The James Webb Still Hasn’t Found Life on K2-18b
Revisiting the rumours of life on K2-18b
A year ago, rumours surfaced of an impending discovery of alien life. They focused on the planet K2-18b, an intriguing world that lies a little more than one hundred light years from Earth. On it, these rumours said, astronomers had spotted the chemical signatures of life. At the time, the supposed findings were said to be secret, awaiting the more conclusive proof such a dramatic discovery would need.
Needless to say, the year 2024 passed without this clinching proof emerging. The research around K2-18b was, as The Quantum Cat argued at the time, interesting but not yet convincing. The supposed chemical signature was weak, the habitability of K2-18b uncertain, and the idea that life was to blame unproven.
Yet this week speculation about K2-18b has once again emerged. In a new paper, published yesterday in The Astrophysical Journal Letters, the researchers behind the original claims say they have found stronger evidence for life. Like last time, the evidence rests on the presence of a chemical known as dimethyl sulphide in the atmosphere of K2-18b. And, like last time, they argue this chemical might have been created by living creatures swimming in a vast ocean.
In their portrayal, K2-18b is a “hycean world”, a planet the size of Neptune surrounded by a thick hydrogen-rich atmosphere and home to a deep ocean of warm water. This kind of planet has never directly been proven to exist – nothing like it, for example, exists in our solar system. But if it does, and K2-18b is indeed one, then it could be a credible place for biology to appear.
The problem is, however, we are not really sure K2-18b has any water at all. No clear evidence of an ocean has ever emerged, and although the Hubble Space Telescope did once detect signs of icy clouds around the planet, the James Webb hasn’t seen any trace of them. Some researchers even think the data we have better fits an inhospitable world covered by oceans of lava rather than warm water.
Neither are other researchers convinced by the signs of dimethyl sulphide. Though the new paper claims to have a stronger signal of this chemical, it is still possible the authors have misinterpreted the data. And even if it is there, there is no guarantee it is coming from biology. The chemistry of such a hycean world is likely to be very different to our own, and there is, perhaps, some non-biological process pumping out the gas. For now, we simply don’t know.
I. The Scent of the Sea
On Earth, dimethyl sulphide is sometimes called the smell of the sea. Large amounts of it are made by algae floating in seawater, and this, indeed, is the primary source of the substance on Earth. Since it is made by life, the presence of it in the atmosphere of a distant planet is generally regarded as a promising sign for alien biology.
The first question, then, is whether the chemical is really present around K2-18b. To find out, researchers used the James Webb Space Telescope. In the original study, published in 2023, they claimed it had spotted the gas, along with some others like carbon dioxide and methane. This was enough to spark the first round of rumours, but in truth the signal was weak and perhaps wasn’t really there at all.
The new study once again used the James Webb telescope. But this time they used a different instrument, studying a different portion of the light coming from the planet. And once again they say they see signs of dimethyl sulphide. Indeed, according to their analysis there is a lot of it: the gas seems to be present in far greater concentrations than those seen on Earth. Alongside this gas, they also think they see dimethyl disulphide. This too is a gas produced by life and is a plausible sign of biology on an alien world.
But even if the gas is there, we still don’t know for sure it is being created by life. Indeed, last year a study reported finding traces of dimethyl sulphide on a comet. Nobody is claiming this comet has oceans and marine life, and so the chemical must have been produced in some other way. Perhaps, the authors of another study propose, ultraviolet light and cosmic rays help to create it in the coldness of space.
And that highlights the biggest criticism of the idea. If K2-18b is a hycean world, it is a very different place to any of the planets we know in detail. We have no real idea of the chemical processes that might be playing out there and thus cannot be sure whether biology or chemistry alone is responsible for the gases we see. Short of further evidence, this doubt will long remain.
II. Is It a Hycean World?
What, then, about the planet itself? From our measurements, K2-18b seems to be a world roughly eight times more massive than the Earth. That makes it about half as massive as Neptune, and as a result it straddles the border between the rocky planets and the gas giants found in our solar system. We also know it orbits its star at about the right point for liquid water to exist on its surface, and thus by simple metrics it is potentially habitable.
