companion. Here I’m dipping into the excellent Pipettepen site at the University of North Carolina, where Mackenna Wood has written up Strand’s work. And as Wood notes, Strand was limited to using glass photographic plates and a ruler to make measurements between the stars. Here’s the illustration Wood ran showing how tricky this would have been:

Image: An example of a photographic plate from one of the telescopes used in the 1943 61 Cygni study. The plate is a negative, showing stars as black dots, and empty space in white. Brighter stars appear as larger dots. Written at the bottom of the plate are notes indicating when the image was taken (Nov. 10, 1963), and what part of the sky it shows. Credit: Mackenna Wood.

Strand’s detection is no longer considered valid because more recent papers using more precise astrometry have found no evidence for a companion in this system. And that was a disappointment for readers of Arthur C. Clarke, who in his hugely exciting The Challenge of the Spaceship (1946) had made this statement in reference to Strand: “The first discovery of planets revolving around other suns, which was made in the United States in 1942, has changed all ideas of the plurality of worlds.”

Can you imagine the thrill that would have run up the spine of a science fiction fan in the late 1940s when he or she read that? Someone steeped in Heinlein, Asimov and van Vogt, with copies of Astounding available every month on the newsstand and the great 1950s era of science fiction about to begin, now reading about an actual planet around another star? I have a lot of issues of Astounding from the late 1930s in my collection though few from the late ‘40s, but I plan to check on Strand’s work to see if it appeared in any fashion in John Campbell’s great magazine in the following decade. Surely there would have been a buzz at least in the letter columns.

Image: Kaj Aage Gunnar Strand (1907-2000) was director of the U.S. Naval Observatory from 1963 to 1977. He specialized in astrometry, especially work on double stars and stellar distances. Credit: Wikimedia Commons / US Navy.

We’re not through with early exoplanet detection yet, though, and we’re staying at the same Sproul Observatory where Strand did the 61 Cygni work. It was in 1960 that another Sproul astronomer, Sarah Lippincott, published work arguing that Lalande 21185 (Gliese 411) had an unseen companion, a gas giant of ten Jupiter masses. A red dwarf at 8.3 light years out, this star is actually bright enough to be seen with even a small telescope. And in fact it does have two known planets and another candidate world, the innermost orbiting in a scant twelve days with a mass close to three times that of Earth, and the second on a 2800-day orbit and a mass fourteen times that of Earth. The candidate planet, if confirmed, would orbit between these two.

The work on Lalande 21185 in exoplanet terms goes back to Peter van de Kamp, who proposed a massive gas giant there in 1945. Lippincott was actually one of van de Kamp’s students, and the duo used astrometrical techniques to study photographic plates taken at Sproul. It turns out that Sproul photographic plates taken at the same time as those Lippincott used in her later paper on the star were later used by van de Kamp in his claim of a planetary system at Barnard’s Star. It was demonstrated later that the photographic plates deployed in both studies were flawed. Systematic errors in the calibration of the telescope were the culprit in the mistaken identifications.

Image: Astronomer Peter van de Kamp (1901-1995). Credit: Rochester Institute of Technology newsletter.

We always knew that exoplanet hunting would push us to the limits, and today’s bounty of thousands of new worlds should remind us of how the landscape looked 75 years ago when Otto Struve delved into detection techniques using the Doppler method. At that time, as far as he knew, there was only one detected exoplanet, and that was Strand’s detection, which as we saw turned out to be false. But Struve had the method down if hot Jupiters existed, and of course they do. He also reminded us of something else, that a large enough planet seen at the right angle to its star should throw a signal:

There would, of course, also be eclipses. Assuming that the mean density of the planet is five times that of the star (which may be optimistic for such a large planet) the projected eclipsed area is about 1/50th of that of the star, and the loss of light in stellar magnitudes is about 0.02. This, too, should be ascertainable by modern photoelectric methods, though the spectrographic test would probably be more accurate. The advantage of the photometric procedure would be its fainter limiting magnitude compared to that of the high-dispersion spectrographic technique.

There, of course, is the transit method which has proven so critical in fleshing out our catalogs of exoplanets. Both radial velocity and transit techniques would prove far more amenable to early exoplanet detection than astrometry of the sort that van de Kamp and Lippincott used, though astrometry definitely has its place in the modern pantheon of detection methods. Back in 1963, when van de Kamp announced the discovery of what he thought were planets at Barnard’s Star, he relied on almost half a century of telescope observations to build his case. No one could fault his effort, and what a shame it is that the astronomer died just months before the discovery of 51 Pegasi b. It would be fascinating to have his take on all that has happened since.

Our recent conversations about the likelihood of life elsewhere in the universe emphasize how early in the search we are. Consider recent work on TRAPPIST-1, which draws on JWST data to tell us more about the nature of the seven planets there. On the surface, this seven-planet system around a nearby M-dwarf all but shouts for attention, given that we have three planets in the habitable zone, all of them of terrestrial size, as indeed are all the planets in the system. Moreover, as an ultracool dwarf star, the primary is both tiny and bright in the infrared, just the thing for an instrument like the James Webb Space Telescope to harvest solid data on planetary atmospheres.

This is a system, in other words, ripe for atmospheric and perhaps astrobiological investigation, and Michaël Gillon (University of Liége), the key player in discovering its complexities, points in a new paper to how much we’ve already learned. If its star is ultracool, the planetary system at TRAPPIST-1 can also be considered ‘ultracompact’ in that the innermost and outermost planets orbit at 0.01 and 0.06 AU respectively. By comparison, Mercury orbits at 0.4 AU from our Sun. The stability of the system through mean motion resonances means that we’re able to deduce tight limits on mass and density, which in turn give us useful insights into their composition.

Image: Measuring the mass and diameter of a planet reveals its density, which can give scientists clues about its composition. Scientists now know the density of the seven TRAPPIST-1 planets with a higher precision than any other planets in the universe, other than those in our own solar system. Credit: NASA/JPL-Caltech/R. Hurt (IPAC).

Because we’ve been talking about SETI recently, I’ll mention that the SETI Institute has already subjected TRAPPIST-1 to a search using the Allen Telescope Array at frequencies of 2.84 and 8.2 gigahertz. The choice of frequencies was dictated by the researchers’ interest in whether a system this compact might have a civilization that had spread between two or more worlds. Searching for powerful broadband communications when planetary alignments between two habitable planets occur as viewed from Earth is thus a hopeful strategy, and as is obvious, the search yielded nothing unusual. A broader question is whether life might spread between such worlds through impacts and subsequent contamination.

What I’m angling for here is the relationship between a bold, unlikely observing strategy and a more orthodox study of planetary atmospheres. Both of these are ongoing, with the investigation of biosignatures a hot topic as we work with JWST but also plan for subsequent space telescopes like the Habitable Exoplanet Observatory (HabEx). The gap in expectations between SETI at TRAPPIST-1 and atmosphere characterization via such instruments highlights what a shot in the dark SETI can be. But it’s a useful shot in the dark. We need to know that there is a ‘great silence’ and continue to poke into it even as we explore the likelihood of abiogenesis elsewhere.

But back to the Gillon paper. Here you’ll find the latest results on planetary dynamics at TRAPPIST-1 and the implications for how these worlds form, along with current data on their densities and compositions. Another benefit of the compact nature of this system is that the planets interact with each other, which means we get strong signals from Transit Timing Variations that help constrain the orbits and masses involved. No other system has rocky exoplanets with such tight density measurements. The three inner planets are irradiated beyond the runaway greenhouse limit, and recent work points to the two inner planets being totally desiccated, with volatiles likely in the outer worlds.

What we’d like to know is whether, given that habitable zone planets are found in M-dwarf systems (Proxima Centauri is an obvious further example), such worlds can maintain a significant atmosphere given irradiation from the parent star. This is tricky work. There are models of the early Earth that involve massive volatile losses, and yet today’s Earth is obviously life supporting. Is there a possibility that rocky planets around M-dwarfs could begin with a high volatile content to counterbalance erosion from stellar bombardment? Gillon sees TRAPPIST-1 as an ideal laboratory to pursue such investigations, one with implications for M-dwarfs throughout the galaxy. From the paper:

Indeed, its planets have an irradiation range similar to the inner solar system and encompassing the inner and outer limits of its circumstellar habitable zone, with planet b and h receiving from their star about 4.2 and 0.15 times the energy received by the Earth from the Sun per second, respectively. Detecting an atmosphere around any of these 7 planets and measuring its composition would be of fundamental importance to constrain our atmospheric evolution and escape models, and, more broadly, to determine if low-mass M-dwarfs, the larger reservoir of terrestrial planets in the Universe, could truly host habitable worlds.

Image: Belgian astronomer Michaël Gillon, who discovered the planetary system at TRAPPIST-1. Credit: University of Liége.

Thus the early work on TRAPPIST-1 atmospheres, conducted with Hubble data and sufficient to rule out the presence of cloud-free hydrogen-dominated atmospheres for all the planets in the system. But now we have early papers using JWST data, and the issues become more stark when we turn to work performed by Gwenaël Van Looveren (University of Vienna) and colleagues. While previous studies of the system have indicated no thick atmospheres on the two innermost planets (b and c), the Van Looveren team focuses specifically on thermal losses occurring as the atmosphere heats as opposed to hard to measure non-thermal processes like stellar winds.

Here the situation clarifies. Working with computer code called Kompot, which calculates the thermo-chemical structure of an upper atmosphere, the team has analyzed the highly irradiated TRAPPIST-1 environment, modeling over 500 photochemical reactions in light of X-Ray, ultraviolet and infrared radiation, among other factors. The results show strong atmospheric loss in the early era of system development, but take into account losses through the different stages of the system’s evolution. It’s important to keep in mind that a star like this takes between 1 and 2 billion years to settle onto the main sequence, a period of high radiation. It’s also true that even main-sequence M-dwarfs can show high levels of radiation activity.

The upshot: X-ray and UV activity declines very slowly in the first several billion years on the main sequence, and stellar radiation in these wavelengths is the main driver of atmospheric loss. Things look dicey for atmospheres on any of the TRAPPIST-1 planets, and the Van Looveren model generalizes to other stars. From the paper:

The results of our models tentatively indicate that the habitable zone of M dwarfs after their arrival on the main sequence is not suited for the long-term survival of secondary atmospheres around planets of the considered planetary masses owing to the high ratio of spectral irradiance of XUV to optical/infrared radiation over a very long time compared to more massive stars. Maintaining atmospheres on planets like this requires their continual replenishment or their formation very late in the evolution of the planets. A further expansion of the grid and more detailed studies of the parameter space are required to draw definitive conclusions for the entire spectral class of M dwarfs.

Image: This is Figure 8 from the paper. Caption: Overview of the planets in the TRAPPIST-1 system and the estimated habitable zone (indicated by the green lines, taken from Bolmont et al. 2017). We added vertical lines at the minimum distances at which atmospheres of various compositions could survive for more than 1 Gyr. Credit: Van Looveren et al.

Note the term ‘primary atmosphere.’ Primary atmospheres of hydrogen and helium give way to secondary atmospheres that are the result of later processes like volcanic outgassing and molecules breaking down under stellar radiation on the planet’s surface. The paper, then, is saying that the kind of secondary atmospheres in which we might hope to find life are unlikely to survive in this environment, although active processes on a given planet might still allow them. The paper ends this way:

Our conclusion from this work is therefore significant for terrestrial planets with a mass that is similar to the Earth’s mass that orbit mid- to late-M dwarfs such as TRAPPIST-1 near or inside the (final) habitable zone. For these planets, substantial N2/CO2 atmospheres are unlikely unless atmospheric gas is continually replenished at high rates on timescales of no more than a few million years (the loss timescales estimated in our work), for example, through volcanism.

I wouldn’t call this the death knell for atmospheric survival at TRAPPIST-1, nor do the authors, but the work points to the factors that have to be addressed in further study of the system, and the results certainly challenge the possibility of life-sustaining atmospheres on any of these planets. The Van Looveren work isn’t included in Michaël Gillon’s paper, which appeared just before its release, but I hope you’ll look at both and keep the Gillon available as the best current overview of TRAPPIST-1.

As to M-dwarf prospects in general, it’s one thing to imagine a high-radiation environment, with the possibilities that life might find an evolutionary path forward, but quite another to strip a planet of its atmosphere altogether. If that is the prospect, then the census of ‘habitable’ worlds drops sharply, for M-dwarfs make up somewhere around 80 percent of all the stars in the Milky Way. A sobering thought to close the morning as I head upstairs to grind coffee beans and rejuvenate myself with caffeine.

The papers are Gillon, “TRAPPIST-1 and its compact system of temperate rocky planets,” to be published in Handbook of Exoplanets (Springer) and available as a preprint. The Van Looveren paper is “Airy worlds or barren rocks? On the survivability of secondary atmospheres around the TRAPPIST-1 planets,” accepted at Astronomy & Astrophysics (preprint).

It’s good now and then to let the imagination soar. Don Wilkins has been poking into the work of Carlo Rovelli at the Perimeter Institute, where the physicist and writer explores unusual ideas, though perhaps none so exotic as white holes. Do they exist, and are there ways to envision a future technology that can exploit them? A frequent contributor to Centauri Dreams, Don is an adjunct instructor of electronics at Washington University, St. Louis, where he continues to track research that may one day prove relevant to interstellar exploration. A white hole offers the prospect of even a human journey to another star, but turning these hypothesized objects into reality remains an exercise in mathematics, although as the essay explains, there are those exploring the possibilities even now.

by Don Wilkins

Among the many concepts for human interstellar travel, one of the more provocative is an offspring of Einstein’s theories, the bright twin of the black hole, the white hole. The existence of black holes (BH), the ultimate compression stage for aging stellar masses above three times the mass of our sun, is announced by theory and confirmed by observation. White holes, the matter spewing counterparts of BHs, escape observation but not the explorations of theorists.

Carlo Rovelli, an Italian theoretical physicist and writer, now the Distinguished Visiting Research Chair at the Perimeter Institute, discusses all this in a remarkably brief book called, simply, White Holes (Riverhead Books, 2023) wherein he travels in company with Dante Alighieri, another author with experience at descents into perilous places. Rovelli makes two remarkable assertions. [1]

1) Rovelli states that another scientist, Daniel Finkelstein, demonstrated that Einstein and other analysts are incorrect when they depict what occurs as one enters a black hole. From the Finkelstein paper (citation below):

The gravitational field of a spherical point particle is then seen not to be invariant under time reversal for any admissible choice of time coordinate. The Schwarzschild surface, r=2m is not a singularity but acts as a perfect unidirectional membrane: causal influences can cross it but only in one direction. [2]

In other words, no time dilation, no spaghettification of trespassers entering a black hole. Schwarzchild’s solution only applies to distant observers; it does not describe the observer crossing the event horizon of the black hole.

2) Rovelli believes in the existence of white holes. His white hole births when the black hole compresses its constituent parts into the realm of quantum mechanics. Rovelli speculates “… a black hole … quantum tunnels into a white one on the inside – and the outside can stay the same.”

In Figure 1 and Rovelli’s intuition, a quantum mesh separates the black hole and white hole. At these minute dimensions, quantum tunneling effects surge matter away from the black hole, into the mouth of the white hole and back into the Universe.

Figure 1. Relationship between a black hole and a white hole. Credit: C. Rovelli/Aix-Marseille University; adapted by APS/Alan Stonebraker.

The outside of a black hole and a white hole are geometrically identical regardless of the direction of time. The horizon is not reversible under the flow of time. As a result the interiors of the black hole and white hole are identical.

In a paper he co-authored with Hal Haggard, Rovelli writes:

We have constructed the metric of a black hole tunneling into a white hole by using the classical equations outside the quantum region, an order of magnitude estimate for the onset of quantum gravitational phenomena, and some indirect indications on the effects of quantum gravity. [3]

Haggard and Rovelli acknowledge that the calculations do not result from first principles. A full theory of quantum gravity would supply that requirement.

Figure 2: Artist rendering of the black-to-white-hole transition. Credit: F. Vidotto/University of the Basque Country. [9]

Efforts to design a stable wormhole require buttressing the entrance or mouth of the wormhole with prodigious amounts of a hypothesized material, negative matter. Although minute amounts have been claimed to form in the narrow confines of a Casimir device, ideas on how to manufacture planetary-sized masses of negative matter are elusive. [4]

According to recent research, the stability of the WH is dependent upon which of the two major families of matter, bosons or fermions, forms the WH. Bosons are subatomic particles which obey Bose-Einstein statistics and whose spin quantum number has an integer value (0, 1, 2, …). Photons, gluons, the Z neutral weak boson and the weakly charged bosons are bosons. The graviton, if it exists, is a boson. Theoretic analysis of stable traversable WHs founded on bosonic fields demonstrates a need for vast amounts of negative matter to hold open the mouth of a WH.

The other family, the fermions, have odd half-integer (1/2, 3/2, etc.) spins. These particles, electrons, muons, neutrinos, and compound particles, obey the Pauli Exclusion Principle. It is this family that is employed by a team of researchers to describe a two fermion stable white hole [5]. Their configuration produces John Wheeler’s “charge without charge”, where an electric field is trapped within the structure without any physical electrical charge present. The opening in the white hole would be too small, a few hundred Planck lengths (a Planck length is 1.62 x 10^-35 meters) to pass gamma rays.

Rovelli reenters the discussion here. [6] The James Webb Space Telescope has identified large numbers of black holes in the early Universe, more black holes than anticipated. Rovelli describes white holes forming from these black holes as Planck-length sized, chargeless entities, unable to interact with the matter except through gravity. In other words, the descendants of the early black holes manifest as the material we describe as dark matter. Rovelli is working on a quantum sensor to detect these white holes.

Once the white holes are detected, it might be possible to capture a white hole. John G. Cramer, professor emeritus of physics at the University of Washington in Seattle, Washington, suggests accelerating the wormhole to almost the speed of light. [7] Aimed at Tau Ceti, he predicts:

The arrival time as viewed through a wormhole is T’ = T/γ , where γ is the Lorentz factor [γ= (1- v/c)^-½] and v is the wormhole-end velocity after acceleration. For reference, the maximum energy protons accelerated at CERN LHC have a Lorentz factor of 6,930. Thus, the arrival time at Tau Ceti of an LHC-accelerated wormhole-end would be 15 hours….Effectively, the accelerated wormhole becomes a time machine, connecting the present with an arrival far in the future.

Spraying accelerated electrons through the wormhole could expand the mouth to a size where it could be used as a sensor portal into another star system. The wormhole becomes a multi light-year long periscope, one that scientists could bend and twist to study up close and in detail the star and its companions. Perhaps the wormhole could be expanded enough to pass larger, physical bodies.

Constantin Aniculaesei and an international team of researchers may have overcome the need for an accelerator as large as the LHC to accelerate the white hole to useful size [8]. Developing a novel wakefield accelerator, wherein an intense laser pulse focused onto a plasma excites nonlinear plasma waves to trap electrons, the team’s machine produced 10 Giga electron Volt (GeV) electron bunches. The wakefield accelerator was only ten centimeters long, although a petawatt laser was needed to excite the wakefields.

Cramer hypothesizes that fermionic white holes formed immediately after the Big Bang and in cosmic rays. The gateways to the stars could be found in the cosmic ray bombardment of the Earth or possibly trapped in meteorites. The heavy particles, if ensnared on Earth, would probably sink to the center of the planet.

All that is needed to find a fermionic white hole, Cramer suggests, is a mass spectrometer. But let me quote him on this:

[Wormholes] might be a super-heavy components of cosmic rays….They might be trapped in rocks and minerals….In a mass spectrograph, they could in principle be pulled out of a vaporized sample by an electric potential but would be so heavy that they would move in an essentially undeflected straight line in the magnetic field. …wormholes might still be found in meteorites that formed in a gravity free environment.

The worm hole is essentially unaffected by a magnetic field. A mass detector would point to an invisible mass. The rest, as non-engineers like to say, is merely engineering.

