And Then There Were Two?
Did we just find the second planetary biosphere?
A cherry-red coal three times wider than the noonday Sun glowers over a hazy sapphire orb. Beneath the haze, between the white patches of cloud, splotches of ochre and violet, umber and sable swirl over the surface like freshly laid bruises.
This is K2-18b. It is a Hycean world: larger and heavier than the Earth, smaller than Neptune, its thick atmosphere composed of methane and hydrogen, its surface entirely swathed in an ocean dozens of kilometres deep.
Could you dive to the lightless depths of that world-spanning ocean, where the crushing pressure is multiples stronger than at the bottom of the Earth’s deepest abyssal trench, you would find the seafloor covered in a thick cladding of ice VII, an exotic form of solid water that forms only under such extreme conditions. Were it not for the world’s powerful volcanoes, the ice VII layer would cut the ocean off completely from the minerals of the submerged rocky surface, thereby sterilizing the ocean by starving it of nutrients. But K2-18b is young, only two and a half billion years old; the nebula that birthed it was heavily enriched with heavy elements, giving it an abundant supply of radioactive isotopes; and it is a large world, easily able to retain its natal heat. The result is robust tectonic activity. Eruptions of magma melt the ice VII from below, occasionally triggering cryvolcanoes from dissolved ammonia which fracture the strong but brittle ice layer, enabling nutrients to seep up through the cracks.
Far above, on the surface, phytoplankton feast on these minerals, blooming wherever concentrations can be found into thick bacterial mats that stretch for kilometres in every direction across the gently undulating swells. Life here is not green, but red, purple, brown, and black – the red dwarf the planet orbits emits most of its light in the infrared part of the spectrum, and life has developed pigments to match. The form of photosynthesis that life here has developed does not produce oxygen, because it takes advantage of the planet’s hydrogen-rich atmosphere, combining molecular hydrogen with carbon dioxide to produce water and various organic compounds. Other organisms survive by combining hydrogen and carbon dioxide, dumping methane into the atmosphere in great stinking clouds that waft up from the ocean’s surface. In deeper layers of the ocean, where light doesn’t reach, other microbes ferment the cells of dead organisms drifting down from above, releasing carbon dioxide to bubble back up to the surface.
On Earth, dying phytoplankton are the primary source of a chemical known as dimethyl sulfide or DMS, with trace amounts also coming from volcanic outgassing, and it has therefore been suggested as a possible biosignature. DMS was recently detected in the atmosphere of K2-18b using the James Webb Space Telescope, along with a similar chemical species, DMDS or dimethyl disulfide; the detection is fairly secure (significant at the 3σ level, or about 99.7% confidence), and the concentration is extremely high (10 parts per million by volume, about 20x the concentration in the Earth’s atmosphere). In case you’re curious, here is a comparison of the observations to the models:
The error bars look huge, which is inevitable because it is extremely difficult to measure the spectrum of an exoplanet. You need to use a method called transmission spectroscopy, which involves comparison of a star’s unoccluded spectrum with the spectrum as seen when it’s passing through a planet’s atmosphere. Since the star is a lot brighter than the planet, the differences between the two spectra are tiny, so it’s hard to do much better than what we’re looking at here. In any case, it might seem like you could fit almost anything to that spectrum, but here’s a breakdown of the models themselves, showing the contributions made by the different chemical species, which demonstrates that the features identified by the team are certainly not caused by methane or carbon dioxide (both of which were already known to be present in the planet’s atmosphere in large amounts).
In Earth’s atmosphere DMS is rapidly destroyed via oxidation, meaning that it must be continuously replenished by decaying microorganisms. There’s no free oxygen in K2-18b’s atmosphere, but it’s likely that DMS is destroyed via photolysis: ultraviolet radiation from the red dwarf star breaking it down as it reaches the stratosphere. This implies that there must be a significant source of DMS constantly pumping it into the planet’s atmosphere. Photolysis is slower than oxidation, removing DMS in weeks rather than days. If DMS survives for a month or so on K2-18b, the much higher concentration would be consistent with a biosphere about as productive as Earth’s, i.e. the average amount of living stuff per square kilometre would be about the same, on average, as our own planet’s.