Beyond this, however, things are much more uncertain. NASA offers only the briefest of description - the world is, they say, “potentially rocky”. But since K2-18b is larger than the Earth, it is possible it has a thick atmosphere, one not quite as thick as that of Neptune, but one rich in hydrogen instead of nitrogen and oxygen.
The hycean idea takes this concept a bit further. Under that atmosphere, it posits, is a deep ocean of liquid water. And since hydrogen is a strong greenhouse gas, that ocean would probably be quite hot. Some early measurements from Hubble suggested this ocean might exist, and some later Webb measurements could be interpreted in the same way.
If this is the case, then the possibilities for life look reasonably good. Under some models the ocean can be kept cool enough for life to thrive, and the water might be rich in the chemicals needed for biology to get going. The hydrogen atmosphere, as long as it is not too thick, is no real impediment for life either.
But to be clear, this is not a world that looks anything like the Earth, or indeed like any planet we are really familiar with. Hycean worlds are something different, a kind of planet that might be widespread in the galaxy, and that might allow life to evolve. But we have no definite proof that they exist.
And this is another problem. There are several alternative models that fit the data we have, and they are not at all pleasant to imagine. In one study last year, researchers sketched out a hellish vision of K2-18b. Instead of a balmy hycean world, they said, it could be a planet of lava oceans boiling under hydrogen-filled skies. No life would be possible there, and none of the measurements we have taken yet rule out this scenario.
Once again, then, we must confront the fact we are looking across a vast stretch of space at a world we really know very little about. The data we have is inconclusive, and there are few things we can say with certainty. K2-18b is certainly worthy of more study. But we cannot yet make grandiose claims of a planet suited for life.
III. See You Again Next Year?
The studies of K2-18b are the work of Nikku Madhusudhan, a professor of astrophysics at Cambridge University. He first started looking at the planet in 2020, when he concluded it could be a warm water world. The next year he published his concept of the hycean worlds, and then, in 2023, claimed that K2-18b was a good candidate for the model. Then, as now, he also claimed to have found the possible fingerprints of life.
Such claims make for good headlines and even better rumours. Newspapers are already labelling the latest discoveries as the strongest evidence yet of life beyond Earth. Perhaps in time they will turn out to be right. Or perhaps they will fade away, as prior claims about life on Mars or Venus have done.
The truth, as always, is muddled and uncertain. We should not forget that we are peering across incredible distances and attempting to spy out molecules floating around faraway worlds. The question of life will always be a hard one to answer, and there will be none of the definite proof the rumours will inevitably hint at.
Most of what I wrote on this subject and this planet a year ago still stands. So too does the conclusion I reached then, which I think is worth repeating in full now:
What all this boils down to is simple. The discovery of life beyond Earth would be the discovery of the century. It is, quite possibly, the biggest claim that any scientist can dare to make; a breakthrough that would change the way we see ourselves and the cosmos, that would raise the serious possibility of encountering alien intelligence and that would send echoes reverberating through society.
Claims of having done so thus face a high standard of proof. As Carl Sagan once said, extraordinary claims require extraordinary evidence. Alien life is certainly an extraordinary claim. Yet the discovery of dimethyl sulphide, or some other similar chemical, does not meet a high enough bar to count as extraordinary evidence. As long as other possibilities exist - geology, chemistry, or whatever else - the threshold to claim discovery of alien life will not be met.
At the same time, we are drawing closer to a point where this changes. Telescopes like the James Webb will keep on finding molecules that might have biological origins. In most cases the data will be weak and the lines only faintly visible. Definitive proof will, in all likelihood, be hard to come by. The conclusions will be controversial. But they will keep coming.
I hope that Nikku Madhusudhan doesn't follow the path of the Harvard professor who sees signs of alien spaceships everywhere.
It seems reasonable that we will eventually find simple life elsewhere. It doesn't seem reasonable to me that life only evolved here. But complex, let alone intelligent, life seems a different question. I think I've mentioned before my crude calculation of the odds of it being 1:10²⁴ (compared to 10²² stars in the visible universe). That said, abiogenesis still seems a deep mystery.