If this line of reasoning is correct – a very large if – enlarged white holes could pass messages and matter through tunnels in the sky to distant stars.

References

1. Carlo Rovelli, translation by Simon Carnell, White Holes, Riverhead Books, USA, 2023

2. David Finkelstein, Past-Future Asymmetry of the Gravitational Field of a Point Particle, Physical Review, 110, 4, pages 965–967, May 1958, 10.1103/PhysRev.110.965

3. Hal M. Haggard and Carlo Rovelli, Black hole fireworks: quantum-gravity effects outside the horizon spark black to white hole tunneling, 4 July 2014, https://arxiv.org/pdf/1407.0989.pdf

4. Matt Visser, Traversable wormholes: Some simple examples, arXiv:0809.0907 [gr-qc], 4 September 2008.

5. Jose Luis Blázquez-Salcedo, Christian Knoll, and Eugen Radu, Traversable Wormholes in Einstein-Dirac-Maxwell theory, arXiv:2010.07317v2, 12 March 2022.

6. What is a white hole? – with Carlo Rovelli, The Royal Institution, https://www.youtube.com/watch?v=9VSz-hiuW9U

7. John G. Cramer, Fermionic Traversable Wormholes, Analog Science Fiction & Fact, January/February 2022.

8. Constantin Aniculaesei, Thanh Ha, Samuel Yoffe, et al, The Acceleration of a High-Charge Electron Bunch to 10 GeV in a 10-cm Nanoparticle-Assisted Wakefield Accelerator, Matter and Radiation at Extremes, 9, 014001 (2024), https://doi.org/10.1063/5.0161687

9). Rovelli, “Black Hole Evolution Traced Out with Loop Quantum Gravity,” Physics 11, 127 (December 10, 2018).
https://physics.aps.org/articles/v11/127

We live in a world that is increasingly at ease with the concept of intelligent extraterrestrial life. The evidence for this is all around us, but I’ll cite what Louis Friedman says in his new book Alone But Not Lonely: Exploring for Extraterrestrial Life (University of Arizona Press, 2023). When it polled in the United States on the question in 2020, CBS News found that fully two-thirds of the citizenry believe not only that life exists on other planets, but that it is intelligent. That this number is surging is shown by the fact that in polling 10 years ago, the result was below 50 percent.

Friedman travels enough that I’ll take him at his word that this sentiment is shared globally, although the poll was US-only. I’ll also agree that there is a certain optimism that influences this belief. In my experience, people want a universe filled with civilizations. They do not want to contemplate the loneliness of a cosmos where there is no one else to talk to, much less one where valuable lessons about how a society survives cannot be learned because there are no other beings to teach us. Popular culture takes many angles into ETI ranging from alien invasion to benevolent galactic clubs, but on the whole people seem unafraid of learning who aliens actually are.

Image: Louis Friedman, Co-Founder and Executive Director Emeritus, The Planetary Society. Credit: Caltech.

The silence of the universe in terms of intelligent signals is thus disappointing. That’s certainly my sentiment. I wrote my first article on SETI back in the early 1980s for The Review of International Broadcasting, rather confident that by the end of the 20th Century we would have more than one signal to decipher from another civilization. Today, each new report from our active SETI efforts at various wavelengths and in varying modes creates a sense of wonder that a galaxy as vast as ours has yet to reveal a single extraterrestrial.

It’s interesting to see how Friedman approaches the Drake equation, which calculates the number of civilizations that should be out there by setting values on factors like star and planet formation and the fraction of life-bearing planets where life emerges. I won’t go through the equation in detail here, as we’ve done that many times on Centauri Dreams. It’s sufficient to note that when Friedman addresses Drake, he cites the estimates for each factor in the current scientific literature and also gives a column with his own guess as to what each of these items might be.

Image: This is Table 1 from Friedman’s book. Credit: Louis Friedman / University of Arizona Press.

This gets intriguing. Friedman comes up with 1.08 civilizations in the Milky Way – that would be us. But he also makes the point that if we just take the first four terms in the Drake equation and multiply them by the time that Earth life has been in existence, we get on the order of two billion planets that should have extraterrestrial life. Thus a point of view I find consistent with my own evolving idea on the matter: Life is all over the place, but intelligent life is vanishingly rare.

Along the way Friedman dismisses the ‘cosmic zoo’ hypothesis that we looked at recently as being perhaps the only realistic way to support the idea that intelligent life proliferates in the Milky Way. Ian Crawford and Dirk Schulze-Makuch see a lot wrong with the zoo hypothesis as well, but argue that the idea we are being observed but not interacted with is stronger than any other explanation for what David Brin and others have called ‘the Great Silence.’ I’ll direct you to Milan M. Ćirković’s The Great Silence: Science and Philosophy of Fermi’s Paradox for a rich explanation both cultural and scientific of our response to the ‘Where are they?’ question.

Before reading Alone But Not Lonely, my own thinking about extraterrestrial intelligence has increasingly focused on deep time. It’s impossible to run through even a cursory study of Earth’s geological history without realizing how tiny a slice our own species inhabits. The awe induced by these numbers tends to put a chill up the spine. The ‘snowball Earth’ episode seems to have lasted, for example, about 85 million years in its entirety. Even if we break it into two periods (accounting for the most severe conditions and excluding periods of lesser ice penetration), we still get two individual eras of global glaciation, each lasting ten million years.

These are matters that are still in vigorous debate among scientists, of course, so I don’t lean too heavily on the precise numbers. The point is simply to cast something as evidently evanescent as our human culture against the inexorable backdrop of geological time. And to contrast even that with a galaxy that is over 13 billion years old, where processes like these presumably occurred in multitudes of stellar systems. What are the odds that, if intelligence is rare, two civilizations would emerge at the same time and live long enough to become aware of each other? And does the lack of hard evidence for extraterrestrial civilizations not make this point emphatic?

But let me quote Friedman on this:

Let’s return to that huge difference between the time scales associated with the start of life on Earth and its evolution to intelligence. The former number was 3.5 to 3.8 billion years ago, a “mere” 0.75 to 1 billion years after Earth formed. Is that just a happenstance, or is that typical of planets everywhere? I noted earlier that intelligence (including the creation of technology) has only been around for 1/2,000,000 of that time—just the last couple thousand years. Life has been on Earth for about 85 percent of its existence; intelligence has been on Earth for about 0.0005 percent of that time. Optimists might want to argue that intelligence is only at its beginning, and after a million years or so those numbers will drastically change, perhaps with intelligence occupying a greater portion of Earth’s history. But that is a lot of optimism, especially in the absence of any other evidence about intelligence in the universe.

Friedman argues that the very fact we can envision numerous ways for humanity to end – nuclear war, runaway climate effects, deadly pandemics – points to how likely such an outcome is. It’s a good point, for technology may well contain within its nature the seeds of its own destruction. What scientists like Frank Tipler and Michael Hart began pointing out decades ago is that it only takes one civilization to overcome such factors and populate the galaxy, but that means we should be seeing some evidence of this. SETI continues the search as it should and we fine-tune our methods of detecting objects like Dyson spheres, but shouldn’t we be seeing something by now?

The reason for the ‘but not lonely’ clause in Friedman’s title is that ongoing research is making it clear how vast a canvas we have to analyze for life in all its guises. Thus the image below, which I swipe from the book because it’s a NASA image in the public domain. What I find supremely exciting when looking at an actual image of an exoplanet is that this has been taken by our latest telescope, which is itself in a line of technological evolution leading to completely feasible designs that will one day be able to sample the atmospheres of nearby exoplanets to search for biosignatures.

Image: This image shows the exoplanet HIP 65426 b in different bands of infrared light, as seen from the James Webb Space Telescope: purple shows the NIRCam instrument’s view at 3.00 microns, blue shows the NIRCam instrument’s view at 4.44 microns, yellow shows the MIRI instrument’s view at 11.4 microns, and red shows the MIRI instrument’s view at 15.5 microns. These images look different because of the ways that the different Webb instruments capture light. A set of masks within each instrument, called a coronagraph, blocks out the host star’s light so that the planet can be seen. The small white star in each image marks the location of the host star HIP 65426, which has been subtracted using the coronagraphs and image processing. The bar shapes in the NIRCam images are artifacts of the telescope’s optics, not objects in the scene. Credit: NASA, ESA, CSA, Alyssa Pagan (STScI).

Bear in mind the author’s background. He is of course a co-founder (with Carl Sagan and Bruce Murray) of The Planetary Society. At the Jet Propulsion Laboratory in the 1970s, Friedman was not only involved in missions ranging from Voyager to Magellan, but was part of the audacious design of a solar ‘heliogyro’ that was proposed as a solution for reaching Halley’s Comet. That particular sail proved to be what he now calls ‘a bridge too far,’ in that it was enormous (fifteen kilometers in diameter) and well beyond our capabilities in manufacture, packaging and deployment at the time, but the concept led him to a short book on solar sails and has now taken him all the way into the current JPL effort (led by Slava Turyshev) to place a payload at the solar gravitational lens distance from the Sun. Doing this would allow extraordinary magnifications and data return from exoplanets we may or may not one day visit.

Friedman is of the belief that interstellar flight is simply too daunting to be a path forward for human crews, noting instead the power of unmanned payloads, an idea that fits with his current work with Breakthrough Starshot. I won’t go into all the reasons for his pessimism on this – as the book makes clear, he’s well aware of all the concepts that have been floated to make fast interstellar travel possible, but skeptical they can be adapted for humans. Rather than Star Trek, he thinks in terms of robotic exploration. And even there, the idea of a flyby does not satisfy, even if it demonstrates that some kind of interstellar payload can be delivered. What he’s angling for beyond physical payloads is a virtual (VR) model in which AI techniques like tensor holography can be wrapped around data to construct 3D holograms that can be explored immersively even if remotely. Thus the beauty of the SGL mission:

We can get data using Nature’s telescope, the solar gravity lens, to image exoplanets identified from Earth-based and Earth-orbit telescopes as the most promising to harbor life. It also would use modern information technology to create immersive and participatory methods for scientists to explore the data—with the same definition of exploration I used at the beginning of this book: an opportunity for adventure and discovery. The ability to observe multiple interesting exoplanets for long times, with high-resolution imaging and spectroscopy with one hundred billion times magnification, and then immerse oneself in those observations is “real” exploration. VR with real data should allow us to use all our senses to experience the conditions on exoplanets—maybe not instantly, but a lot more quickly than we could ever get to one.

The idea of loneliness being liberating, which Friedman draws from E. O. Wilson, is a statement that a galaxy in which intelligence is rare is also one which is entirely open to our examination, one which in our uniqueness we have an obligation to explore. He lists factors such as interplanetary smallsats and advanced sail technologies as critical for a mission to the solar gravitational lens, not to mention the deconvolution of images that such a mission would require, though he only hints at what I consider the most innovative of the Turyshev team’s proposals, that of creating ‘self-assembling’ payloads through smallsat rendezvous en-route. In any case, all of these are incremental steps forward, each yielding new scientific discoveries from entirely plausible hardware.

Such virtual exploration does not, of course, rule out SETI itself, including the search for other forms of technosignature than radio or optical emissions. Even if intelligence ultimately tends toward machine incarnation, evidence for its existence might well turn up in the work of a mission to the gravitational lens. So I don’t think a SETI optimist will find much to argue with in this book, because its author makes clear how willing he is to continue to learn from the universe even when it challenges his own conceptions.

Or let’s put that another way. Let’s think as Friedman does of a program of exploration that stretches out for centuries, with not one but numerous missions exploring through ever refined technologies the images that the bending of spacetime near the Sun creates. We keep hunting, in other words, for both life and intelligence, for we know that the cosmos seems to have embedded within it the factor of surprise. A statement sometimes attributed to Asimov comes to mind: “The most exciting phrase to hear in science, the one that heralds new discoveries, is not “Eureka!” (I found it!) but “That’s funny…” The history of astronomy is replete with such moments. There will be more.

The book is Friedman, Alone but Not Lonely: Exploring for Extraterrestrial Life, University of Arizona Press, 2023.

Globular clusters, those vast ‘cities of stars’ that orbit our galaxy, get a certain amount of traction in SETI circles because of their age, dating back as they do to the earliest days of the Milky Way. But as Henry Cordova explains below, they’re a less promising target in many ways than the younger, looser open clusters which are often home to star formation. Because it turns out that there are a number of open clusters that likewise show considerable age. A Centauri Dreams regular, Henry is a retired map maker and geographer now living in southeastern Florida and an active amateur astronomer. Here he surveys the landscape and points to reasons why older open clusters are possible homes to life and technologies. Yet they’ve received relatively short shrift in the literature exploring SETI possibilities. Is it time for a new look at open clusters?

by Henry Cordova

If you’re looking for signs of extra-terrestrial intelligence in the cosmos, whether it be radio signals or optical beacons or technological residues, doesn’t it make sense to observe an area of sky where large numbers of potential candidates (particularly stars) are concentrated? Galaxies, of course, are large concentrations of stars, but they are so remote that it is doubtful we would be able to detect any artifacts at those distances. Star clusters are concentrations of stars gathered together in a small area of the celestial sphere easily within the field of view of a telescope or radio antenna. These objects also have the advantage that all their members are at the same distance, and of the same age,

Ask any amateur astronomer; “How many kinds of star cluster are there?” and he will answer; “Two, Open Clusters (OCs) and Globular Clusters (GCs)”. The terms “Globular” and “Open” refer to both their general morphology as well as their appearance through the eyepiece. It’s important to keep in mind that both are collections of stars presumably born at the same time and place (and hence, from the same material) but they are nevertheless very different kinds of objects. There does not seem to be a clearly defined transitional or intermediate state between the two. One type does not evolve into the other. Incidentally, the term ‘Galactic Cluster’ is often encountered when researching this field. It is an obsolete term for an OC and should be abandoned. It is too easily misunderstood as meaning a ‘cluster of galaxies’ and can lead to confusion.

GCs are in fact globular. They are collections of thousands, if not hundreds of thousands, of stars forming spheroidal aggregates much more densely packed towards their centers. OCs are amorphous and irregular in shape, random clumps of several hundred to several thousand stars resembling clouds of buckshot flying through space. Their distribution throughout the galaxy is different as well. GCs orbit the galactic center in highly elliptical orbits scattered randomly through space. They are, for the most part, located at great distances from us. OCs, on the other hand, appear to be restricted to mostly circular orbits in the plane of the Milky Way. Due to the obscuring effects of interstellar dust in the plane of the galaxy, most are seen relatively near Earth. although they are scattered liberally throughout the spiral arms.

Image: The NASA/ESA Hubble Space Telescope has captured the best ever image of the globular cluster Messier 15, a gathering of very old stars that orbits the center of the Milky Way. This glittering cluster contains over 100 000 stars, and could also hide a rare type of black hole at its center. The cluster is located some 35 000 light-years away in the constellation of Pegasus (The Winged Horse). It is one of the oldest globular clusters known, with an age of around 12 billion years. Very hot blue stars and cooler golden stars are seen swarming together in this image, becoming more concentrated towards the cluster’s bright center. Messier 15 is also one of the densest globular clusters known, with most of its mass concentrated at its core. Credit: NASA, ESA.

Studies of both types of clusters in nearby galaxies confirm these patterns are general, not a consequence of our Milky Way’s history and architecture, but a feature of galactic structure everywhere. Other galaxies are surrounded by clouds of GCs, and swarms of OCs circle the disks of nearby spirals. It appears that the Milky Way hosts several hundred GCs and several thousand OCs. It is now clear that not only is the distribution and morphology of star clusters divided into two distinct classes but their populations are as well. OCs are often associated with clouds of gas and dust, and are sometimes active regions of star formation. Their stellar populations are often dominated by massive bright, hot stars evolving rapidly to an early death. GCs, on the other hand, are relatively dust and gas free, and the stars there are mostly fainter and cooler, but long-lived. Any massive stars in GCs evolved into supernovae, planetary nebulae or white dwarfs long ago.

It appears that the globulars are very old. They were created during the earliest stages of the galaxy’s evolution. Conditions must have been very different back then; indeed, globulars may be almost as old as the universe itself. GC stars formed during a time when the interstellar medium was predominantly hydrogen and helium and their spectra now reveal large concentrations of heavy elements (“metals”, in astrophysical jargon). The metals have been carried up from the stellar cores by convective processes late in the stars’ life. Any planets formed around this early generation of stars would likely be gas giants, composed primarily of H and He—not the rocky Earth-type worlds we tend to associate with life.

Open Clusters, on the other hand, are relatively new objects. Many of them we can see are still in the process of formation, condensing from molecular clouds well enriched by metals from previous cycles of nucleogenesis and star formation. These clouds have been seeded by supernovae, solar winds and planetary nebulae with fusion products so that subsequent generations of stars will have the higher elements to incorporate in their own retinue of planets.

Image; Some of our galaxy’s most massive, luminous stars burn 8,000 light-years away in the open cluster Trumpler 14. Credit: NASA, ESA, and J. Maíz Apellániz (Institute of Astrophysics of Andalusia, Spain); Acknowledgment: N. Smith (University of Arizona).

Older OCs may have broken up due to galactic tidal stresses but new ones seem to be forming all the time, and there appears to be sufficient material in the galactic plane to ensure a continuous supply of new OCs for the foreseeable future. In general, GCs are extremely old and stable, but not chemically enriched enough to be suitable for life. OCs are young, several million years old, and they usually don’t survive long for life to evolve there. Any intelligent life would probably evolve after the cluster broke up and its stars dispersed. BUT…there are exceptions.

The most important parameter that determines a star’s history is its initial mass. All stars start off as gravitationally collapsing masses of gas, glowing from the release of gravitational potential energy. Eventually, temperatures and pressures in the stars’ cores rise to the point where nuclear fusion reactions start producing light and heat. This energy counteracts gravity and the star settles down to a long period of stability, the main sequence. The terminology arises from a line of stars in the color-magnitude diagram of a star cluster. Main sequence stars stay on this line until they run out of fuel and wander off the main sequence.

All stars follow the same evolutionary pattern, but where on the main sequence they wind up, and how long they stay there, depend on their initial mass. Massive stars evolve quickly, lighter ones tend to stay on the main sequence a long time. Our Sun has been a main sequence star for about 4.6 billion years, and it will remain on the main sequence for about another 5 billion years. When it runs out of nuclear fuel it will wander off the main sequence, getting brighter and cooler as it evolves.

All stars evolve in a similar way, but the amount of time they spend in that stable main sequence state is highly dependent on their mass at birth. Studying the point on the color-magnitude diagram of a cluster’s main sequence where stars start to “peel-off” from the MS allows astrophysicists to determine the age of the cluster. It is not necessary to know the absolute brightness, or distance, of the stars since, by definition, all the stars in a cluster are at the same distance. The color-magnitude (or Hertzprung-Russell) diagram is as important to astronomy as the periodic table is to chemistry. It allows us to visualize stellar evolution using a simple graphic model to interpret the data. It is one of the triumphs of 20th century science.

It is this ability to determine the age of a cluster that allows us to select a set of OCs that meet the criterion of great age needed for biological evolution to take place. Although open clusters tend to quickly lose their stars through gravitational interactions with molecular clouds in the disc of the galaxy, a surprising number seem to have survived long enough for biological, and possibly technologically advanced, species to evolve. Although less massive stars, such as main sequence red dwarfs, tend to be preferentially ejected from OCs due to gravitational tides, more massive F, G, and K stars are more likely to remain.

Sky Catalog 2000.0 (1) lists 32 OCs of ages greater than 1.0 Gyr. A more up-to-date reference, the Wikipedia entry (2), lists others. No doubt, a thorough search of the literature will reveal still more. A few of these OCs are comparable in age to the globulars. They are relics of an ancient time. But many others are comparable to our Sun in age (indeed, our own star, like many others, was born in an open cluster).