It’s worth noting that K2-18b’s high levels of methane have also been pointed to as a possible biosignature, although the paper in which this possibility was presented was careful to note that an entirely lifeless mini-Neptune with no ocean at all was also fully consistent with the available data ... however, it’s also worth noting that the new JWST data are in flat contradiction to numerous predictions of the mini-Neptune model (according to the team which reported the new data, at least). In any case, here’s a comparison of the old JWST data to the three models: a lifeless Hycean world, a living Hycean world, and a mini-Neptune.
Personally, I think the living Hycean world looks like a better fit. Compare the carbon dioxide feature near 4 microns, for example. Statistically, however, the fits are identical (for stats nerds, that’s given by the chi^2 statistic in the upper right corner of each panel).
So, we’ve got a couple of different papers coming to the independent conclusion that K2-18b, alien as it is from the Earth, shows strange atmospheric signatures which can be easily explained by a planetary biosphere. If it holds up, that’s huge.
Whether it holds up is, as always, the question. As you’d expect, the result is being questioned from every conceivable angle. Does the planet even have an ocean, or is it really a ball of magma with a thick atmosphere? Is the detection of DMS even real, or could the signal be explained by other chemical species? If it is real, does it necessarily imply a biogenic source, or could there be some chemical pathway that can produce it in the required amount? DMS has been made in the lab, after all, and volcanism is known to produce at least a little bit of it even on Earth ... although the volcanism hypothesis, it’s worth pointing out, is extremely difficult in this case since any volcanoes would be buried under both several kilometres (if not hundreds of kilometres) of high-pressure ocean and, in all probability, a kilometres-thick sheet of exotic, high-pressure ice (as described above). It’s hard to see how complex organic molecules generated in volcanic eruptions would survive the trip to the surface. Then again, who knows ... perhaps there is some exotic chemistry in the ocean itself that hasn’t been figured out yet.
Anyone who’s followed the history of detections of extraterrestrial life will recognize this pattern, with Mars as the usual whipping boy. Back in 1996 there was the martian meteorite Alan Hills 84001, in which structures resembling fossilized nanobacteria were discovered using electron microscopy. These were later written off as having more likely formed via chemical and minerological processes having nothing to do with life.
Earlier, in 1976, the Viking lander conducted a series of chemical experiments in which soil samples were placed in sealed compartments, provided with moisture and nutrients, and monitored for signs of respiration and metabolism. The experiments produced mixed results – one positive, one inconclusive, and one clearly indicative abiotic chemistry. Curiously, despite numerous subsequent missions, there have been no follow-up experiments.
Our ambitions for the scope of Martian life have declined dramatically over time. During the 19th century, it was widely believed by astronomers that Mars had canals, with Percival Lowell speculating at length – he wrote three books on the subject – that the canals were giant engineering works built by a dying civilization, intended to irrigate the drying planet.
Mars continues to tease us to this day. Less than a year ago, NASA’s Perseverance rover investigated a curious rock formation dubbed Chevaya Falls, which exhibits white spots with black rims dubbed ‘leopard spots’. On Earth these spots are associated with organic activity. Various organic molecules have also been confirmed in the sample, while the area in which it was found is thought to have been a river delta feeding into a lake. It’s certainly very suggestive.
While Mars has probably been the source of most claimed detections of extraterrestrial life, there have been some others. For example, in the 1980s Fred Hoyle and Chandra Wickramasinghe reported that the infrared and ultraviolet spectral properties of interstellar dust grains were fully consistent with dessicated E. coli bacteria. They proposed, in effect, that the nebulae from which stars and planets form are packed with bacterial spores. That sounds crazy, and the scientific community considered it to be crazy, but it’s worth pointing out that Hoyle and Wickramasinghe largely founded the study of interstellar dust grains.
Wickramasinghe was the one who first proposed that interstellar dust was largely organic, a prediction which has been fully vindicated with subsequent widespread detection of polycylic aromatic hydrocarbons, but while the widespread existence of these organic molecules is now widely accepted, E. coli spores are considered a step too far. Wickramasinghe doesn’t care. He insists to this day that there are bacteria in space. Since then, he’s published investigations of anomalies such as ‘red rains’ of biological material coming down from the sky.
Scientists are deeply reluctant to accept evidence of extraterrestrial life. I suspect this might be partly due to the traumatic disappointment that came from the collision, in the space of about six decades, between Lowell’s grand visions of an ancient, dying Martian civilization, and the Viking mission’s revelation of a barren, lifeless wasteland.
