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).