Regardless of the observing technique or wavelength utilized, an OC provides the opportunity to examine a large number of stars simultaneously, stars which have been pre-selected as being of a suitable age to support life or a technically advanced civilization. It will also be assured that, as members of an OC, all the stars sampled were formed in a metal-rich environment, and that any planets formed about those stars may be rocky or otherwise Earthlike.

If a technical civilization has arisen on any of those stars, it is possible that they have explored or colonized other stars in the cluster and we have the opportunity to eavesdrop on intra-cluster communications. And from the purely practical point of view, when acquiring scarce funding or telescope time for such a project, it will be possible to piggy-back a SETI program onto non-SETI cluster research. Other than SETI, there are very good reasons to study OCs. They provide a useful laboratory for investigations into stellar evolution.

References

1) Sky Catalog 2000.0, Vol II, Sky Publishing Corp, 1985.

2) https://en.wikipedia.org/wiki/List_of_open_clusters

Suggestions for Additional reading

1. H. Cordova, The SETI Potential of Open Star Clusters, SETIQuest, Vol I No 4, 1995

2. R. De La Fuente Marcos, C. De La Fuente Marcos, SETI in Star Clusters: A Theoretical Approach, Astrophysics and Space Science 284: 1087-1096, 2003

3. M.C. Turnbull, J.C. Tarter, Target Selection for SETI II: Tycho-2 Dwarfs, Old Open Clusters, And the Nearest 100 Stars, ApJ Supp. Series 149: 423-436, 2003

Working in the field has its limitations, as Alex Tolley reminds us in the essay that follows, but at least biologists have historically been on the same planet with their specimens. Today’s hottest news would be the discovery of life on another world, as we saw in the brief flurries over the Viking results in 1976 or the Martian meteorite ALH84001. We rely, of course, on remote testing and will increasingly count on computer routines that can make the fine distinctions needed to choose between biotic and abiotic reactions. A new technique recently put forward by Robert Hazen and James Cleaves holds great promise. Alex gives it a thorough examination including running tests of his own to point to the validity of the approach. One day using such methods on Mars or an ice giant moon may confirm that abiogenesis is not restricted to Earth, a finding that would have huge ramifications not just for our science but also our philosophy.

by Alex Tolley

Perseverance rover on Mars – composite image.

Cast your mind back to Darwin’s distant 5-year voyage on HMS Beagle. He could make very limited observations, make drawings and notes, and preserve his specimen collection for his return home to England.

Fifty years ago, a field biologist might not have much more to work with. Hours from a field station or lab with field guides and kits to preserve specimens, with no way to communicate. As for computers to make repetitive calculations, fuggedaboutit.

Fast forward to the late 20th and early 21st centuries, and fieldwork is extending out to the planets in our solar system to search for life. Like Darwin’s voyage, the missions are distant and long. Unlike Darwin, samples have not yet been returned from any planets, only asteroids and comets. Communication is slow, more on the order of field experiences. But instead of humans, our robot probes are “Going where no one has gone before” and humans may not go until much later. The greater the communication lag, the more problematic the central command to periphery control model. Reducing this delay demands a need for more peripheral autonomy at the periphery to make local decisions.

The 2006 Astrobiology Field Laboratory Science Steering Group report recommended that the Mars rover be a field laboratory, with more autonomy [17]. The current state of the art is the Perseverance rover taking samples in the Jezero crater, a prime site for possible biosignatures. Its biosignature instrument, SHERLOC, uses Raman spectrography and luminescence to detect and identify organic molecules [6]. While organic molecules may have been detected [19], the data had to be transmitted to Earth for interpretation, maintaining the problem of lag times between each sample to be chosen and analyzed.

As our technology improves, can these robots operating on planetary surfaces be able to do more effective in situ analyses in the search for extant or extinct life, so that they can operate more quickly like a human field scientist, in the search for life?

While we “know life when we see it”, nevertheless we still struggle to define what life is, although with terrestrial life we have sufficient characteristics except for edge cases like viruses and some ambiguous early fossil material. However, some defining characteristics do not apply to dead, or fossilized organisms and their traces. Fossil life does not metabolize, reproduce, or move, and molecules that are common to life no longer exist in their original form. Consider the “fossil microbes” on the Martian meteorite ALH84001 that caused such a sensation when announced but proved ambiguous.

Historically, for fossil life, we have relied on detecting biosignatures, such as C13/C12 ratios in minerals (due to chlorophyll carbon isotope preference), long-lasting biomolecules like lipids, homochirality of organic compounds, and disequilibria in atmospheric gases. Biomolecules can be ambiguous, as the amino acids detected in meteorites are most likely abiotic, something the Miller-Urey experiment demonstrated many decades ago.

Ideally, we would like a detection method that is simple, robust, and whose results can be interpreted locally without requiring analysis on Earth.

A new method to try to identify the probably biotic nature of samples with organic material is the subject of a new paper from a collaboration under Prof. Robert Hazen and James Cleaves. The team not only uses an analytical method—pyrolysis gas chromatography coupled to electron impact ionization mass spectrometry (Pyr-GC-EI-MS) to heat (pyrolyze), fractionate volatile components (gas chromatography), and determine their mass (mass spectrometry), but also analyzes the data to classify whether the new samples contain organic material of biological origin. Their reported early results are very encouraging [10, 11, 12].

The elegance of Hazen et al’s work has been to apply the Pyr-GC-EI-MS technique [3, 15, 18] that is not only available in the laboratory, but is also designed for planetary rovers to meet the need for local analysis. Their innovation has been to couple this process with computationally lightweight machine learning models to classify the samples, thereby bypassing the time lags associated with distant terrestrial interpretation. A rover could relatively rapidly take samples in an area and determine whether any might have a biosignature based on a suite of different detected compounds and make decisions locally on how to proceed.

The resulting data of masses and extraction time can be reduced and then classified using the pre-trained Random Forest [4], which is a suite of Decision Trees (see Figure 3) using samples of the feature set of masses, to provide a classification, which with the currently tested samples, provides a better than 90% probability of correct classification. The reported experiment used 134 samples, 75 labeled as abiotic and 59 as biotic or of biotic origin. The data set ranged in mass from 50 to 700 and several thousand scans over time. This data was reduced to a manageable size by reducing the mass and time ranges to 8147 values. The samples were then run against several machine learning methods, of which the Random Forest worked best.

To provide a visualization of which mass and time values were most instrumental in classifying the data, the 20 most informative data points were extracted and overlaid on the MS data as shown in Figure 1.

Figure 1 above shows 2 samples of data. One is the Allende meteorite which is classified as a 4.5 billion years old abiotic sample. This is contrasted with one of the microbial samples. While the details of the meteorite sample were not provided, older analyses by others indicated that the surface was contaminated with terrestrial material, whilst the interior matrix included polycyclic aromatic hydrocarbons, a common material found in samples from space missions [7,8]. The bacterial sample, as expected, shows many more compounds after pyrolysis, as the organism is composed of a large variety of organic compounds including amino acids, nucleobases, lipids, and sugars which will decompose with heating. A key point is that the discriminant features are not coincident with the most common masses in the samples, but rather in the rarer compounds as indicated by their intensities. [The lower bound mass bin ensures that common pyrolysis products such as low carbon number compounds will be excluded from the analysis and visualization. The data is normalized to the highest peak so that relative values rather than absolutes are analyzed to eliminate sample amounts.] Most of the defining compounds are in the 140 – 200 mass range, which would imply all-carbon compounds with 12-16 atoms.

Figure 2 shows a 2-dimensional Principal Components Analysis (PCA) using the 20 most informative features that illustrate the separation of the sample types. The expanded box encompasses all the abiotic samples.

I note that even though the biotic and natural samples were given the same classification label, these samples separate quite clearly too, indicating that the natural samples appear almost distinct from the biotic samples. What is perhaps surprising is that biological materials like cedarwood (oils derived from the bark) cluster with the abiotic samples, and even cyanobacteria seem similar in this view. Notice that the dried oak leaf, clearly a degraded living material, is remarkably similar to a cysteine (amino acid) and glucose Maillard reaction (used in the searing of foods to create flavors). A number of the natural materials that were classified as of biological origin or containing material of biological origin, also cluster closely with the abiotic samples, such as Quisqueite and Asphaltum. The peat sample (labeled natural) is placed between the bulk of both biological and natural samples.

Why should this technique work to classify samples according to the type labels? It has been argued that living things are both complex, but composed of molecules that occupy a relatively small space of possible diversity. [Work by Lee Cronin’s group has looked at the way biology restricts the possible structures of organic molecules to create complex macromolecules using few subunits. For example, the amino acid glycine is both important as a constituent of proteins, forming much of the structure of collagen, and is central to several biosynthesis pathways, that include the synthesis of porphyrins and thence to heme in red blood corpuscles. Some macromolecules such as cellulose are formed entirely of D-glucose, as are most complex sugar macromolecules. Cronin calls his technique Assembly Theory [1].]

But larger molecules constructed of a small number of simpler molecules alone are insufficient. Cellulose is a polymer of D-glucose molecules, but clearly, we would not state that a sheet of wet paper was once living, or formed by natural processes. A minimal complexity is required. Life relies on a suite of molecules connected by metabolic pathways that exquisitely restrict the possible number of resulting molecules, however complex, such as proteins that are constructed from just 20 of the possible much greater number of amino acids. At the heart of all life is the Krebs cycle which autotrophs use in the reverse direction to oxidation as part of carbon fixation to build biomass, often glucose to build cellulose cell walls.

The Pyr-GC-EI-MS technique detects a wide range of organic molecules, but the machine learning algorithm uses a set of specific ones to detect the requisite complexity as well as the abiotic randomness. In other words, this is complementary to Cronin’s “Assembly Theory” of life.

I would note that the PCA uses just 20 variables to separate the abiotic and biotic/natural samples. This appears adequate in the majority of the sample set but may be fewer than the variables used in the Random Forest machine learning algorithm. [A single Decision Tree using my reduced data uses just 12 rules – (masses and normalized frequency), but the accuracy is far lower. The Random Forest using different rules (masses and quantities, would be expected to use more features.]

How robust is this analysis?

The laboratory instrument generates a large amount of data for each sample, over 650 mass readings repeated over 6000 times over the scan time. The data was reduced for testing which in this case was 8149 values. There were 134 samples, 59 were classed as biotic or natural, and 75 were abiotic samples. A Random Forest (a suite of Decision Trees) algorithm proved the best method to classify the samples. This resulted in a 90+% correct classification of the sample types. The PCA visualization in Figure 2 is instructive as it shows how the samples were likely classified by the Random Forest model, and which samples were likely misclassified. The PCA used just 20 of the highest-scoring variables to separate the 2 classes of samples.

Generally, the Pyr-GC-EI-MS technique is considered robust with respect to masses extracted from different samples of the same material. The authors included replicates in the samples which should, ideally, be classified together in the same leaf in each Decision Tree in the Random Forest. That this is the case in this experiment is hinted by the few labels that point to 2 samples that are close together in the PCA shown in Figure 2, e.g. the cysteine-glucose Maillard reaction. That replicates are very similar is important as it indicates that the sample processing technique reliably produces the same output and therefore single samples are producing reliable mass and time signals with low noise. [In my experiment (see Appendix A) where K-means clustering was used, in most cases, the replicate pairs were collected together in the same cluster indicating that no special data treatment was needed to keep the replicates together.]

The pyrolysis of the samples transforms many of the compounds, often with more species than the original. For example, cellulose composed purely of D-Glucose will pyrolyze into several different compounds [18]. The assumption is that pyrolysis will preserve the differences between the biotic and abiotic samples, especially for material that has already undergone heating, such as coal. As the pyrolysis products in the mass range of 50 to 200 may no longer be the same as the original compounds, this technique can be applied to any sample containing organic material.

The robustness of the machine learning approach can be assessed by the distribution of the accuracy of the individual runs of the Random Forest. This is not indicated in the article. However, the high accuracy rate reported does suggest that the technique will report this level of accuracy consistently. What is not known is whether this existing trained model would continue to classify new samples accurately. This will also indicate the likely boundary conditions where this model works and whether retraining will be needed after the sample set is increased. This will be particularly important when assessing the nature of any confirmed extraterrestrial organic material that is materially different from that recovered from meteorites.

The robustness may be dependent on the labeling to train the Random Forest model. The sample set labels RNA and DNA as abiotic because they were sourced from a laboratory supply, while the lower complexity insect chitin exoskeleton was labeled biotic. But note that the chitin sample is within the abiotic bounding box in Figure 2, as well as the DNA sample.

Detecting life from samples that are fossils, degraded material, or parts of an organism like a skeletal structure, probably requires being able to look for both complexity and material that is composed of fewer, simpler subunits. In extremis, a sample with few organic molecules even after pyrolysis will likely not be complex enough to be identified as biotic (e.g. the meteorite samples), while a large range of organic molecules may be too varied and indicate abiotic production (e.g. Maillard reactions caused by heating). There will be intermediate cases, such as the chitinous exoskeleton of an insect that has relatively low molecular complexity but which the label defines as biotic.

What is important here is that while it might be instructive to know what the feature molecules are, and their likely pre-heated composition, the method does not rely on anything more than the mass and peak appearance time of the signal to classify the material.

Why does the Random Forest algorithm work well, and exceed that of a single Decision Tree or 2-layer Perceptron [a component of neural networks used in binary classification tasks]? A single Decision Tree requires that the set of features have a strong common overlap for all samples in the class. The greater the overlap, the fewer rules are needed. However, a single Decision Tree model is brittle in the face of noise. This is overcome with the Random Forest by using different subsets of the features to build each tree in the forest. With noisy data, this builds robustness as the predicted classification is based on a majority vote. (See Appendix A for a brief discussion on this.)

Is this technique agnostic?

Now let me address the important issue of whether this approach is agnostic to different biologies, as this is the crux of whether the experimental results will detect not just life, but extraterrestrial life. Will this approach address the possibly very different biologies of life evolved from a different biogenesis?

Astrobiology, a subject with no examples, is currently theoretical. There is almost an industry trying to provide tests for alien life. Perhaps the most famous example is the use of the disequilibria of atmospheric gases, proposed by James Lovelock. The idea is that life, especially autotrophs like plants on Earth, will create an imbalance in reactive gases such as oxygen and methane that keeps them apart from their equilibrium. This idea has since been bracketed with constraints and additional gases, but the basic idea remains a principal approach for exoplanets where only atmospheric gas spectra can be measured.

As life is hypothesized to require a complex set of molecules, yet far fewer than a random set of all possible molecules, or as Cronin has suggested, reuse of molecules to reduce the complexity of building large macromolecules, it is possible that there could be fossil life, either terrestrial or extraterrestrial, that has the same apparent complexity, but largely non-overlapping molecules. The Random Forest could therefore build some Decision Trees that could select different sets of molecules to make the same biotic classification, suggesting that this is an agnostic method. However, this has yet to be tested as there are no extraterrestrial biotic samples to test. It may require such samples, if found and characterized as biotic, to be added to a new training set should they not be classified as biotic using the current model.

As this experiment assumes that life is carbon-based, clearly truly exotic life based on other key elements such as silicon would be unlikely, but not impossible, to be detected if volatile non-organic materials in a sample could be classified correctly.

The authors explain what agnostic in their experiment means:

Our Proposed Biosignature is Agnostic. An important finding of this study is that abiotic, living, and taphonomic suites of organic molecules display well-defined clusters in their high-dimensional space, as illustrated in Fig. 2. At the same time, large “volumes” of this attribute space are unpopulated by either abiotic suites or terrestrial life. This topology suggests the possibility that an alien biochemistry might be recognized by forming its own attribute cluster in a different region of Fig. 2—a cluster that reflects the essential role in selection for function in biotic systems, albeit with potentially very different suites of functional molecules. Abiotic systems tend to cluster in a very narrow region of this phase space, which could in principle allow for easy identification of anomalous signals that are dissimilar to abiotic geochemical systems or known terrestrial life.

What they are stating is that their approach will detect the signs of life in both extant organisms and the resulting decay of their remains when fossilized, such as shales and fossil fuels like coal and oil. As the example PCA of Figure 2 shows, the abiotic samples are tightly clustered in a small space compared to the far greater space of the biotic and once-biotic samples. The authors’ Figure 1 shows that their chosen method results in fewer different molecules found in the Allende meteorite compared to a microbe. I note that the dried oak leaf that is also within the abiotic cluster of the PCA visualization is possibly there because the bulk of the material is cellulose. Cellulose is made of chains of polymerized D-glucose, and while the pyrolysis of cellulose is a physical process that creates a wider assortment of organic compounds [18], this still limits the possible pyrolysis products.

This analysis is complementary to Cronin’s Assembly Theory which theorizes a reduced molecular space of life compared to the randomness and greater complexity of purely chemical and physical processes. This is because life constrains its biochemistry to enzyme-mediated reaction pathways. Assembly Theory [1] and other complexity theories of life [15] would be expected to reduce the molecular space compared to the possible arrangements of all the atoms in an organism.

The authors’ method is probably detecting the greater space of molecules from the required complexity of life compared to the simpler samples and reactions that were labeled as abiotic.

For any extraterrestrial “carbon units” that are theorized to follow organizing principles, this method may well detect extraterrestrial life, whether extant or fossilized, from a unique abiogenesis. However, I would be cautious of this claim simply because there were no biotic extraterrestrial samples used, because we have none, only presumed abiotic samples such as the organic material inside meteorites that should not be contaminated with terrestrial life.

The authors suggest that an alien biology using very different biological molecules might form their own discrete cluster and therefore be detectable. In principle, this is true, but I am not sure that the Random Forest machine learning model would detect the attributes of this cluster without training examples to define the rules needed. Any such samples might simply expose any brittleness in the model and either cause an error or be classified as a false positive for either a biotic or abiotic sample. Ideally, as Asimov once stated, the phrase most associated with interesting discoveries “is not ‘Eureka’ but ‘That’s funny . . .’”, might be associated with an anomalous classification. This might be particularly noticeable if the technique indicates that the sample is abiotic, while a direct observation by microscope clearly shows wriggling microbes.

In summary, it is yet to be tested against new, unknown samples to confirm whether it is both robust, and also agnostic, for other carbon-based life.

The advantage of this technique for remote probes

While the instrument data would likely be sent to Earth regardless of local processing and any subsequent rover actions, the trained Random Forest model is computationally very lightweight and easy to run on the data. Inspection of the various Decision Trees in the Random Forest allows an explanation for which features best classify the samples. As the Random Forest is updated by larger sample sets, it is easy to update the model to analyze samples in the lab or on a remote robotic instrument, in contrast to Artificial Neural Network architectures (ANN) that are computationally intensive. Should a sample that looks like it could be alien life but produces an anomalous result (That’s funny…”), the data can be analyzed on Earth and then assigned a classification, and the Random Forest model rerun with the new data either on Earth and the model uploaded, or locally on the probe.

Let me stress again that the instrumentation needed is already available for life-detection missions on robotic probes. The most recent is the Mars Organic Molecule Analyzer (MOMA) [9] which is to be one of the suite of instruments on the Rosalind Franklin rover as part of the delayed ExoMars mission which is now planned for a 2028 launch. MOMA will use both the Pyr-GC-EI-MS sample processing approach, plus a UV laser on the organic material extracted from 2-meter subsurface drill cores to characterize the material. I would speculate that it might make sense to calibrate the sample set with the MOMA instruments to determine if the approach is as robust with this instrument as the lab equipment for this study. The sample set can be increased and run on the MOMA instruments and finalized well before the launch date.

[If the Morningstar Mission to Venus does detect organic material in the temperate Venusian clouds, perhaps in 2025, this type of analysis using instruments flown on a subsequent balloon mission might offer the fastest way to determine if that material is from a life form before any later sample return.]