Yet at the same time, some of this seems to be nothing more than prejudice – a strong prior that the cosmos is fundamentally lifeless, that it is a wasteland which is fundamentally hostile to life, in which biology is vanishingly rare. Some go so far as to claim that the Earth is the only living world in the Galaxy, if not in the observable universe. According to this view, life is so fantastically unlikely, requiring not only such incredibly fine-tuned conditions to emerge, but a series of such deeply improbable chemical coincidences, that it is a miracle that there is any life in the universe at all.
If you look into most of the claimed detections of extraterrestrial life – Lowell’s canals very much excepted – and examine the claimed deboonkings, a reliable pattern emerges. Some signal is found which matches a chemical signature or fossilized remnant of terrestrial life, most commonly at the microbial level. Other scientists then come forward to show that this putative biosignature could have originated with a non-biological process. If it was found on Earth, it would be assumed to be biological without a second thought; because it was not, the identification of a non-biological process that could have caused the biosignature is generally assumed to constitute proof that the putative biosignature was caused by a non-biological process. Since virtually every biosignature could be caused by some ordinary chemical reaction, it follows that no biosignature is ever accepted, and the question of extraterrestrial life is thereby kept permanently open.
Personally, I’ve always found the view that life is random and rare to be not only bleak, but rather unlikely. Life arose on Earth almost immediately after the end of the Hadean epoch. The first fossil evidence of terrestrial life is dated to between 3.7 billion years ago (from stromatolites, considered entirely uncontroversial) and 4.3 billion years ago (from microfossils in hydothermal vent deposits, considered much more controversial). The most recent reconstruction of the genome of the Last Universal Common Ancestor of all terrestrial life pegs it to 4.2 billion years ago. The point is that, almost as soon as the Earth cooled enough for life to be possible, life seems to have swarmed all over its surface. That’s quite remarkable if life started on Earth from scratch due to random chemical processes that would presumably have taken quite a long time to accidentally stumble across the right sequence of amino acids to build a self-replicating chain of nucleotides.
On the other hand, if Wickramasinghe and Hoyle are correct, and interstellar space is full of bacterial spores, essentially every planetary body that condenses from the interstellar gas and dust would come pre-loaded with bacterial spores, and we’d therefore expect life to blossom more or less immediately whenever and wherever it finds the right combination of temperature, pressure, water, nutrients, and free energy. In this scenario, the rapid appearance of terrestrial life is an entirely mundane occurrence, as reliable as new shoots sprouting in springtime ... and the existence of other planetary biospheres, indeed a profusion of other planetary biospheres, would also be expected, as a matter of course.
But just how many biospheres would we expect?
Well, if we accept the detection of biosignatures on K2-18b, then we can say that there is at least one living world within 124 light-years of the Sun (this being the distance to K2-18). Yes, we are doing statistics starting with N = 2. I know, I know. It’s the best we’ve got to work with right now, so be quiet. So, if we then assume that the distance separating the Sun and K2-18 is the average distance between planetary biospheres in our region of the Galaxy, we have one living world per 124-light-year diameter sphere. Within that volume there are about 100,000 stars. The Milky Way itself has about 100 billion stars, from which we can then infer a galactic population of one million planetary biospheres.
That number seems huge, but it might actually be a severe underestimate. The method used to find K2-18b in the first place is called the transit method, which relies on the planet passing directly between the star it orbits and our line of sight. This requires a nearly perfect alignment of the planet’s orbit, which is true for only a very small fraction of systems, something on the order of 1% or less. That would imply that there could be 100 living worlds within our 124 light-year volume that simply can’t be seen with the transit method, from which we would extrapolate a hundred million living worlds in the Galaxy – roughly one living world for every thousand stars.
Now, most of those worlds probably aren’t inhabited by anything much more interesting than patches of ill-smelling slime. That was true of the Earth for roughly 87% of its history: multicellular life didn’t emerge until 600 million years ago, so even when life takes root in ideal conditions, it takes a very long time for evolutionary processes to work themselves up to anything particularly charismatic. Furthermore, since we know that microbial life will eke out a meagre existence under the most marginal of conditions, we should probably expect that ‘living world’ encompasses quite a few environments that are very marginal indeed – radiation-blasted desert worlds nearly devoid of water, iceballs where rapid metabolisms are impossible and bacterial cells take ten thousand years to divide, or Hycean worlds like K2-18b.
