Centauri Dreams
Imagining and Planning Interstellar Exploration
Can the ‘Zoo Hypothesis’ Be Saved?
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).
Life Elsewhere? Relaxing the Copernican Principle
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 & Schulze-Makuch 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).
Holiday Thoughts on Deep Time
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.
A Novel Strategy for Catching Up to an Interstellar Object
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.
Forbidden Worlds? Theory Clashes with Observation
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
Interstellar Precursor? The Statite Solution
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.
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