While this is an exciting, innovative approach to classifying organic molecules and classifying them as biotic or abiotic, it is not the only approach and should be considered complementary. For example, terrestrial fossils may be completely mineralized, with their form indicating origin. A low-complexity fragment of an insect’s exoskeleton would have a form indicative of biotic origin. The dried oak leaf in the experiment that clusters with the abiotic samples would leave an impression in the sediment indicative of life, just as we see occasionally in coal seams. Impressions left by soft-bodied creatures that have completely decayed would not be detectable by this method even though their shape may be obviously from an organism. [Although note that shape alone was insufficient for determining the nature of the “fossils” in the Martian meteorite, ALH84001.]

Earlier, I mentioned that the cellulose of paper represents an example with low complexity compared to an organism. However, if a robot probe detected a fragment of paper buried in a Martian sediment, we would have little hesitation in identifying it as a technosignature. Similarly, a stone structure on Mars might have no organic material in its composition but clearly would be identified as an artifact built by intelligent beings.

Lastly, isotopic composition of elements can be indicative of origin when compared to the planetary background isotopic ratios. If we detected methane (CH4) with isotope ratios indicative of production by subsurface methanogens, that would be an important discovery, one that would be independent of this experimental approach.

Despite my caveats and cautions, local life detection, rather like the attempts with the 1976 Viking landers may be particularly important now that the Mars Sample Return mission costs are ballooning and may result in a cancelation, stymying the return to Earth of the samples Perseverance is collecting [16]. One of the major benefits of training the Apollo astronauts to understand the geology and identify important rock samples was the local decisions made by the astronauts over which rock samples to collect, rather than taking random samples and hoping the selection was informative. A mission to an icy moon would benefit from such local life detection efforts if multiple attempts need to be made in fairly rapid succession without requiring communication delays with Earth for analysis and decision-making and where no sample return to Earth was likely. This innovative technique appears to be an important contribution to the search for extraterrestrial life in our system, and possibly even beyond if our probes capture samples from interstellar objects.

The paper is “Cleaves, J et al, Hazen, R, “A robust, agnostic molecular biosignature based on machine Learning,” PNAS 120 (41) (September 25, 2023) e2307149120. Abstract.

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Appendix A. My experiment with the supplied data. [12]

Method

To test some of the feedback from the authors, I ran some simple machine-learning experiments on the data. Rather than reduce the data to the number of variables in the paper, I used a simple data reduction by collapsing the scan data dimension so that only the single mass values remained. I normalized to the largest mass value in a sample that was set to 100 and all normalized floating point numbers were reduced to integers. All the resulting values of less than 1 were therefore set to 0. I used the classification labels as given. I also shuffled the class labels to test that the information in the data was lost with this operation. I used the Weka ML software package for running Decision Trees, Random Forests, and other ML methods [20].

Results and Discussion

Using the example I ran [figure 3] it is clear that the presence of a molecule[s] of mass 280 is sufficient to classify 14 of the 59 biological samples with no other rules needed, and if that rule fails, passing a rule with the presence of a molecule about ½ the mass of the first rule, adds a further 8 samples correctly classified as biological. However, it takes a further 6 rules to classify another 22 biological samples, and 7 rules to select 48 (1 sample was a false positive) of the 75 abiotic samples. The rules used mostly used larger molecules to determine the classifications because they had the most discriminatory power, as suggested by the number of the larger molecules of the 20 used in the PCA visualization. Of the 12 rules in my experiment, all but 3 used masses of 100 or greater, with 3 rules of 200 or greater. It should be noted that many rules simply needed the presence or absence (less than 1% of the peak frequency) of a molecule. The 2 largest biotic and abiotic leaves each required 7 rules, but about half required some non-zero value. The biotic leaf with 22 samples had just 3 rules with peak values that were present, while the abiotic leaf with 49 classified samples had all 7 rules with no peak value or values below a threshold.

Figure 3. The model for a Decision Tree output for a reduced collapsed set of data. It shows the rule tree of different mass normalized frequencies to classify abiotic [A], and biotic and natural [B], samples as leaves. There are 134 samples, For training, all the samples were used, 75 are classed abiotic, and 59 and biotic/natural. [The few misclassified samples were excluded for simplicity and clarity]. As all samples were used, there was no out-of-sample testing of the model.

The best classifier was the Random Forest, as found by the authors. This far exceeded a single Decision Tree. It even exceeded a 2 layer Perceptron. The Random Forest managed to reach a little more than 80% correct classification, which fell to random with the shuffled data. While the results using the more greatly reduced data were less accurate than those of the paper, this is expected by the data reduction method.

To test whether the data had sufficient information to separate the 2 classes simply by clustering, I ran a K-Means clustering [14] to determine how the data separated.

1. The 2 clusters were each comprised of about 60% of one class. Therefore while the separation was poor, there was some separation using all the data. Shuffling the labels destroyed any information in the samples as it did with the Decision Tree and Random Forest tests.

2. The replicate pairs almost invariably stayed in the same cluster together, confirming the robustness of the data.

3. The natural samples, i.e. those with a biogenic origin, like coal, tended mostly to cluster with the abiogenic samples, rather than the biotic ones.

I would point out that the PCA in Figure 2 was interpreted to mean that abiotic samples clustered tightly together. However, an alternative interpretation is that the abiotic and natural samples separate from the biotic if a separation is drawn diagonally to separate the biotic samples from all the rest.

One labeling question I have was placing the commercially supplied DNA and RNA samples in the abiotic class. If we detected either as [degraded] samples on another world, we would almost certainly claim that we had detected life once the possibility of contamination was ruled out. Switching these labels made very little difference to my Random Forest classification overall, but it did switch more samples to be classified as biotic, in excess of the switch of the 2 samples to biotic labels. It did make a difference for a simpler Decision Tree. It increased the correct classifications (92 to 97 of 134), mostly reducing the misclassification of abiotic to biotic classes, (23 to 16). The cost of this improvement was 2 extra nodes and 1 leaf in the Decision Tree.

The poor results of the 2-layer Perceptron indicate that the nested rules used in the Decision Trees are needed to classify the data. Perceptrons are 2-layer artificial neural networks (ANNs) that have an input and output layer, but no hidden neural layers. Perceptons are known to fail the exclusive-OR test (XOR) although the example Decision Tree in Figure 3 does not require any variables to overcome this issue. A multilayer neural net with at least 1 hidden layer would be needed to match the results of the Random Forest.

In conclusion, my results show that even with a dimensionally reduced data set, the data contains some information in total that allows a weak separation of the 2 classification labels and that the random Forest is the best classifier of many that were available in the WEKA ML software package.

References

1. Assembly Theory (AT) – A New Approach to Detecting Extraterrestrial Life Unrecognizable by Present Technologies www.centauri-dreams.org/2023/05/16/assembly-theory-at-a-new-approach-to-detecting-extraterrestrial-life-unrecognizable-by-present-technologies/

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5. PCA “Principal Component Analysis” en.wikipedia.org/wiki/Principal_component_analysis accessed 10/05/2023

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9. Goesmann, F et al The Mars Organic Molecule Analyzer (MOMA) Instrument: Characterization of Organic Material in Martian Sediments. Astrobiology. 2017 Jul 1; 17(6-7): 655–685.
Published online 2017 Jul 1. doi: 10.1089/ast.2016.1551

10. Cleaves, J et al, Hazen, R, A robust, agnostic molecular biosignature based on machine Learning, PNAS September 25, 2023, 120 (41) e2307149120
doi.org/10.1073/pnas.2307149120

11. __ Supporting information. www.pnas.org/action/downloadSupplement?doi=10.1073%2Fpnas.2307149120&file=pnas.2307149120.sapp.pdf

12. __ Mass Spectroscopy data: osf.io/ubgwt

13. Gold, T. The Deep Hot Biosphere: The Myth of Fossil Fuels. Springer Science and Business Media, 2001.

14. K-means clustering en.wikipedia.org/wiki/K-means_clustering

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20. Weka 3: Machine Learning Software in Java https://www.cs.waikato.ac.nz/ml/weka/

The challenges involved in sending gram-class probes to Proxima Centauri could not be more stark. They’re implicit in Kevin Parkin’s analysis of the Breakthrough Starshot system model, which ran in Acta Astronautica in 2018 (citation below). The project settled on twenty percent of the speed of light as a goal, one that would reach Proxima Centauri b well within the lifetime of researchers working on the project. The probe mass is 3.6 grams, with a 200 nanometer-thick sail some 4.1 meters in diameter.

The paper we’ve been looking at from Marshall Eubanks (along with a number of familiar names from the Initiative for Interstellar Studies including Andreas Hein, his colleague Adam Hibberd, and Robert Kennedy) accepts the notion that these probes should be sent in great numbers, and not only to exploit the benefits of redundancy to manage losses along the way. A “swarm” approach in this case means a string of probes launched one after the other, using the proposed laser array in the Atacama desert. The exciting concept here is that these probes can reform themselves from a string into a flat, lens-shaped mesh network some 100,000 kilometers across.

Image: Figure 16 from the paper. Caption: Geometry of swarm’s encounter with Proxima b. The Beta-plane is the plane orthogonal to the velocity vector of the probe ”at infinity” as it approaches the planet; in this example the star is above (before) the Beta-plane. To ensure that the elements of the swarm pass near the target, the probe-swarm is a disk oriented perpendicular to the velocity vector and extended enough to cover the expected transverse uncertainty in the probe-Proxima b ephemeris. Credit: Eubanks et al.

The Proxima swarm presents one challenge I hadn’t thought of. We have to be able to predict the position of Proxima b to within 10,000 kilometers at least 8.6 years before flyby – this is the time for complete information cycle between Earth, Proxima and back to Earth. Effectively, we need to figure out the planet’s velocity to a value of 1 meter per second, with a correspondingly tight angular position (0.1 microradians).

Although we already have Proxima b’s period (11.68 days), we need to determine its line of nodes, eccentricity, inclination and epoch, and also its perturbations by the other planets in the system. At the time of flyby, the most recent Earth update will be at least 8.5 years old. The Proxima b orbit state will need to be propagated over at least that interval to predict its position, and that prediction needs to be accuracy to the order of the swarm diameter.

The authors suggest that a small spacecraft in Earth orbit can refine Proxima b’s position and the star’s ephemeris, but note that a later paper will dig into this further.

In the previous post I looked at the “Time on Target” and “Velocity on Target” techniques that would make swarm coherence possible, with variations in acceleration and velocity allowing later-launched probes to reach higher speeds, but with higher drag so that as they reach the craft sent before them, they slow to match their speed. From the paper again:

A string of probes relying on the ToT technique only could indeed form a swarm coincident with the Proxima Centauri system, or any other arbitrary point, albeit briefly. But then absent any other forces it would quickly disperse afterwards. Post-encounter dispersion of the swarm is highly undesirable, but can be eliminated with the VoT technique by changing the attitude of the spacecraft such that the leading edge points at an angle to the flight direction, increasing the drag induced by the ISM, and slowing the faster swarm members as they approach the slower ones. Furthermore, this approach does not require substantial additional changes to the baseline BTS [Breakthrough Starshot] architecture.

In other words, probes launched at different times with a difference in velocity target a point on their trajectory where the swarm can cohere, as the paper puts it. The resulting formation is then retained for the rest of the mission. The plan is to adjust the attitude of the leading probes continually as they move through the interstellar medium, which means variations in their aspect ratio and sectional density. A probe can move edge-on, for instance, or fully face-on, with variations in between. The goal is that the probes lost later in the process catch up with but do not move past the early probes.

All this is going to take a lot of ‘smarts’ on the part of the individual probes, meaning we have to have ways for them to communicate not just with Earth but with each other. The structure of the probes discussed here is an innovation. The authors propose that key components like laser communications and computation should be concentrated, so that whereas the central disk is flat, the ‘heart of the device,’ as they put it, is concentrated in a 2-cm thickened rim around the outside of the sail disk.

The center of the disk is optical, or as the paper puts it, ‘a thin but large-aperture phase-coherent meta-material disk of flat optics similar to a fresnel lens…’ which will be used for imaging as well as communications. Have a look at the concept:

Image: This is Figure 3a from the paper. Caption: Oblique view of the top/forward of a probe (side facing away from the launch laser) depicting array of phase-coherent apertures for sending data back to Earth, and optical transceivers in the rim for communication with each other. Credit: Eubanks et al.

So we have a sail moving at twenty percent of lightspeed through an incoming hydrogen flux, an interesting challenge for materials science. The authors consider both aerographene and aerographite. I had assumed these were the same material, but digging into the matter reveals that aerographene consists of a three-dimensional network of graphene sheets mixed with porous aerogel, while aerographite is a sponge-like formation of interconnected carbon nanotubes. Both offer extremely low density, so much so that the paper notes the performance of aerographene for deceleration is 10⁴ times better than conventional mylar. Usefully, both of these materials have been synthesized in the laboratory and mass production seems feasible.

Back to the probe’s shape, which is dictated by the needs not only of acceleration but survival of its electronics – remember that these craft must endure a laser launch that will involve at least 10,000 g’s. The raised rim layout reminds the authors of a red corpuscle as opposed to what has been envisioned up to now as a simple flat disk. The four-meter central disk contains 247 25-cm structures arranged, as the illustration shows, like a honeycomb. We’ll use this optical array for both imaging Proxima b but also returning data to Earth, and each of the arrays offers redundancy given that impacts with interstellar hydrogen will invariably create damage to some elements.

Remember that the plan is to build an intelligent swarm, which demands laser links between the probes themselves. Making sure each probe is aware of its neighbors is crucial here, for which purpose it will use the optical transceivers around its rim. The paper calculates that this would make each probe detectable by its closest neighbor out to something close to 6,000 kilometers. The probes transmit a pulsed beacon as they scan for neighboring probes, and align to create the needed mesh network. The alignment phase is under study and will presumably factor into the NIAC work.

The paper backs out to explain the overall strategy:

…our innovation is to use advances in optical clocks, mode-locked optical lasers, and network protocols to enable a swarm of widely separated small spacecraft or small flotillas of such to behave as a single distributed entity. Optical frequency and reliable picosecond timing, synchronized between Earth and Proxima b, is what underpins the capability for useful data return despite the seemingly low source power, very large space loss and low signal-to-noise ratio.

For what is going to happen is that the optical pulses between the probes will be synchronized, meaning that despite the sharp constraints on available energy, the same signal photons are ‘squeezed’ into a smaller transmission slot, which increases the brightness of the signal. We get data rates through this brightening that could not otherwise be achieved, and we also get data from various angles and distances. On Earth, a square kilometer array of 796 ‘light buckets’ can receive the pulses.

Image: This is Figure 13 from the paper. Caption: Figure 13: A conceptual receiver implemented as a large inflatable sphere, similar to widely used inflatable antenna domes; the upper half is transparent, the lower half is silvered to form a half-sphere mirror. At the top is a secondary mirror which sends the light down into a cone-shaped accumulator which gathers it into the receiver in the base. The optical signals would be received and converted to electrical signals – most probably with APDs [avalanche photo diodes] at each station and combined electrically at a central processing facility. Each bucket has a 10-nm wide band-pass filter, centered on the Doppler-shifted received laser frequency. This could be made narrower, but since the probes will be maneuvering and slowing in order to meet up and form the swarm, and there will be some deceleration on the whole swarm due to drag induced by the ISM, there will be some uncertainty in the exact wavelength of the received signal. Credit: Eubanks et al.

If we can achieve a swarm that is in communication with its members using micro-miniaturized clocks to keep operations synchronous, we can thus use all of the probes to build up a single detectable laser pulse bright enough to overcome the background light of Proxima Centauri and reach the array on Earth. The concept is ingenious and the paper so rich in analysis and conjecture that I keep going back to it, but don’t have time today to do more than cover these highlights. The analysis of enroute and approach science goals and methods alone would make for another article. But it’s probably best that I simply send you to the paper itself, one which anyone interested in interstellar mission design should download and study.

The paper is Eubanks et al., “Swarming Proxima Centauri: Optical Communication Over Interstellar Distances,” submitted to the Breakthrough Starshot Challenge Communications Group Final Report and available online. Kevin Parkin’s invaluable analysis of Starshot is Parkin, K.L.G., “The Breakthrough Starshot system model,” Acta Astronautica 152 (2018), 370–384 (abstract / preprint).

NIAC’s award of a Phase I grant to study a ‘swarm’ mission to Proxima Centauri naturally ties to Breakthrough Starshot, which continues its interstellar labors, though largely out of the public eye. The award adds a further research channel for Breakthrough’s ideas, and a helpful one at that, for the NASA Innovative Advanced Concepts program supports early stage technologies through three levels of funding, so there is a path for taking these swarm ideas further. An initial paper on swarm strategies was indeed funded by Breakthrough and developed through Space Initiatives and the UK-based Initiative for Interstellar Studies.

Centauri Dreams readers are by now familiar with my enthusiasm for swarm concepts, and not just for interstellar purposes. Indeed, as we develop the technologies to send tiny spacecraft in their thousands to remote targets, we’ll be testing the idea out first through computer simulation but then through missions within our own Solar System. Marshall Eubanks, the chief scientist for Space Initiatives, a Florida-based startup focused on 50-gram femtosatellites and their uses near Earth, talks about swarm spacecraft covering cislunar space or analyzing a planetary magnetosphere. Eubanks is lead author of the aforementioned paper.

But the go-for-broke target is another star, and that star is naturally Proxima Centauri, given Breakthrough’s clear interest in the habitable zone planet orbiting there. The NIAC announcement sums up the effort, but I turn to the paper for discussion of communications with such swarm spacecraft. As Starshot has continued to analyze missions at this scale, it explores probes with launch mass on the scale of grams and onboard power restricted to milliwatts. The communications challenge is daunting indeed given the distances and power available.

If we want to reach a nearby star in this century, so the thinking goes, we should build the kind of powerful laser beamer (on the order of 100 GW) that can push our lightsails and their tiny payloads to speeds that are an appreciable fraction of the speed of light. Moving at 20 percent of c, we reach Proxima space within 20 years, to begin the long process of returning data acquired from the flybys of our probes. Eubanks and colleagues estimate we’ll need thousands of these, because we need to create an optical signal strong enough to reach Earth, one coordinated through a network that is functionally autonomous. We’re way too far from home to control it from Earth.

Image: Artist’s impression of swarm passing by Proxima Centauri and Proxima b. The swarm’s extent is ∼10 larger than the planet’s, yet the ∼5000-km spacing is such that one or more probes will come close to or even impact the planet (flare on limb). It should be possible to do transmission spectroscopy with such swarms. Green 432/539-nm beams are coms to Earth; red 12,000-nm laser beacons are for intra-swarm probe-to-probe coms. Conceptual artwork courtesy of Michel Lamontagne.

The engineering study that has grown out of this vision describes the spacecraft as being ‘operationally coherent,’ meaning they will be synchronized in ways that allow data return. The techniques here are fascinating. Adjusting the initial velocity of each probe (this would be done through the launch laser itself) allows the string of probes to cohere. The laser also allows clock synchronization, so that we wind up with what had been a string of probes traveling together through the twenty year journey. In effect, the tail of the string catches up with the head. What emerges is a network.

As the NIAC announcement puts it:

Exploiting drag imparted by the interstellar medium (“velocity on target”) over the 20-year cruise keeps the group together once assembled. An initial string 100s to 1000s of AU long dynamically coalesces itself over time into a lens-shaped mesh network 100,000 km across, sufficient to account for ephemeris errors at Proxima, ensuring at least some probes pass close to the target.

The ingenuity of the communications method emerges from the capability of tiny spacecraft to travel with their clocks in synchrony, with the ability to map the spatial positions of each member of the swarm. This is ‘operational coherence,’ which means that while each probe returns the same data, the transmission time is related to its position within the swarm. The result; The data pulses arrive at the same time on Earth, so that while the signal from any one probe would be undetectable, the combined laser pulse from all of them can become bright enough to detect over 4.2 light years.