If only one in a thousand stars is orbited by a living world, perhaps only one in a thousand of those biospheres has complex, multicellular life. That gives us around a hundred thousand worlds with interesting flora and fauna in the Galaxy at any given time. If intelligent, tool-using life emerges on only one in a thousand of those worlds, that would imply a hundred such species in the Galaxy ... or, with our first lower bound of a million biospheres, exactly one: us.
Of course, I pulled those numbers out of my butt. If we adjust our ratios of living:multicellular:intelligent to the fractions of time occupied by each during Earth’s own history, which amounts to pulling different numbers out of my butt, then maybe one in ten living worlds develop complex multicellular life, and one in six hundred of those go on to produce intelligent life. In that case, we’d have well over ten thousand intelligent species occupying the Galaxy at any given time, which is basically Star Trek (though even then, there are about 10 million stars for every intelligent species).
If the seeds of life are effectively ubiquitous in the universe, living worlds consequently common as grass, and intelligent life therefore as abundant as bugs in a meadow, the Fermi paradox becomes a lot more concerning, and only really admits one of two plausible answers, both of which are rather terrifying.
The first plausible resolution of the Fermi paradox is that there is a Great Filter which wipes out intelligent species long before they can conquer Galactic empires, most likely before they even get off their home planets. In that case, we are probably doomed. That’s not a possibility we should write off lightly, as there are many pathways to doom that we can see in our own near future. But then, we should never assume that we’re doomed, lest it become a self-fulfilling prophecy.
The other possibility is simply that there is no Fermi paradox: the aliens have been here all along. In other words, Unidentified Anomalous Phenomena – what used to be called UFOs – are entirely real, and constitute observable – and indeed somewhat routinely observed – evidence of nonhuman intelligence visiting our world and, occasionally, interacting with its inhabitants.
UAPs are, of course, generally considered to be deeply unserious, lunatic conspiratainment mythology. Yet this evaluation seems to me to arise from precisely the same epistemic prejudice that leads scientists to immediately search for, and reflexively accept as a priori more likely, non-biological explanations every time a biosignature is detected. The assumption is that we are almost certainly alone, that there is no other biological or intelligent life in the universe, and that any claim to the contrary is definitionally extraordinary and therefore required to meet the ‘extraordinary evidence’ bar ... a bar which somehow always seems to be raised just above the claim’s ability to jump. In just the same way, every UAP sighting is swamp gas no matter how far from a swamp, or a hallucination no matter how sober and psychologically stable or numerous the men who saw it, or a radar malfunction no matter how sound the equipment. The minimally plausible ‘mundane’ explanation is always accepted, no matter how implausible ... because the idea that we might be just one intelligent species amongst many in a vast and thickly populated galaxy is, for some reason, not considered to be ‘mundane’.
I once chatted with a scientist who was deeply skeptical of the available evidence for UAPs, whether the sightings consisted of lights dancing strangely in the sky or strange creatures doing strange things to ordinary people, and asked him what he, personally, would accept as convincing evidence. He considered a moment, and declared that he would need to personally meet the aliens – the craft would need to land, the aliens walk out of the craft, approach, shake his hand, and make small talk. But, I replied, we have innumerable reports of exactly such encounters, all over the world, from countless people, and scientists write them all off as lunatics. Are you not saying that the only evidence you personally would accept, is precisely the evidence that no one else would accept?
He didn’t know what to say to that, except to agree that I’d made a good point.
I don’t know what’s going to happen with the K2-18b detection. If I had to guess, it will be picked apart in the usual fashion – papers will be published proposing non-biological explanations for the dimethyl sulfide and the methane, and these will be accepted as deboonking yet another irresponsible claim of extraterrestrial life from an insufficiently grumpy professional scientist.
But you never know. There are signs in the culture of a thaw on the question: small, but growing communities of scientists are beginning to make more systematic study of UAPs, while Congressional hearings on the subject have already entered some wild things into the official record, stories of salvaged alien spacecraft and recovered ‘non-human biologics’. On a more conventional front, other planets are being studied using the same techniques used for K2-18b, and our ability to study them is only improving. So far maybe 100 planets have been observed using transmission spectroscopy – that we’ve found biosignatures, plural, in one of them may be very significant. It’s entirely possible that at some point, a ‘slam dunk’ detection – say, of the spectral signatures of chlorophyll and oxygen – will be made, which will simply be too hard to explain away. An observation like that could crack the consensus about extraterrestrial life, and suddenly all that other evidence – everything from interstellar bacteria to, who knows, the Face on Mars – could come flooding back through into serious consideration very quickly.