The paper cites a ‘time-on-target’ technique to allow the formation of effective swarm topologies, while a finer-grained ‘velocity-on-target’ method is what copes with the drag imparted by the interstellar medium. This one stopped me short, but digging into it I learned that the authors talk about adjusting the attitude of individual probes as needed to keep the swarm in coherent formation. The question of spacecraft attitude also applies to the radiation and erosion concerns of traveling at these speeds, and I think I’m right in remembering that Breakthrough Starshot has always contemplated the individual probes traveling edge-on during cruise with no roll axis rotation.

Image; This is Figure 2a from the paper. Caption: A flotilla (sub-fleet) of probes (far left), individually fired at the maximum tempo of once per 9 minutes, departs Earth (blue) daily. The planets pass in rapid succession. Launched with the primary ToT technique, the individual probes draw closer to one another inside the flotilla, while the flotilla itself catches up with previously-launched flotillas exiting the outer Solar system (middle) ∼100 AU. For the animation go to https://www.youtube.com/watch?v=jMgfVMNxNQs (Hibberd 2022).

Figure 2b takes the probe ensemble into the Oort Cloud.

Image: Figure 2b caption: Time sped up by a scale factor of 30. The last flotilla launched draws closer to the earlier flotillas; the full fleet begins to coalesce (middle), now under both the primary ToT and secondary VoT techniques, beyond the Kuiper-Edgeworth Belt and entry into the Oort Cloud ∼1000–10,000 AU.

When we talk about using collisions with the interstellar medium to create velocities transverse to the direction of travel, we’re describing a method that again demands autonomy, or what the paper describes as a ‘hive mind,’ a familiar science fiction trope. The hive mind will be busy indeed, for its operations must include not just cruise control over the swarm’s shape but interactions during the data return phase. From the paper;

With virtually no mass allowance for shielding, attitude adjustment is the only practical means to minimize the extreme radiation damage induced by traveling through the ISM at 0.2c. Moreover, lacking the mass budget for mechanical gimbals or other means to point instruments, then controlling attitude and rate changes of the entire craft in pitch, yaw, roll, is the only practical way [to] aim onboard sensors for intra-swarm communications, interstellar comms with Earth and imagery acquisition / distributed processing at encounter.

I gather that other techniques for interacting with the interstellar medium will come into play in the NIAC work, for the paper speaks of using onboard ‘magnetorquers,’ an attitude adjustment mechanism currently in use in low-mass Cubesats in low Earth orbit. It’s an awkward coinage, but a magnetorquer refers to magnetic torquers or torque rods that have been developed for attitude control in a given inertial frame. The method works through interaction between a magnetic field and the ambient magnetic field (in current cases, of the Earth). Are magnetic fields in the interstellar medium sufficient to support this method? The paper explores the need for assessment.

A solid state probe has no moving parts, but it’s also clear that further simulations will explore the use of what the paper calls MEMS (micro-electromechanical systems) trim tabs that could be spaced symmetrically to provide dynamic control by producing an asymmetric torque. This sounds like a kludge, though one that needs exploring given the complexities of adjusting attitudes throughout a swarm. We’ll see where the idea goes as it matures in the NIAC phase. All this will be critical if we are to connect interswarm to create the signaling array that will bring the Proxima data home.

Interestingly, the kind of probes the paper describes may vary in some features:

We note for the record that although all probes are assumed to be identical, implicitly in the community and explicitly in the baseline study, there is in fact no necessity for them to be “cookie cutter” copies, since the launch laser must be exquisitely tunable in the first place, capable of providing a boost tailored to every individual probe. At minimum, probes can be configured and assigned for different operations while remaining dynamically identical, or they can be made truly heterogeneous wherein each probe could be rather different in form and function, if not overall mass and size.

There is so much going on in this paper, particularly the issue of the orbital position of Proxima b, which you would think would be known well enough by now (but guess again). The question of carrying enough stored energy for the two decade mission is a telling one. But the overwhelming need is to get information back to Earth. How data would be received from these distances has always bedeviled the Starshot idea, and having followed the conversation on this for some time now, I find the methods proposed here seriously intriguing. We’ll dig into these issues in the next post.

The paper is Eubanks et al., “Swarming Proxima Centauri: Optical Communication Over Interstellar Distances,” submitted to the Breakthrough Starshot Challenge Communications Group Final Report and available online.

Speculating about the diffusion of intelligent species through the galaxy, as we’ve been doing these past few posts, is always jarring. I go back to the concept of ‘deep time,’ which is forced on us when we confront years in their billions. I can’t speak for anyone else, but for me thinking on this level is closer to mathematics than philosophy. I can accept a number like 13.4 × 10⁹ years (the estimate for the age of globular cluster NGC 6397 and a pointer to the Milky Way’s age) without truly comprehending how vast it is. As biological beings, a century pushes us to the limit. What exactly is an aeon?

NGC 6397 and other globular clusters are relevant because these ancient stellar metropolises are the oldest large-scale populations in the Milky Way. But I’m reminded that even talking about the Milky Way can peg me as insufferably parochial. David Kipping takes me entirely out of this comparatively ‘short-term’ mindset by pushing the limits of chronological speculation into a future so remote that elementary particles themselves have begun to break down. Not only that – the Columbia University astrophysicist finds a way for human intelligence to witness this.

You absolutely have to see how he does this in Outlasting the Universe, a presentation on his Cool Worlds YouTube channel. Now Cool Worlds is a regular stop here because Kipping is a natural at rendering high-level science into thoughtful explanations that even the mathematically challenged like me can understand. Outlasting the Universe begins with Kipping the narrator saying “We are in what you would call the future…the deep future” and takes human evolution through the end of its biological era and into a computer-borne existence in which a consciousness can long outlive a galaxy.

Image: Astrophysicist, author and indeed philosopher David Kipping. Credit: Columbia University.

Along the way we remember (and visit in simulation) Freeman Dyson, who once speculated that to become (almost) immortal, a culture could slow down the perceived rate of time. “Like Zeno’s arrow,” says Kipping, “we keep dialing down the speed.” The visuals here are cannily chosen, the script crisp and elegant, imbued with the ‘sense of wonder’ that brought so many of us to science fiction. Outlasting the Universe is indeed science fiction of the ‘hard SF’ variety as Kipping draws out the consequences of deep time and human consciousness in ways that make raw physics ravishing. I envy this man’s students.

With scenarios like this to play with, where do we stand with the ‘zoo hypothesis?’ It must, after all, reckon with years by the billions and the spread of intelligence. Science fiction writer James Cambias responded to my Life Elsewhere? Relaxing the Copernican Principle post with a tight analysis of the notion that we may be under observation from a civilization whose principles forbid contact with species they study. This is of course Star Trek’s Prime Directive exemplified (although the lineage of the hypothesis dates back decades), and it brings up Jim’s work because he has been so persistent a critic of the idea of shielding a population from ETI contact.

Jim’s doubts about the zoo hypothesis go back to his first novel. A Darkling Sea posits an Europa-like exoplanet being studied by a star-faring species called the Sholen, who are employing a hands-off policy toward local intelligence even as they demand that human scientists on the world’s sea bottom do the same. Not long after publication of the novel (Tor, 2014), he told John Scalzi that he saw Prime Directives and such as “ …a mix of outrageous arrogance and equally overblown self-loathing, a toxic brew masked by pure and noble rhetoric.” The arrogance comes from ignoring the desires of the species under study and denying them a choice in the matter.

In a current blog post called The Zoo Hypothesis: Objections, Jim lays this out in rousing fashion:

…we deduce that you can’t hide a star system which contains a civilization capable of large-scale interstellar operations, which the Zookeepers are by definition. They’re going to be emitting heat, EM radiation, laser light, all the spoor of a Kardashev Type I or higher civilization. And the farther away they are, the more they’re going to be emitting because they need to be bigger and more energy-rich in order to have greater reach.

This gives us one important lesson: if the goal of a Zoo is to keep the civilizations inside from even knowing of the existence of other civilizations, the whole thing is impossible. You can’t have a Zoo without Zookeepers, and the inhabitants of the Zoo will detect them.

Jim’s points are well-taken, and he extends the visibility issue by noting that we need to address time, which must be deep indeed. For a civilization maintaining all the apparatus of a protected area around a given star has to do so on time frames that are practically geological in length. Here we can argue a bit, for a ‘zoo’ set up for reasons we don’t understand in the first place might well come into existence only when the species being studied has reached the capability of detecting its observers.

I referenced Amri Wandel (Hebrew University of Jerusalem) on this the other day. Wandel argues that our own industrial lifespan is currently on the order of a few centuries, and who knows what level of technological sophistication a ‘zoo-keeping’ observer culture might want us to reach before it decides it can initiate contact? That would drop the geological timeframe down to a more manageable span, although the detectability problem still remains. So does the issue of interaction with other star-faring species who might conceivably need to be warned off entering the zoo. Cambias again:

If Captain Kirk or whoever shows up on your planet and says “I’m from another planet. Let’s talk and maybe exchange genetic material — or not, if you want me to leave just say so,” that’s an infinitely more reasonable and moral act than for Captain Kirk to sneak around watching you without revealing his own existence. The first is an interaction between equals, the second is the attitude of a scientist watching bacteria. Is that really a moral thing to do? Why does having cooler toys than someone else give you the right to treat them like bacteria?

This is lively stuff, and speculation of this order is why many people begin reading and writing science fiction in the first place. A hard SF writer, a ‘world builder,’ will make sure that he or she has thought through implications for every action he attributes not only to his characters but the non-human intelligences they may interact with. One thing that had never occurred to me was the issue of visibility when translated to the broader galaxy. Because a zoo needs to be clearly marked. Here’s Jim’s view:

If you’re going to exclude other civilizations from a particular region of the Galaxy, you have to let them know. Shooting relativistic projectiles or giant laser beams at incoming starships is a very ham-fisted way of communicating “keep out!” — and it runs the risk of convincing the grabby civilization that you’re shooting at to start shooting back. And if they’re grabby and control a lot of star systems, that’s going to be a lot of shooting.

Jim’s points are telling, and the comments on my recent Centauri Dreams posts also reflect readers’ issues with the zoo hypothesis. My partiality to it takes these issues into account. If the zoo hypothesis is the best of the solutions to the Fermi question, then the likelihood that other intelligent species are in our neighborhood is vanishingly small. Which lets me circle back to the paper by Ian Crawford and Dirk Shulze-Makuch that set off this entire discussion. It asked, you’ll recall, whether the zoo hypothesis wasn’t the last standing alternative to the idea that technological civilizations are, at the least, rare. It’s not a good alternative, but there it is.

In other words, I’d like the zoo hypothesis to have some traction, because it’s the only way I can find to imagine a galaxy in which intelligent civilizations are common.

Consider the thinking of Crawford and Schulze-Makuch on other hypotheses. Interstellar flight might be impossible for reasons of distance and energy, but this seems a non-starter given that we know of ways within known physics to send a payload to another star even in this century. A slow exploration front moving at Voyager speeds could do the trick in a fraction of the time available given the age of the Milky Way. The lack of SETI detections likewise points to technologies that are physically feasible (various kinds of technosignatures) but are not yet observed.

Is the answer that civilizations don’t live very long, and the chances of any two existing at the same brief time in the galaxy are remote? The nagging issue here is that we would have to assume that all civilizations are temporally limited. It takes only one to find a way through whatever ‘great filter’ is out there and survive into a star-faring maturity to get the galaxy effectively visited and perhaps colonized by now. Crawford and Schulze-Makuch reject models that result in volumes of the galactic disk being unvisited during the four billion years of Earth’s existence, considering them valid mathematically but implausible as solutions to the larger Fermi puzzle.

Many of the hypotheses to explain the Great Silence go even further into the unknowable. What, for example, do we make of attempts to parse out an alien psychology, which inevitably is seen, wittingly or not, as reflecting our own human instincts and passions? Monkish cultures that choose not to expand for philosophical reasons will remain unknowable to us, for example, as will societies that self-destruct before they achieve interstellar flight. We can still draw a few conclusions, though, as Crawford and Schulze-Makuch do, all pointing at least to intelligence being rare.

Although we know nothing of alien sociology, it seems inevitable that the propensity for self-destruction, interstellar colonization and so on must be governed by probability distributions of some kind. The greater the number of ETIs that have existed over the history of the Galaxy, the more populated will be the non-self-destructed and/or pro-colonization wings of these distributions, and it is these ETIs that we do not observe. On the other hand, if the numbers of ETIs have always been small, these distributions will have been sparsely populated and the non-observation of ETIs in their expansionist wings follows naturally.

Image: Are ancient ruins the only thing we may expect to find if we reach other star systems? Are civilizations always going to destroy themselves? The imposing remains of Angkor Wat. Credit: @viajerosaladeriva.

Likewise, we still face the problem that, as Stapledon long ago noted, different cultures will choose different priorities. Why assume that in a galaxy perhaps stuffed with aliens adopting Trappist-like vows of silence there will not be a few societies that do want to broadcast to the universe, a METI-prone minority perhaps, but observable in theory. We have no paradox in the Fermi question if we assume that aliens are rare, but if they are as common as early science fiction implied, the paradox is only reinforced.

So Crawford and Schulze-Makuch have boiled this down to the zoo hypothesis or nothing, with the strong implication that technological life must indeed be rare. I rather like my “one to ten” answer to the question of how many technological species are in the galaxy, because I think it squares with their conclusions. And while we can currently only speculate on reasons for this, it’s clear that we’re on a path to draw conclusions about the prevalence of abiogenesis probably in this century. How often technologies emerge after unicellular life covers a planet is a question that may have to wait for the detection of a technosignature. And as is all too clear, it’s possible this will never come.

The paper is Crawford & Schulze-Makuch, “Is the apparent absence of extraterrestrial technological civilizations down to the zoo hypothesis or nothing?” Published online in Nature Astronomy 28 December 2023 (abstract). James Cambias’ fine A Darkling Sea (Tor, 2014) is only the first of his novels, the most recent of which is The Scarab Mission (Baen, 2023), part of his ‘billion worlds’ series. Modesty almost, but not quite, forbids me from mentioning my essay “Ancient Ruins” which ran in Aeon a few years back.

If we were to find life other than Earth’s somewhere else in the Solar System, the aftershock would be substantial. After all, a so-called ‘second genesis’ would confirm the common assumption that life forms often, and in environments that range widely. The implications for exoplanets are obvious, as would be the conclusion that the Milky Way contains billions of living worlds. The caveat, of course, is that we would have to be able to rule out the transfer of life between planets, which could make Mars, say, controversial. But find living organisms on Titan and the case is definitively made.

Ian Crawford and Dirk Schulze-Makuch point out in their new paper on the Fermi question and the ‘zoo hypothesis’ that this issue of abiogenesis could be settled relatively soon as our planetary probes gain in sophistication. We could settle it within decades if we found definitive biosignatures in an exoplanet atmosphere, but here my skepticism kicks in. My guess is that once we have something like the Habitable Worlds Observatory in place (and a note from Dominic Benford informs me that NASA has just put together teams to guide the development of HWO, the flagship mission after the Nancy Grace Roman Space Telescope), the results will be immediately controversial.

In fact, I can see a veritable firestorm of debate on the question of whether a given biosignature can be considered definitive. Whole journals a few decades from now will be filled with essays pushing abiotic ways to produce any signature we can think of, and early reports that support abiogenesis around other stars will be countered with long and not always collegial analysis. This is just science at work (and human nature), and we can recall how quickly Viking results on Mars became questioned.

So I think in the near term we’re more likely to gain insights on abiogenesis through probing our own planetary system. Life on an ice giant moon may turn up, or around a gas giant like Saturn in an obviously interesting moon like Enceladus, and we can strengthen our hunch that abiogenesis is common. In which case, where do we stand on the development of intelligence or, indeed, consciousness? What kind of constraints can we put on how often technology is likely to be the result of highly evolved life? Absent a game-changing SETI detection, we’re still left with the Fermi question. We have billions of years of cosmic history to play with and a galaxy that over time could be colonized.

Image: JWST’s spectacular image of M51 (NGC 5194), some 27 million light-years away in the constellation Canes Venatici. Taken with the telescope’s Near-InfraRed Camera (NIRCam), the image is so lovely that I’ve been looking for an excuse to run it. This seems a good place, for we’re asking whether a universe that can produce so many potential homes for life actually gives rise to intelligence and technologies on a galaxy-wide scale. Here the dark red features trace warm dust, while colors of red, orange, and yellow flag ionized gas. How long would it take for life to emerge in such an environment, and would it ever become space-faring? Credit: ESA/Webb, NASA & CSA, A. Adamo (Stockholm University) and the FEAST JWST team.

Crawford and Schulze-Makuch ask a blunt question in the title of their paper in Nature Astronomy: ”Is the apparent absence of extraterrestrial technological civilizations down to the zoo hypothesis or nothing?” The zoo hypothesis posits that we are being studied by beings that for reasons of their own avoid contact. David Brin referred in his classic 1983 paper “The Great Silence” (citation below) to this as one variation of a quarantine, with the Solar System something like a nature preserve whose inhabitants have no idea that they are under observation.

Quarantines can come in different flavors, of course. Brin notes the possibility that observers might wait for our species to reach a level of maturity sufficient to join what could be a galactic ‘club’ or network. Or perhaps the notion is simply to let planets early in their intellectual development lie fallow as their species mature. Wilder notions include the idea that we could be quarantined because we represent a danger to the existing order, though it’s hard to imagine a scenario in which this occurs.

But the Crawford / Schulze-Makuch paper is not exactly a defense of the zoo hypothesis. Rather, it asks whether it is the only remaining alternative to the idea that the galaxy is free of other civilizations. The paper quickly notes the glaring issue with the hypothesis, and it’s one anticipated by Olaf Stapledon in Star Maker. While any species with the ability to cross interstellar distances might remain temporarily hidden, wouldn’t there be larger trends that mitigate the effectiveness of their strategy? Can you hide one or more civilizations that have expanded over millions of years to essentially fill the galaxy? At issue is the so-called ‘monocultural fallacy’:

…to explain the Fermi paradox in a Galaxy where ETIs are common, all these different, independently evolved civilizations would need to agree on the same rules for the zoo. Moreover, to account for the apparent non-interference with Earth’s biosphere over its history, these rules may have had to remain in place, and to have been adhered to, ever since the first appearance of colonizing ETI in the Galaxy, which might be billions of years if ETIs are common. Indeed, Stapledon (ref. 29, p.168) anticipated this problem when he noted, from the point of view of a future fictional observer, that “different kinds of races were apt to have different policies for the galaxy”.

I always return to Stapledon with pleasure. I dug out my copy of Star Maker to cite more from the book. Here the narrator surveys the growth and philosophies of civilizations in their multitudes during his strange astral journey:

Though war was by now unthinkable, the sort of strife which we know between individuals or associations within the same state was common. There was, for instance, a constant struggle between the planetary systems that were chiefly interested in the building of Utopia, those that were most concerned to make contact with other galaxies, and those whose main preoccupation was spiritual. Besides these great parties, there were groups of planetary systems which were prone to put the well-being of individual world-systems above the advancement of galactic enterprise. They cared more for the drama of personal intercourse and the fulfillment of the personal capacity of worlds and systems than for organization or exploration of spiritual purification. Though their presence was often exasperating to the enthusiasts, it was salutary, for it was a guarantee against extravagance and against tyranny.

That’s a benign kind of strife, but it has an impact. The matter becomes acute when we consider interacting civilizations in light of the differential galactic rotation of stars, as Brin pointed out decades ago. The closest species to us at any given time would vary as different stars come into proximity. That seems to imply a level of cultural uniformity that is all but galaxy-wide if the zoo hypothesis is to work. But Crawford and Schulze-Makuch are on this particular case, noting that a single early civilization (in galactic history) might be considered a ‘pre-emptive civilization’ (this is Ronald Bracewell’s original idea), thus enforcing the rules of the road to subsequent ETIs. In such a way we might still have a galaxy filled with technological societies.

An interesting digression here involves the age of likely civilizations. We know that the galaxy dates back to the earliest era of the universe. European Southern Observatory work on the beryllium content of two stars in the globular cluster NGC 6397 pegs their age at 13,400 ± 800 million years. Extraterrestrial civilizations have had time to arise in their multitudes, exacerbating the ‘monocultural’ issue raised above. But the authors point out that despite its age, the galaxy’s habitability would have been influenced by such issues as “a possibly active galactic nucleus, supernovae and close stellar encounters.” Conceivably, the galaxy at large evolved in habitability so that it is only within the last few billion years that galaxy-spanning civilizations could become possible.

Does that help explain the Great Silence? Not really. Several billion years allows ample time for civilizations to develop and spread. As the paper notes, we have only the example of our Earth, in which it took something like two billion years to develop an atmosphere rich in the oxygen that allowed the development of complex creatures. You don’t have to juggle the numbers much to realize that different stellar systems and their exoplanets are going to evolve at their own pace, depending on the growth of their unique biology and physical factors like plate tectonics. There is plenty of room even in a galaxy where life only emerged within the last billion years for civilizations to appear that are millions of years ahead of us technologically.

Image: The globular cluster NGC 6397. A glorious sight that reminds us of the immensity in both space and time that our own galaxy comprehends. Credit: ESO.

Back to the zoo hypothesis. Here’s one gambit to save it that the paper considers. A policy of non-interference would only need to be enforced for a few thousand years – perhaps only a few hundreds – if extraterrestrials were interested primarily in technological societies. This is Amri Wandel’s notion in an interesting paper titled “The Fermi paradox revisited: technosignatures and the contact era” (citation below). Wandel (Hebrew University of Jerusalem) eases our concern over the monocultural issue by compressing the time needed for concealment. Crawford and Schulze-Makuch cite Wandel, but I don’t sense any great enthusiasm for pressing his solution as likely.

The reasons for doubt multiply:

Even if they can hide evidence of their technology (space probes, communications traffic and so forth), hiding the large number of inhabited planets in the background implied by such a scenario would probably prove challenging (unless they are able to bring an astonishingly high level of technical sophistication to the task). In any case, advanced technological civilizations may find it difficult to hide the thermodynamic consequences of waste heat production, which is indeed the basis of some current technosignature searches. Moreover, any spacefaring civilization is likely to generate a great deal of space debris, and the greater the number of ETIs that have existed in the history of the Galaxy the greater the quantity of debris that will drift into the Solar System, where a determined search may discover evidence for it.

Why then highlight the zoo hypothesis when it has all these factors working against it? Because in the view of the authors, other solutions to the Fermi question are even worse. I’m running out of time this morning, but in the next post I want to dig into some of these other answers to see whether any of them can still be salvaged. For the more dubious our solutions to the ‘where are they’ question, the more likely it seems that there are no civilizations nearby. We’ll continue to push against that likelihood with technosignature and biosignature searches that could change everything.

The paper is Crawford & Schulze-Makuch, “Is the apparent absence of extraterrestrial technological civilizations down to the zoo hypothesis or nothing?” Published online in Nature Astronomy 28 December 2023 (abstract). David Brin’s essential paper is “The Great Silence – the Controversy Concerning Extraterrestrial Intelligent Life,” Quarterly Journal of the Royal Astronomical Society Vol. 24, No.3 (1983), pp. 283-309 (abstract/full text). Amri Wandel’s paper is “The Fermi Paradox revisited: Technosignatures and the Contact Era,” Astrophysical Journal 941 (2022), 184 (preprint).

Most people I know are enthusiastic about the idea that other intelligent races exist in the galaxy. Contact is assumed to be an inevitable and probably profoundly good thing, with the exchange of knowledge possibly leading to serious advances in our own culture. This can lead to a weighting of the discourse in favor of our not being alone. The ever popular Copernican principle swings in: We can’t be unique, can we? And thus every search that comes up empty is seen as an incentive to try still other searches.

I’m going to leave the METI controversy out of this, as it’s not my intent to question how we should handle actual contact with ETI. I want to step back further from the question. What should we do if we find no trace of extraterrestrials after not just decades but centuries? I have no particular favorite in this race. To me, a universe teeming with life is fascinating, but a universe in which we are alone is equally provocative. Louis Friedman’s new book Alone But Not Lonely (University of Arizona Press, 2023) gets into these questions, and I’ll have more to say about it soon.

I’ve thought for years that we’re likely to find the galaxy stuffed with living worlds, while the number of technological civilizations is tiny, somewhere between 1 and 10. The numbers are completely arbitrary and, frankly, a way I spur (outraged) discussion when I give talks on these matters. I’m struck by how many people simply demand a galaxy that is alive with intelligence. They want to hear ‘between 10,000 and a million civilizations,’ or something of that order. More power to them, but it’s striking that such a lively collection of technological races would not have become apparent by now. I realize that the search space is far vaster than our efforts so far, but still…

Image: The gorgeous M81, 12 million light years away in Ursa Major, and seen here in a composite Spitzer/Hubble/Galaxy Evolution Explorer view. Blue is ultraviolet light captured by the Galaxy Evolution Explorer; yellowish white is visible light seen by Hubble; and red is infrared light detected by Spitzer. The blue areas show the hottest, youngest stars, while the reddish-pink denotes lanes of dust that line the spiral arms. The orange center is made up of older stars. Should we assume there is life here? Intelligence? Credit: NASA/JPL.

So when Ian Crawford (Birkbeck, University of London) was kind enough to send me a copy of his most recent paper, written with Dirk Schulze-Makuch (Technische Universität Berlin), I was glad to see the focus on an answer to the Fermi question that resonates with me, the so-called ‘zoo hypothesis.’ A variety of proposed resolutions to the ‘where are they’ question exist, but this one is still my favorite, a way we can save all those teeming alien civilizations, and a sound reason for their non-appearance.

As far as I know, Olaf Stapledon first suggested that intelligent races might keep hands off civilizations while they observed them, in his ever compelling novel Star Maker (1937). But it appears that credit for the actual term ‘zoo hypothesis’ belongs to John Ball, in a 1973 paper in Icarus. From Ball’s abstract:

Extraterrestrial intelligent life may be almost ubiquitous. The apparent failure of such life to interact with us may be understood in terms of the hypothesis that they have set us aside as part of a wilderness area or zoo.

That’s comforting for those who want a galaxy stuffed with intelligence. I want to get into this paper in the next post, but for now, I want to note that Crawford and Schulze-Makuch remind us that what is usually styled the Fermi ‘paradox’ is in fact no paradox at all if intelligent races beyond our own do not exist. We have a paradox because we are uneasy with the idea that we are somehow special in being here. Yet a universe devoid of technologies other than ours will look pretty much like what we see.

The angst this provokes comes back to our comfort with the ‘Copernican principle,’ which is frequently cited, especially when we use it to validate what we want to find. Just as the Sun is not the center of the Solar System, so the Solar System is not the center of the galaxy, etc. We are, in other words, nothing special, which makes it more likely that there are other civilizations out there because we are here. If we can build radio telescopes and explore space, so can they, because by virtue of our very mediocrity, we represent what the universe doubtless continues to offer up.

But let’s consider some implications, because the Copernican principle doesn’t always work. It was Hermann Bondi, for example, who came up with the notion that we could apply the principle to the cosmos at large, noting that the universe was not only homogeneous but isotropic, and going on to add that it would show the exact same traits for any observer not just at any place but at any time. The collapse of the Steady State theory put an end to that speculation as we pondered an evolving universe where time’s vantage counted critically in terms of what we would see.

Our position in time matters. So, for that matter, does our position in the galaxy.

But physics seems to work no matter where we look, and the assumption of widespread physical principles is essential for us to do astronomy. So as generalizations go, this Copernican notion isn’t bad, and we’d better hang on to it. Kepler figured out that planetary orbits weren’t circular, and as Caleb Scharf points out in his book The Copernicus Complex: Our Cosmic Significance in a Universe of Planets and Probabilities (Farrar, Straus and Giroux, 2014), this was a real break from the immutable universe of Aristotle. So too was Newton’s realization that the Sun itself orbits around a variable point close to its surface and well offset from its core.

So even the Sun isn’t the center of the Solar System in any absolute sense. As we move from Ptolemy to Copernicus, from Tycho Brahe to Kepler, we see a continuing exploration that pushes humanity out of any special position and any fixed notions that are the result of our preconceptions. I think the problem comes when we make this movement a hard principle, when we say that no ‘special places’ can exist. We can’t assume from a facile Copernican model that each time we apply the principle of mediocrity, we’ve solved a mystery about things we haven’t yet proven.

Consider: We’ve learned how unusual our own Solar System appears to be; indeed, how unusual so many stellar systems are as they deviate hugely from any ‘model’ of system development that existed before we started actually finding exoplanets. This is why the first ‘hot Jupiters’ were such a surprise, completely unexpected to most astronomers.

Is the Sun really just another average star lost in the teeming billions that accompany it in its 236 million year orbit of the galaxy? There are many G-class stars, to be sure, but if we were orbiting a more average star, we would have a red dwarf in the sky. These account for 75 percent, and probably more, of the stars in the Milky Way. We’re not average on that score, not when G-class stars amount to a paltry 7 percent of the total. Better to say that we’re only average, or mediocre, up to a point. If we want to take this to its logical limit, we can back our view out to the scale of the cosmos. Says Scharf::

The fact that we are so manifestly located in a specific place in the universe — around a star, in an outer region of a galaxy, not isolated in the intergalactic void, and at just this time in cosmic history — is simply inconsistent with ‘perfect’ mediocrity.

And what about life itself? Let me quote Scharf again (italics mine). Here he works in the anthropic idea that our observations of the universe are not truly random but are demanded by the fact that the universe can produce life in the first place:

…a Copernican worldview at best suggests that the universe should be teeming with life like that on Earth, and at worst doesn’t really tell us one way or the other. The alternative — anthropic arguments — require only a single instance of life in the universe, which would be us. At best, some fine-tuning studies suggest that the universe could be marginally suitable for heavy-element-based-life-forms, rather than being especially fertile. Neither view reveals much about the actual abundance of life to be expected in our universe, or much about our own more parochial significance or insignificance.

So when we speculate about the Fermi question, we need to be frank about our assumptions and, indeed, our personal inclinations. If we relax our Copernican orthodoxy, we have to admit that because we are here does not demand that they are there. Let’s just keep accumulating data to begin answering these questions.

And as we’ll discuss in the next post, Crawford and Schulze-Makuch point out that we’re already entering the era when meaningful data about these questions can be gathered. One key issue is abiogenesis. How likely is life to emerge even under the best of conditions? We may have some hard answers within decades, and they may come from discoveries in our own system or in biosignatures from a distant exoplanet.

If abiogenesis turns out to be common (and I would bet good money that it is), we still have no knowledge of how often it evolves into technological societies. An Encyclopedia Galactica could still exist. Could John Ball be right that other civilizations may be ubiquitous, but hidden from us because we have been sequestered into ‘nature preserves’ or the like? Are we an example of Star Trek’s ‘Prime Directive’ at work? There are reasons to think that the zoo hypothesis, out of all the Fermi ‘solutions’ that have been suggested, may be the most likely answer to the ‘where are they’ question other than the stark view that the galaxy is devoid of other technological societies. We’ll examine Crawford and Schulze-Makuch’s view on this next time.

Caleb Scharf’s The Copernicus Complex: Our Cosmic Significance in a Universe of Planets and Probabilities is a superb read, highly recommended. The Ball paper is “The Zoo Hypothesis,” Icarus Volume 19, Issue 3 (July 1973), pp. 347-349 (abstract). The Crawford & McKuch paper we’ll look at next time is “Is the apparent absence of extraterrestrial technological civilizations down to the zoo hypothesis or nothing?” Nature Astronomy 28 December, 2023 (abstract).

An old pal from high school mentioned in an email the other day that he had an interest in Adam Frank’s work, which we’ve looked at in these pages a number of times. Although my most recent post on Frank involves a 2022 paper on technosignatures written with Penn State’s Jason Wright, my friend was most intrigued by a fascinating 2018 paper Frank wrote for the International Journal of Astrobiology (citation below). The correspondence triggered thoughts of other, much earlier scientists, particularly of Charles Lyell’s Principles of Geology (1830-1833), which did so much to introduce the concept of ‘deep time’ to Europe and played a role in Darwin’s work. Let’s look at both authors, with a nod as well to James Hutton, who largely originated the concept of deep time in the 18th Century.

Adam Frank is an astrophysicist at the University of Rochester, and one of those indispensable figures who meshes his scientific specialization (stellar evolution) with a broader view that encompasses physics, cultural change and their interplay in scientific discourse. He fits into a niche of what I think of as ‘big picture’ thinkers,’ scientists who draw out speculation to the largest scales and ponder the implications of what we do and do not know about astrophysics for a species that may spread into the cosmos.

Now in the case of my friend’s interest, the picture is indeed big. Frank’s 2018 paper asked whether our civilization is the first to emerge on Earth. Thus the ‘Silurian’ hypothesis, explored on TV’s Doctor Who in reference to a race of intelligent reptiles by that name who are accidentally awakened. The theme pops up occasionally in science fiction, though perhaps less often that one might expect. James Hogan’s 1977 novel Inherit the Stars, for example, posits evidence for unknown technologies discovered on the Moon that apparently have their origin in an earlier geological era.

Image: Astrophysicist Adam Frank. Credit: University of Rochester.

I won’t go through this paper closely because I’ve written it up before (see Civilization before Homo Sapiens?), but this morning I want to reflect on the implications of the question. For it turns out that if, say, a species of dinosaur had evolved to the point of creating technologies and an industrial civilization, finding evidence of it would be an extremely difficult thing. So much so that I find myself reflecting on deep time in much the same way that I reflect on the physical cosmos and its seemingly endless reach.

Consider that we can trace our species back in the Quaternary (covering the last 2.6 million years or so) and find evidence of non-Homo Sapiens cultures, among which the Neanderthals are the most famous, along with the Denisovians. Bipedal hominids show up at least as far back as the Laetoli footprints in Tanzania, which date to 3.7 million years ago and were apparently produced by Australopithecus afarensis. Frank and co-author Gavin Schmidt also note that the largest ancient surface still available for study on our planet is in the Negev Desert, dating back about 1.8 million years.

These are impressive numbers until we put them into context. The Earth is some 4.5 billion years old, and complex life on land has existed for about 400 million of those years. Let’s also keep in mind that agriculture emerged perhaps 12,000 years ago in the Fertile Crescent, and in terms of industrial technologies, we’ve only been active for about 300 years (the authors date this from the beginning of mass production methods). Tiny slivers of time, in other words, amidst immense timeframes.

So as Frank and Schmidt point out, we’re talking about fractions of fractions here. There is a fraction of life that gets fossilized, which in all cases is tiny and also varies according to tissue, bone structure, shells and so forth, and also varies from an extremely low rate in tropical environments to a higher rate in dry conditions or river systems. The dinosaurs were active on Earth for an enormous period of time, from the Triassic to the end-Cretaceous extinction event, something in the range of 165 million years. Yet only a few thousand near-complete dinosaur specimens exist for this entire time period. Would homo sapiens even show up in the future fossil record?

From the paper:

The likelihood of objects surviving and being discovered is similarly unlikely. Zalasiewicz (2009) speculates about preservation of objects or their forms, but the current area of urbanization is <1% of the Earth’s surface (Schneider et al., 2009), and exposed sections and drilling sites for pre-Quaternary surfaces are orders of magnitude less as fractions of the original surface. Note that even for early human technology, complex objects are very rarely found. For instance, the Antikythera Mechanism (ca. 205 BCE) is a unique object until the Renaissance. Despite impressive recent gains in the ability to detect the wider impacts of civilization on landscapes and ecosystems (Kidwell, 2015), we conclude that for potential civilizations older than about 4 Ma, the chances of finding direct evidence of their existence via objects or fossilized examples of their population is small.

Image: The Cretaceous-aged rocks of the continental interior of the United States–from Texas to Montana–record a long geological history of this region being covered by a relatively shallow body of marine water called the Western Interior Seaway (WIS). The WIS divided North America in two during the end of the age of dinosaurs and connected the ancient Gulf of Mexico with the Arctic Ocean. Geologists have assigned the names “Laramidia” to western North America and “Appalachia” to eastern North America during this period of Earth’s history. If a species produced a civilization in this era, would we be able to find evidence of it? Credit; National Science Foundation (DBI 1645520). The Cretaceous Atlas of Ancient Life is one component of the overarching Digital Atlas of Ancient Life project. CC BY-NC-SA 4.0 DEED.

Intriguing stuff. The authors advocate exploring the persistence of industrial byproducts in ocean sediment environments, asking whether byproducts of common plastics or organic long-chain synthetics will be detectable on million-year timescales. They also propose a deeper dive into anomalies in current studies of sediments, the same sort of analysis that has been done, for example, in exploring the K-T boundary event but broadened to include the possibility of an earlier civilization. I send you to the paper, available in full text, for discussion of such testable hypotheses.

Back to deep time, though, and the analogy of looking ever deeper into the night sky. In asking how long a civilization can survive (Drake’s L term in the famous equation), we ask whether we are likely to find other civilizations given that over billion year periods, they may last only as a brief flicker in the night. We have no good idea of what the term L should be because we are the only civilization we know about. But if civilizations can emerge more than once on the same world, the numbers get a little more favorable, though still daunting. A given star may be circled by a planet which has seen several manifestations of technology, a greater chance for our detection.

A cycle of civilization growth and collapse might be mediated by fossil fuel availability and resulting climate change, which in turn could feed changes in ocean oxygen levels. Frank has speculated that such changes could trigger the conditions for creating more fossil fuels, so that the demise of one culture actually feeds the energy possibilities of the next after many a geological era. How biospheres evolve – how indeed they have evolved on our own world – is a question that exoplanet research may help to answer, for we have no shortage of available worlds to examine as our biosignature technologies develop.

Culturally, we must come to grips with these things. In an essay for The Geological Society, British paleontologist Richard Fortey discusses the seminal work of James Hutton and Charles Lyell in the 18th and 19th Centuries in developing the concept of geological time, which John McPhee would present wonderfully in his 1981 book Basin and Range (I remember reading excerpts in The New Yorker). The Scot James Hutton had literary ambitions, publishing his Theory of the Earth in 1795 and changing our conception of time forever. Hutton knew Adam Smith and spent time with David Hume; he would also have been aware of French antecedents to his ideas. But despite its importance, even Lyell would admit that he found Hutton’s book all but unreadable.

It took a friend named John Playfair to turn Hutton’s somnolent prose into the simplified but clear Illustrations of the Huttonian Theory of the Earth in 1802, making the idea of deep time available to a large audience and leading to Lyell. Which goes to show that sometimes it takes a careful popularizer to gain for a scientist the traction his or her work deserves. The emphasis there is on ‘careful.’

Lyell’s Principles of Geology, published in three volumes between 1830 and 1833, famously traveled with Darwin on the Beagle and, as Fortey says, “donated the time frame in which evolution could operate.” He goes on:

“…once the time barrier had been breached, it was only a question of how much time. The stratigraphical divisions of the geological column, the periods such as Devonian or Cambrian, with which we are now so familiar, were themselves being refined and put into the right sequence through the same historical period. Just to have a sequence of labels helped geologists grapple with time, and, in a strange way, labels domesticate time.

But domestication co-exists with wonder. I imagine the most hardened geologist of our day occasionally quakes at the realization of what all those sedimentary layers point to, a chronological architecture — time’s edifice — in which our entire history as a species is but a glinting mote on a rockface of the future. Our brief window today is reminiscent of Hutton and Lyell’s. Like them, we are compelled to adjust to a cosmos that seems to somehow enlarge every time we probe it, inspired by new technologies that give birth to entire schools of philosophy.