Before this century is out, we may be inhabiting a radically different conceptual universe, one in which our planet is not a lonely, isolated dust mote of life in a dead cosmic desert, but a living island woven into a biocultural tapestry that spans the Galaxy, and possibly beyond. Wickramasinghe’s interstellar bacteria don’t only imply that the seeds of life are everywhere, but that life wherever it is found will be related: based on RNA and DNA, sharing the same metabolic pathways, far less alien than we’d expect. Including UAPs in this picture expands the scope for interaction still further, suggesting the possibility of not only common biological origins but ongoing biological and cultural interactions across interstellar distances.
If we come to live in such a world, the wildest aspect of it is that it will not seem strange at all – it will simply be taken as ordinary and banal, just as we now consider it wholly unremarkable that the Earth orbits the Sun, that the continents drift, that one life-form evolves into another, that the universe is expanding. Extraterrestrial organiss and non-human intelligence will simply be part of the background scenery of life, and life will go on, by and large, as it always has.
I hope you enjoyed this little excursion up out of the grubby mud of politics to consider, for a change, the grand questions of the universe. I know I enjoyed writing it ... this is one of my favourite subjects, after all. I’ve written on similar things a time or two before. Here, for instance, I consider the extraterrestrial intelligence question from the perspective of the full scope, in time and space, of our universe, and try to show that the possibilities are quantifiably greater than our imagination can fill:
Metempsychotic Dragons in the Cosmic Dungeon
Think of the universe as we know it to be while making somewhat optimistic assumptions about the prevalence of life.
Here I discuss the epistemological questions relating to UAPs, in the context of other phenomena that were once considered pseudo-scientific old wives’ tales:
The moral evidence for sprites
A few years ago a girlfriend texted to ask me about something weird she’d seen. The lights had been arranged in a line, and had gone whipping across the sky. She’d never seen anything like it before. Could they have been a UFO? I asked her if maybe she’d seen a StarLink constellation, and a few minutes later she responded sheepishly that, …
And here I look at what we could do with a proper space telescope, and how we might build one:
What Big Eyes You Have
I’m an absolute sucker for insanely ambitious engineering projects. Little projects like draining the Mediterranean to irrigate the Sahara, for instance.
That’s all for now. Thank you for your attention, and for that glorious minority who support this project, my gratitude for your patronage is as deep as the sky, and my thank-yous as numerous as the stars shining within it.
As our equipment improves, and as we begin to investigate remote planets without the filter of Earth's atmosphere, we’re going to get increasingly refined analysis of our galactic neighborhood. If it turns out we live in a completely lifeless universal desert, or if we live in a galaxy filled with countless life-filled worlds, either way, human beings will carry on pretty much as they always have. Nonetheless, on the remote chance we someday find a way to actually transit interstellar space, I prefer a universe with many habitable worlds. Allowing the human race multiple bites at the apple, rather than just relying on old Manhome, Planet Dirt, and the lifeless worlds of our solar system, would be a great boon.
Loved your point that the only proof acceptable to that UAP skeptic would be unacceptable to any other skeptic. He is making the very unscientific confession that only *subjective* evidence will convince him!
Hoyle & Wickramasinghe's Panspermia theory, indeed any theory that holds that life is common in the universe, contradicts one of the basic (though long obsolete) metaphysical axioms of science: that the universe tends toward disorder, that order, design, intelligence comes only from human beings (or a supernatural God that imposes such from without). Scientists therefore are uncomfortable with the idea that life is an inherent property of the universe, that the universe is replete with life, whether in protostellar clouds or spontaneously arising wherever conditions make it possible. LIfe must be an extremely rare statistical anomaly in order to maintain the exile of God from creation. The mechanists' formula is Determinism + Randomness = Creation. They cannot countenance any kind of teleological principle. I discussed this in a book 20 years ago: https://ascentofhumanity.com/text/chapter-6-08/. The whole chapter is relevant to this discussion.