John Playfair would write upon visiting Siccar Point, the promontory in Berwickshire that inspired Hutton’s ideas, that “The mind seemed to grow giddy looking so far into the abyss of time.” We are similarly dwarfed by the vistas of the Hubble Ultra Deep Field and the exquisite imagery from JWST. Who knows what we have yet to discover in Earth’s deep past?

The paper is Schmidt and Frank, “The Silurian Hypothesis: Would it be possible to detect an industrial civilization in the geological record?” published online by the International Journal of Astrobiology 16 April 2018 (full text). Gregory Benford’s Deep Time: How Humanity Communicates Across Millennia (Bard, 2001) is a valuable addition to this discourse. For a deeper dive, Fortey mentions Martin Rudwick’s Bursting the Limits of Time: The Reconstruction of Geohistory in the Age of Revolution ( University of Chicago Press, 2007). Fortey’s own Life: A Natural History of the First Four Billion Years of Life on Earth (Knopf, Doubleday 1999) is brilliant and seductively readable.

Reaching ‘Oumuamua through some kind of statite technology, an idea we’ve been kicking around recently, brings up the interesting work of Richard Linares at MIT, who has been working on a “dynamic orbital slingshot” for rendezvous with future objects from the interstellar depths (ISOs). Linares received a Phase I grant from the NASA Innovative Advanced Concepts (NIAC) Program to pursue the idea of a network of statites on sentry duty, any one of which could release the stored energy of the sail to enter a trajectory that would take it to a flyby of an object entering our system on a hyperbolic orbit.

The concept is simplicity itself, once you realize that a statite balances the pressure of solar photons against the Sun’s gravitational pull, and essentially hovers in place. As I mentioned when covering Greg Matloff and Les Johnson’s paper on using statites to achieve fast rectilinear trajectories to reach interstellar interlopers, Robert Forward was the one who came up with the idea and practical uses for it. He could envision, for example, communications satellites in polar position to cover high latitudes on Earth.

Here’s what Forward said about the statite concept in his wonderful essay collection Indistinguishable from Magic (1995):

…I have the patent on it — U.S. Patent 5,183,225 “Statite: Spacecraft That Utilizes Light Pressure and Method of Use”… The unique concept described in the patent is to attach a television broadcast or weather surveillance spacecraft to a large highly reflective lightsail, and place the spacecraft over the polar regions of the Earth with the sail tilted so the light pressure from the sunlight reflecting off the lightsail is exactly equal and opposite to the gravity pull of the Earth.

You can see why we need a new term here. If you deploy a sail in the configuration Forward describes, it essentially sits over the polar region while the Earth rotates below it. In other words, technically it is not a satellite. ‘Statite’ is a Forward coinage to describe such a hovering object in space. He wrote of a statite he dubbed the ‘Hovering Hawke’ in one of his short stories. It would be placed too far from the surface to be effective as a communications satellite, but could offer direct broadcasting to places on Earth that are without that capability. Weather surveillance is another use.

Polesitters become interesting when we consider the nature of a geostationary orbit. Put a satellite directly over the equator at 35,786 kilometers altitude and it will appear stationary over the Earth, a useful trait for communications. But the satellite must be positioned directly above the equator, matching Earth’s rotation, to maintain its position relative to the surface.

If we put our satellite at an angle relative to the equator, its apparent motion on Earth will be a figure eight, in what is called an inclined geosynchronous (not geostationary) orbit. That’s useful for areas not covered by geostationary satellites but not good enough for continuous coverage of a specific area, especially the more latitudinally challenged regions like the poles, and that’s why the polesitter is attractive. It can give us continuous coverage even when the region it sits above is far from the equator.

Image: Analog‘s December, 1990 issue contained an article by Robert Forward describing the ‘polesitter’ concept, one of many innovative ideas the scientist introduced to a broad audience. Credit: Condé Nast.

There’s always a catch, and here’s the catch with polesitters, as Forward explained it in his article. When the summer months arrive and the polar regions are in sunlight, keeping the statite precisely balanced (to maintain the hover) becomes quite tricky. He saw that such seasonal instability demanded that a statite be relatively far from Earth, and calculated that it cannot get any closer than 250 Earth radii to the surface.

But Linares and team are not thinking about statites supplying services to Earth. The NIAC work explores using statites to set up an early warning system for interstellar objects, one that will allow fast intercepts before these interlopers blow through our system and return to interstellar space. Consider what happens when we ‘turn off’ the statite capability on our satellite (as from rotating the sail to an edge-on position, for example, or simply releasing a CubeSat). At this point, the released object has no forces impinging upon it but gravity. Let me quote Linares from a white paper on the subject:

…a statite at 1 AU has a free-fall trajectory of about 64 days. This fast response time to a potential ISO can be thought of as a slingshot effect, since the solar sail is used to “store energy” that is released when desired. Additionally, to achieve a flyby some Delta-V is required to adjust from the free-fall path to a flyby trajectory. The proposed mission for the statite concept is to utilize a constellation of such devices to achieve wider coverage over a spherical region of 1 AU for potential ISO missions. Additionally, the orbital plane can be adjusted with relatively low Delta-V.

Image; A constellation of statites as envisioned by Richard Linares for intercepting a future interstellar interloper. Credit: MIT/Richard Linares.

The levitating sail has an inertial velocity of zero, and when released from ‘hover,’ it enters a Keplerian orbit. So as Linares points out, we can turn any one of the statites in our constellation of statites into a ‘sundiver,’ hurtling toward the Sun before its trajectory is adjusted by use of the sail (or perhaps other propulsion). Which statite is deployed simply depends upon the optimum trajectory to the incoming ISO.

We are now on a fast track toward reaching the interstellar object with at least a flyby. Linares calls this a “dynamic orbital slingshot for rendezvous with interstellar objects.” And the idea is to have a constellation of these statites always at the ready for the next ‘Oumuamua. Or, considering how odd ‘Oumuamua seems to be, perhaps I should say “the next Borisov.” Even so, with this net, who knows what we might catch?

The paper makes the case that a statite free-falling toward the Sun from an initial position at 1 AU and then deploying its sail away from the Sun at perihelion can achieve speeds of up to 25 AU/year, making it possible to deliver payloads to the outer Solar System. Now we’re in Matloff/Johnson ‘sundiver’ territory. Voyager 1 has reached 3.6 AU per year by comparison, making the statite concept attractive beyond its value as a station-keeper for quick response missions to interstellar comets/asteroids.

For more on Richard Linares’ work, see “Rendezvous Mission for Interstellar Objects Using a Solar Sail-based Statite Concept,” a white paper available on arXiv.

Back before we knew for sure there were planets around other stars, the universe seemed likely to be ordered. If planet formation was common, then we’d see systems more or less like our own, with rocky inner worlds and gas giants in outer orbits. And if planet formation was a fluke, we’d find few planets to study. All that has, of course, been turned on its head by the abundant discoveries of exoplanets galore. And our Solar System turns out to be anything but a model for the rest of the galaxy. In today’s essay, Don Wilkins looks at several recent discoveries that challenge planet formation theory. We can bet that the more we probe the Milky Way, the more we’ll find anomalies that challenge our preconceptions.

by Don Wilkins

The past few decades have not been easy on planet formation theories. Concepts formed on the antiquated Copernican speculation, the commonality of star systems identical to the Solar System, have given way to the strangeness and variety uncovered by Kepler, Hubble, and the other space borne telescopes. The richness of the planetary arrangements defies easy explanation.

Penn State University researchers uncovered another oddity challenging current understanding of stellar system development. [1] Study of the LHS 3154 system reveals a planet so massive in comparison to its star that generally accepted theories of planet formation cannot explain the existence of the planet, Figure 1. LHS 3154, an “ultracool” star with a “chilly” surface temperature of 2,700 °K (2,430 °C; 4,400 °F), is an M-dwarf, a category that comprises three quarters of the stars in the Milky Way. Most of the light of LHS 3154 is in the infrared band. The M- dwarf star is nine times less massive than the Sun yet it hosts a planet 13 times more massive than Earth.

Figure 1. An artist rendition of the mass comparison between the Earth and Sun and the star LHS 3154, and its companion, LHS 3154b. Credit: Pennsylvania State University.

In current theories, stars form from condensing large clouds of gas and dust into smaller volumes. After the star forms, the left-over gas and dust which is a much smaller fraction of the original cloud, settles into a disk around the new star. From this much smaller mass, planets will condense, completing the star system. In these theories, the star consumes the major proportion of the progenitor clouds.

The Sun, for example, contains an estimated 99.8% of the mass of the Solar System. Only 0.2% is left over for the eight planets, various moons and asteroids.

The mass ratio comparing LHS 3154b to LHS 3154 is 117 times greater than mass ratio comparing the Earth to the Sun. LHS 3154b probably is Neptune-like in composition, completes its orbit in 3.7 Earth days and, the researchers believe, is a very rare world. Typically M-dwarves host small rocky bodies rather than gas giants.

According to current theories, once the star formed, there should not have been enough mass to form a planet as large as LHS 3154b. A young LHS 3154 disk dust-mass and dust-to-gas ratio must be ten times greater than what is typically observed surrounding an M-dwarf star to birth a giant such as LHS 3154b.

“The planet-forming disk around the low-mass star LHS 3154 is not expected to have enough solid mass to make this planet,” Suvrath Mahadevan, the Verne M. Willaman Professor of Astronomy and Astrophysics at Penn State and co-author on the paper said. “But it’s out there, so now we need to reexamine our understanding of how planets and stars form.”

Mahadevan’s team built a novel spectrograph, the Habitable Zone Planet Finder (HPF), with the intention of detecting planets orbiting the coolest of stars. Planets orbiting low temperature stars might have surfaces cool enough to support liquid water and life. In looking for planets with liquid water, the team found, as often happens in research, something new, a massive planet to challenge current theories of stellar system formation.

Another discovery, this time by a Carnegie Institution for Science team, uncovered another challenging world. [2]

Figure 2. Artist’s conception a small red dwarf star, TOI-5205, and its out-sized companion TOI-5205b. Credit: Katherine Cain, the Carnegie Institution for Science.

“The host star, TOI-5205, is just about four times the size of Jupiter, yet it has somehow managed to form a Jupiter-sized planet, which is quite surprising,” observed Shubham Kanodia, who led the team which found TOI-5205b.

When TOI-5205b crosses in front of TOI-5205, the planet blocks about seven percent of the star’s light—a dimming among the largest known exoplanet transit signals.

The rotating disk of gas and dust that surrounds a young star gives birth to its planetary companions. More massive planets require more of the gas and dust left over as the star ignites. Gas planet formation, in the accepted theories, requires about 10 Earth masses of rocky material to produce the massive rocky core of the gas giant. Once the core is formed, it gathers gas from the surrounding clouds, resulting in the mammoth atmosphere of the giant planet.

“TOI-5205b’s existence stretches what we know about the disks in which these planets are born,” Kanodia explained. “In the beginning, if there isn’t enough rocky material in the disk to form the initial core, then one cannot form a gas giant planet. And at the end, if the disk evaporates away before the massive core is formed, then one cannot form a gas giant planet. And yet TOI-5205b formed despite these guardrails. Based on our nominal current understanding of planet formation, TOI-5205b should not exist; it is a ‘forbidden’ planet.”

Not all mysteries are confined to M-dwarfs. A sun-like star, an infant of 14 million years some 360 light years from Earth, hosts a gas giant six times more massive than Jupiter, that orbits the star at a distance twenty times greater than the distance separating Jupiter and the Sun, Figure 3. [3]

Figure 3. A direct image of the exoplanet YSES 2b (bottom right) and its star (center). The star is blocked by a coronagraph. Credit: ESO/SPHERE/VLT/Bohn et al.

The large distance from YSES 2b to the star does not fit either of the two most well-known models describing large gaseous planet formation. If YSES 2b formed by means of core accretion at such an enormous distance far from the star, the planet should be much lighter than what is observed as a result of scarcity of disk material at that distant location. YSES 2b is too massive to satisfy this theory.

Gravitationally instability, the second theorized method for producing gas giants, postulates very massive protostellar disks that are unstable, splintering into large clumps from which gas giants are directly formed. YSES 2b appears not massive enough to have been formed in this fashion.

In a third possibility, YSES 2b might have formed by core accretion much closer to its host star and migrated outwards. A second planet is needed to pull YSES 2b into the outer regions of the system, but no such planet has been located.

Observations by the current generation of space-borne telescopes have upset the theories of planet formation. Hot Jupiters, worlds orbiting pulsars, odd arrangements of worlds, super Earths, and wandering worlds flung close to a star then flying back have complicated the ideas of Laplace, See, Chamberlin and Moulton. Further study by the James Webb Space Telescope and its successors will only enliven the debate surrounding the origin of the planets.

References

[1] Guðmundur Stefánsson, Suvrath Mahadevan, Yamila Miguel, et al, “A Neptune-mass exoplanet in close orbit around a very low-mass star challenges formation models,” Science, 30 Nov 2023, Vol. 382, Issue 6674, pp. 1031-1035, DOI: 10.1126/science.abo0233.

[2] Shubham Kanodia et al, “TOI-5205b: A Short-period Jovian Planet Transiting a Mid-M Dwarf,” The Astronomical Journal (2023). DOI: 10.3847/1538-3881/acabce

[3] Alexander J. Bohn et al. “Discovery of a directly imaged planet to the young solar analog YSES 2.” Accepted for publication in Astronomy & Astrophysics, www.aanda.org/10.1051/0004-6361/202140508

What an interesting object Methone is. Discovered by the Cassini imaging team in 2004 along with the nearby Pallene, this moon of Saturn is a scant 1.6 kilometers in radius, orbiting between Mimas and Enceladus. In fact, Methone, Pallene and another moon called Anthe all orbit at similar distances from Saturn and are dynamically jostled by Mimas. What stands out about Methone is first of all its shape and, perhaps even more strikingly, the smoothness of its surface. We’d like to know what produces this kind of object and would also like to retrieve imagery of both Pallene and Anthe. If something this strange has equally odd companions, is there something about its relationship with both nearby moons and Saturn’s rings that can produce this kind of surface?

Image: It’s difficult not to think of an egg when looking at Saturn’s moon Methone, seen here during a Cassini flyby of the small moon. The relatively smooth surface adds to the effect created by the oblong shape. NASA/JPL-Caltech/Space Science Institute.

Our path to interstellar missions will see us ramp up the velocities of our probes to objects in our own system, made more accessible by shorter mission times, sail technologies and miniaturization. There is no shortage of targets between high-interest moons like Europa, Titan and Enceladus and Kuiper Belt Objects like Arrokoth. For that matter, the interstellar interloper ‘Oumuamua may yet be within range of faster missions (and in fact we’ll be examining ‘Oumuamua prospects in at least one upcoming article). But the point is that intermediate steps to interstellar will enhance exploration of objects we’ve already visited and take us to numerous others.

One way to proceed is discussed by Greg Matloff and Les Johnson in a recent paper for the Journal of the British Interplanetary Society that grew out of a presentation at the 6th International Space Sailing Symposium this summer. Here the idea is to adjust the parameters of a solar sail so that a balance is achieved between the gravitational force of the Sun and the solar photon radiation impinging upon it. The parameters are clear enough: We need a sail of a specific thickness (areal density), and tightly constrained figures for its reflectance and absorbance. We want to cancel out the gravitational acceleration imposed by the Sun through the propulsive effects of solar photons, allowing us to effectively ‘hover’ in place.

Hovering isn’t traveling, but bear with me. We’ve looked at this kind of sail configuration before and discussed its development in the hands of Robert Forward. It was Forward who dubbed the configuration a ‘statite,’ implying that when the force on the sail from solar radiation exactly balances the gravitational force acting upon it, the spacecraft is effectively in what the paper calls a ‘force-free environment.’

This gets interesting in terms of fast probes because while the statite is normally considered to remain stationary (and it will do so when the sail is stationary relative to the Sun during sail deployment), something else happens when the craft is orbiting the Sun when the sail is deployed. The sail now moves in a straight line at its orbital velocity at the time of deployment. The authors style this ‘rectilinear sun-diving.’ As Matloff noted in an email the other day:

“To do this operationally, it is necessary to maintain the sail normal to the Sun – broadside facing the Sun – during the acceleration process. The sail moves off at its velocity relative to the Sun at sail deployment because radiation pressure force on the sail balances solar gravitational attraction. This is a consequence of Newton’s First Law.”

Using this method we can fling the sail and payload outward. What is known as the sail’s lightness factor is the ratio of solar radiation forces divided by the solar gravitational force, and in the case of the rectilinear trajectory described above, the lightness factor is 1. So consider a sail being deployed from a circular orbit of the Sun at 1 AU. The statite, free of other forces, now moves out on a rectilinear trajectory at 30 kilometers per second, which is the Earth’s orbital velocity. The number is noteworthy because it practically doubles the interstellar velocity of Voyager 1. Matloff and Johnson point out that at this velocity, the Sun’s gravitational focus at 550 AU is reachable in 87 years.

Moving at the same pace gets us to Saturn (and the interesting Methone) in 1.5 years. I’m going to run through the other two scenarios the scientists consider to show the range of possibilities. Assume an orbit that is not circular but rather one having a perihelion of 0.7 AU and aphelion at 1 AU. Deploying the sail at perihelion allows the spacecraft to reach 38 kilometers per second, getting to the inner gravitational focus in about 66 years. Finally, with an aphelion at 1 AU and perihelion at 0.3, our craft achieves a velocity after sail deployment of 66 km/sec, reaching the focus in 38 years.

As regards to ‘Oumuamua, the third scenario, with sail deployment at perihelion some 0.3 AU out from the Sun, achieves enough interstellar cruise velocity to catch the object roughly around 2045, when it will be some 220 AU from the Sun. To these times, of course, must be added the time needed to move the sail from aphelion to the sail deployment point at perihelion, but the numbers are still quite satisfactory.

This is especially true given that we are talking about relatively near-term technologies that are under active development. Matloff and Johnson calculate using an areal mass thickness of 1.46 X 10^-3kg/m² for the proposed missions. They show current state of the art solar sail film as 1.54 X 10^-3kg/m² (this does not include deployment mechanisms, structure, etc). The point is clear, however: Achieving 30 km/sec or more offers us fast passage to targets within the outer Solar System as we analyze options for missions beyond it, using technologies that are not far removed from present capability.

The authors note that we can’t assume a constant value for solar radiation; the solar constant actually varies by about 0.1% in response to the Sun’s activity cycle. Hence the need to explore options like adjusting the curvature of the sail or using reflective vanes for fine-tuning. Controlling the sail will obviously be critical. The paper continues:

Control of the sail depends upon the ability of the system to dynamically adjust the center of mass (CM) versus the center of (photon) pressure (CP). Any misalignment of the CM versus the CP will induce torques in the sail system that have to be actively managed lest the offset result in an eventual loss of control. The sail will encounter micrometeorites and interplanetary dust during flight that will create small holes in the fabric, changing its reflectivity asymmetrically and inducing unwanted torques. Depending upon how the sail is packaged and deployed, there may also be fold lines, wrinkles, and small tears that occur with similar end results.

Hence the need for a momentum management system, which could involve possibilities like reflective control devices for roll or diffractive sail materials that manipulate the exit direction of incoming photons as needed to counter these effects. The authors point out that the solar sail propulsion systems for this kind of mission are at TRL-6 despite recent failures such as the loss of the Near-Earth Asteroid Scout Cubesat mission, which carried an 86 square meter solar sail that was lost after launch in late November 2022. With solar sails under active development, however, the prospect for exploring rectilinear sundiver missions in the near term seems quite plausible.

The paper is Matloff & Johnson, “Breakthrough Sun Diving: The Rectilinear Option,” Journal of the British Interplanetary Society Vol. 76 (2023), 283-287.

John Barrow has been on my mind these past few days, for reasons that will become apparent in a moment. In my eulogy for Barrow (1952-2020), I quoted from his book The Left Hand of Creation (Oxford, 1983). I want to revisit that passage for its clarity, something that always inspired me about this brilliant physicist. For it seemed he could render the complex not only accessible but encouragingly pliable, as if scientific exploration always unlocked doors of possibility we could use to our advantage. His was a bright vision. The notion that animated him was that there was something in the sheer process of research that held its own value. Thus:

Could there be any shortcuts to the answers to the cosmological questions? There are some who foolishly desire contact with advanced extraterrestrials in order that we might painlessly discover the secrets of the universe secondhand and prematurely extend our understanding. Such a civilization would surely resemble a child who receives as a gift a collection of completed crossword puzzles. The human search for the structure of the universe is more important than finding it because it motivates the creative power of the human imagination.

You can see that for Barrow, the question of values was not separated from scientific results, and in a sense transcended the data we actually gathered. He goes on:

About 50 years ago a group of eminent cosmologists were asked what single question they would ask of an infallible oracle who could answer them with only yes or no. When his opportunity came, Georges Lemaître made the wisest choice. He said, “I would ask the Oracle not to answer in order that a subsequent generation would not be deprived of the pleasure of searching for and finding the solution.”

Image: Cosmologist, mathematician and physicist John D. Barrow, whose books have been a personal inspiration for many years. Credit: Tom Powell.

Leave it to Lemaître (and Barrow to quote him) as we reach beyond the immediately practical to unlock what it is about human experience that compels us to push into new terrain, whether it be physical exploration or flights of the imagination as we pursue a new hypothesis about nature. Barrow comes to mind because we’ve just been talking about the scales by which a civilization can be measured. Some of these are well established, as for example the Kardashev scale, with its familiar Types I, II and III keyed to the scale of a civilization’s energy use. In Clarke’s The Fountains of Paradise we find an alien scale based on the use of tools. It’s possible to imagine other scales, and Barrow’s own contribution takes us into the nano-realm.

As best I can determine, Barrow first floated the scale in his 1998 book Impossibility: The limits of science and the science of limits (Oxford University Press). Inverting Kardashev, Barrow was interested in a civilization’s ability to control smaller and smaller things, relying on the observed fact that as we have explored such micro-realms, our technologies have proliferated. Nanotechnology and biotechnology are drawn out of our ability to manipulate matter at small scales, and in fact the development of nanotech is one marker for a Barrow scale IV culture.

Barrow I: The ability to manipulate objects at the same scale as the person or being involved. In other words, simple activities involving basic tools.

Barrow II: The control of genetic information.

Barrow III: The ability to control molecules.

Barrow IV: The ability to control individual atoms.

Barrow V: The manipulation of atomic nuclei..

Barrow VI: Control of elementary particles.

Barrow Omega (Ω): The ability to control fundamental elements of spacetime.

Table: Energetic and inward civilization development. Kardashev’s (1964) types refer to energy consumption; Barrow’s (1998, 133) types refer to a civilization’s ability to manipulate smaller and smaller entities. Credit: Clément Vidal.

I’ve drawn the above table from a paper by French philosopher and SETI scientist Clément Vidal, who is one of the few who have explored this realm (citation below). Here we get both Kardashev and Barrow at once, a convenience, and central to Vidal’s argument that black holes are going to draw advanced civilizations to extract their energies and explore what he calls “the computational density of matter.” On this score, it’s interesting to note that Freeman Dyson proposed in 1979 that a civilization exploiting time dilation effects near black holes could survive effectively forever (a later revision had to take into account the accelerating expansion of the universe).

What all this means for SETI is intriguing – almost punchy – and I’ll send you to Vidal’s superb The Beginning and the End: The Meaning of Life in a Cosmological Perspective (Springer, 2014) for a deep dive into the concepts involved. But consider this for a starter: Dysonian SETI assumes civilizations far more advanced than our own, the reasoning being that their works should be apparent even at astronomical scales. Thus searching our astronomical data as far back as we can could conceivably flag an anomaly that merits investigation as a possible civilizational marker.

What Clément Vidal has been investigating is where such markers would turn up, and for this he deploys the scales of both Kardashev and Barrow. I think the easiest assumption is that we would find an alien civilization at its home world, but of course this needn’t be the case. Vidal speaks of ‘attractors’ as those sources of energy that an advanced civilization would increasingly exploit. Take a culture a billion years older than our own and ponder energy needs that might require it to exploit things like the energies of close binary neutron stars or black holes themselves. Such a civilization would be far flung, with operations well beyond its local group of stars.

Now ponder Barrow Type Ω. This ‘omega’ culture is free of the constraints of spacetime, having achieved the ability to manipulate both. It’s anyone’s guess whether such a civilization would be noted by achievements on a truly celestial scale, or whether its works would actually be embedded in the nature of space and time themselves, so that to us they appear the simple functioning of nature. In this mode of thinking, the more advanced a civilization becomes as it moves up the Barrow scale, the more it begins to effectively disappear. Barrow thus channels Richard Feynman and anticipates Lee Smolin’s notions about cosmological evolution, a kind of self-selection for universes.

I’m going to swipe the chart below from Vidal’s 2010 paper on black hole attractors, showing the entertaining fact that as he puts it, “from the relative human point of view, there is more to explore in small scales than in large scales.”

Table: That humans are not in the center of the universe is also true in terms of scales. This implies that there is more to explore in small scales than in large scales. Richard Feynman (1960) popularized this insight when he said “there is plenty of room at the bottom”. Figure adapted from (Auffray and Nottale 2008, 86). Credit: Clément Vidal.

Futurist John Smart has dug into what he calls STEM Compression, with STEM in this case meaning Space/Time/Energy/Matter, and the compression being the idea that in terms of density and efficiency, we can as Vidal puts it “do more with less.” For going deeper into the Barrow scale, we see that as things get smaller, we are not hampered by the speed of light problem. In fact, our endgame barrier is at the Planck scale. A Kardashev II civilization extracting energy from a rotating black hole using technologies far up the Barrow scale may well be indistinguishable from an X-ray binary of the sort that has been cataloged in the astronomical literature.

Such speculations are on the far edge of SETI (and again, I refer you to Vidal’s book), but it’s also true that whether or not extraterrestrial civilizations exist, our own ability to chart futures for an expanding civilization may well come in handy if we can somehow punch through whatever ‘great filter’ may be out there and become a species that survives on the scale of deep time. There is no knowing whether this is even possible, and it may be that the galaxy is filled with the ruins of those who have gone before us.

It is also true, of course, that no one may have gone before us. Maybe N really does equal 1. But I return to Barrow: “The human search for the structure of the universe is more important than finding it because it motivates the creative power of the human imagination.” And the human imagination is currency of the realm in matters like these.

The Vidal paper is “Black Holes: Attractors for Intelligence?” presented at the Kavli Royal Society International Centre, “Towards a scientific and societal agenda on extra-terrestrial life”, 4-5 Oct 2010 (abstract). The Dyson paper is “Time Without End: Physics and Biology in an Open Universe,” Review of Modern Physics 51: 447-460 (abstract). My eulogy for Barrow is On John Barrow. John Smart contributed a fascinating essay on cosmic evolution in these pages in The Goodness of the Universe.

When we’ve discussed interstellar ‘interlopers’ like ‘Oumuamua and 2I/Borisov, the science fiction-minded among us have now and then noted Arthur Clarke’s Rendezvous with Rama (Gollancz, 1973). Although we’ve yet to figure out definitively what ‘Oumuamua is (2/I Borisov is definitely a comet), the Clarke reference is an imaginative nod to the possibility that one day an alien craft might enter our Solar System during a gravitational assist maneuver and be flung outward on whatever its mission was (in Rama’s case, out in the direction of the Large Magellanic Cloud).

Since we’ll never see ‘Oumuamua again, we wait with great anticipation the work of the Legacy Survey of Space and Time (LSST), which will be run via the Vera Rubin Telescope (first light in 2025). Estimates vary widely but the consensus seems to be that with a telescope capable of imaging the entire visible sky in the southern hemisphere every few nights, the LSST should produce more than a few interstellar objects, perhaps ten or more, every year. We probably won’t find a Rama, but who knows?

Meanwhile, I’m reminded of another Clarke novel that rarely gets the attention in this regard that Rendezvous with Rama does. This is 1979’s The Fountains of Paradise (BCA/Gollancz). Although known primarily for its exploration of space elevators (and its reality-distorting geography), the novel includes as a separate theme another entry into the Solar System, this time by a craft that, unlike Rama, is willing to take notice of us. Starglider is its name, and it represents a civilization that is cataloging planetary systems through probes scattered across a host of nearby stars.

Starglider has a 500 kilometer antenna to communicate with its home star (humans name this Starholme), and in the words of a report on its activities within the novel, it more or less ‘charges its batteries’ each time it makes a close stellar pass. Having explored the Alpha Centauri trio, its next destination after the Sun is Tau Ceti. The game plan is that each stellar encounter will gather data and open communications with any civilization found there as a precursor to long-term radio contact and, presumably, entry into some kind of interstellar information network.

This is rather fascinating. For Starglider is smart enough to have studied human languages and is able to converse, after a fashion. From the novel:

It was obvious from its first messages that Starglider understood the meaning of several thousand basic English and Chinese words, which it had deduced from an analysis of television, radio, and especially broadcast video-text services. But what it had picked up during its approach was a very unrepresentative sample from the whole spectrum of human culture; it contained little advanced science, still less advanced mathematics, and only a random selection of literature, music, and the visual arts.

Like any self-taught genius,therefore, Starglider had huge gaps in its education. On the principle that it was better to give too much than too little, as soon as contact was established, Starglider was presented with the Oxford English Dictionary, the Great Chinese Dictionary (Mandarin edition), and the Encyclopedia Terrae. Their digital transmission required little more than fifty minutes, and it was notable that immediately thereafter Starglider was silent for almost four hours — its longest period off the air. When it resumed contact, its vocabulary was immensely enlarged, and more than ninety-nine percent of the time it could pass the Turing test with ease — that is, there was no way of telling from the messages received that Starglider was a machine, and not a highly intelligent human.

Clarke slyly notes the cultural differences between species as opposed to the commonality of, say, mathematics, saying that Starglider had little comprehension of lines like this from Keats:

Charm’d casements, opening on the foam
Of perilous seas, in faery lands forlorn…

And it drew a blank on Shakespeare as well:

Shall I compare thee to a summer’s day?
Thou art more lovely and more temperate…

Well, these are aliens, after all. We have enough trouble with cross-cultural references here on Earth. Humans broadcast thousands of hours of music and video drama to Starglider to help it out, but here, of course, we run into the messaging problem. Just how much do we want to reveal of ourselves to a culture about which we have all too little information other than that it is markedly more advanced than our own? You’ll find that aspect of the METI debate explored as a core part of the Starglider subplot.

Some have panned Starglider’s appearance in the novel because it seems intrusive to the plot (although I suppose I could argue that autonomous probes cataloging stellar systems almost have to be intrusive to get their job done). But in the midst of the Starglider passages, we learn that the chatty aliens, now freely talking to humans via radio, catalog the civilizations they find on a scale based on their technological accomplishments. Is this Clarke channeling Nikolai Kardashev?

Whatever the case, Clarke as always takes the long view, and the long view by its very nature always pushes out into mystery. Consider the scale used by Starglider:

I. Stone Tools

II. Metals, fire

III. Writing, handicrafts, ships

IV. Steam power, basic science

V. Atomic energy, space travel

VI. “…the ability to convert matter completely into energy, and to transmute all elements on an industrial scale.”

On this scale of one through six we can place our species at level 5, as Starglider sees us. But are there further levels? Clarke is wise to imply their existence without exploring it any further, as this lets the reader’s imagination do the job. He’s expert at this:

“And is there a Category Seven?” Starglider was immediately asked. The reply was a brief “Affirmative.” When pressed for details, the probe explained: “I am not allowed to describe the technology of a higher-grade culture to a lower one.” There the matter remained, right up to the moment of the final message, despite all the leading questions designed by the most ingenious legal brains of Earth.

When the University of Chicago’s Department of Philosophy transmits the whole of Thomas Aquinas’ Summa Theologica to Starglider, all hell breaks loose. I turn you to the novel for more.

Image; Hubble took this image on Oct. 12, 2019, when comet 2I/Borisov was about 418 million kilometers from Earth. The image shows dust concentrated around the nucleus, but the nucleus itself was too small to be seen by Hubble. We are on the cusp of a windfall of ‘interstellar interloper’ data as the LSST comes online within a few years. Will we ever find a Rama, or a Starglider, amidst our observations? Credit: NASA, ESA and D. Jewitt (UCLA).

As I mentioned, some critics fault The Fountains of Paradise for Starglider’s very presence, noting that there are essentially two plots at work here. In fact there are in fact three plots taking place on different timescales here, one of them dating back several thousand years, and recall that the voyage of Starglider itself spans millennia, the mission having began some 60,000 years before the events of the main part of the novel – construction of the space elevator – take place. This kind of chronological juggling, allows Clarke to inspire deeper reflection on humanity’s place in the universe and I find it enormously effective.

Wonders fairly pop out of Clarke’s early novels and much of his later work. On that score, I likewise refuse to fault him severely because he cannot achieve complex characterization. A case can be made (James Gunn makes it strongly) that science fiction of Clarke’s ilk needs to put the wonder first. Rich, strange and complicated characters confronting rich, strange and wondrous events may lead to one richness to many. For we, the readers, to absorb the mystery, we need to see how a relatively straightforward character reacts. It’s that contrast that Clarke aims to mine.

That’s only one way of doing science fiction, but much science fiction of the 1950s, which I consider the genre’s true golden age (with a nod to the late 1930s, as one must) often operated with precisely this conceit. And that’s okay, because when writers of greater literary style began to emerge – writers like Alfred Bester, say, with his staggering The Stars My Destination (1956) we were able to see complex characters confronting the deeply strange in ways that simply added depth to the experience. Look at Robert Silverberg in the 1960s as an exemplar of an almost magical insight into what makes the individual human tick. Once you’ve begun on that journey, the field is altered forever, but that doesn’t negate its rich past.

In fact, none of this subsequent growth nullifies Clarke’s accomplishment in the realm of big ideas. Consider him a writer of a kind of SF that flourished and fed a mighty stream into what has now become a river of wildly untamed ideas and insights. And sometimes only Clarke will do. Thus when i read, for the umpteenth time, The City and the Stars, I’m again dazzled by the very title, and the first few pages take me back into a realm where there are suns not quite our own casting a numinous glow over landscapes we learn to navigate through characters who learn with us. Like Stapledon’s, like Asimov’s, Clarke’s is a voice we’ll celebrate deep into the future.

The ancient notion of the ‘music of the spheres’ sounds primitive until you learn something about planetary dynamics. Gravity is wondrous and can nudge planets in a given system into orbits that show an obvious mathematical ratio. Two planets in resonance can emerge, for instance, in a 2:1 ratio, where one goes around its star twice in the time it takes the second to orbit it once. Such linkages might seem almost coincidental to the casual observer until the coincidences begin to pile up.

In the exoplanet system at HD 110067, for example, resonance flourishes, so much so that we have six planets moving in a ‘resonance chain.’ No coincidence here, just gravity at work, although an actual coincidence is that just when I finished a post highlighting system dynamics in closely packed environments like TRAPPIST-1 as a ‘brake’ on inbound comets, an international team should reveal HD 110067’s resonance chain. It’s a beauty, for all six planets not only move in harmonic rhythm but also turn out to be transiting worlds. An orbital dance this complex is rare, but even more so is the ability to study such worlds thanks to the happenstance of our viewing angle.

Transits allow us to extract information, and plenty of it, including analysis of planetary atmospheres as light from the central star passes through them. Because complex resonances are in some sense ‘self-correcting,’ they tell us something about the history of the system, for planet migration during the period when the resonance is being established influences the final state of the system. In HD 110067 we have a mother lode of system harmonics around a star that, usefully enough, is fifty times brighter than TRAPPIST-1, where we have seven rocky planets in a resonant chain.

HD 110067 offers up all of this for that highly interesting category of planets called ‘sub-Neptunes,’ about which we’d like to know a lot more. 100 light years away in the constellation Coma Berenices, HD 110067’s resonance chain is obviously complex. The innermost planet makes three orbital revolutions as the second world makes two – a 3:2 resonance. But the chain continues: 3:2, 3:2, 3:2, 4:3, and 4:3, with the innermost planet making six orbits as the outermost planet completes one.

Image: A rare family of six exoplanets has been unlocked with the help of ESA’s Cheops mission. The planets in this family are all smaller than Neptune and revolve around their star HD110067 in a very precise waltz. When the closest planet to the star makes three full revolutions around it, the second one makes exactly two during the same time. This is called a 3:2 resonance. The six planets form a resonant chain in pairs of 3:2, 3:2, 3:2, 4:3, and 4:3, resulting in the closest planet completing six orbits while the outermost planet does one. CHEOPS confirmed the orbital period of the third planet in the system, which was the key to unlocking the rhythm of the entire system. This is the second planetary system in orbital resonance that CHEOPS has helped reveal. The first one is called TOI-178. Credit and copyright: ESA.

Untangling this particular chain was not easy. The astronomers used data from both ESA’s CHEOPS mission and the TESS space observatory to nail down the system architecture. Data from TESS determined the orbital periods of the innermost worlds to be 9 and 14 days. Observations from CHEOPS tagged planet d at 20.5 days and thus demonstrated that while the innermost planet revolves 9 times around the star, the second revolves six, and the third planet four times. The periods of the three outer planets could then be deduced as 31, 41 and 55 days respectively, with further analysis of the TESS data showing that no solution other than the 3:2, 3:2, 3:2, 4:3, 4:3 chain would work. Ground-based observations supplemented the TESS and CHEOPS data.

The analysis was led by Rafael Luque (University of Chicago) and published in Nature. Says Luque:

“This discovery is going to become a benchmark system to study how sub-Neptunes, the most common type of planets outside of the solar system, form, evolve, what are they made of, and if they possess the right conditions to support the existence of liquid water in their surfaces.”

TOI-178 offers a five-planet resonance chain that may include a sixth world in this system of transiting planets in the constellation Sculptor, some 200 light years out. The paper on HD 110067 takes note of the fact that resonant architectures like these imply a situation that has remained unchanged since the birth of the system, making them useful laboratories for planet formation and evolution. The planetary radii at HD 110067 range from 1.94 that of Earth to 2.85 times as large (1.94R_⊕ to 2.85R_⊕), and the low densities found in the three planets whose mass has been measured point to the likelihood of large atmospheres dominated by hydrogen.

Image: Tracing a link between two neighbor planets at regular time intervals along their orbits creates a pattern unique to each couple. The six planets of the HD110067 system create together a mesmerizing geometric pattern due to their resonance-chain. © CC BY-NC-SA 4.0, Thibaut Roger/NCCR PlanetS.

Ann Egger (a graduate student at the University of Bern and a co-author of the paper on this work) notes what is ahead in the study of this system:

“The sub-Neptune planets of the HD110067 system appear to have low masses, suggesting they may be gas- or water-rich. Future observations, for example with the James Webb Space Telescope (JWST), of these planetary atmospheres could determine whether the planets have rocky or water-rich interior structures.”

The sheer beauty of the HD 110067 system comes across in the animation below:

Image: To-scale animation of the orbits of the six resonant planets in the HD110067 system. The pitch of the notes played when each planet transits matches the resonant change in orbital frequencies between each subsequent planet. The relative sizes of the planets is accurate, although their true size compared to the star is much smaller. Also available at https://www.youtube.com/watch?v=2rrODAG7nmI.

The paper is Luque et al., “A resonant sextuplet of sub-Neptunes transiting the bright star HD 110067,” Nature 623 (November 29, 2023), 932-937 (abstract).