If you鈥檙e the kind of person who keeps an eye on astronomical news 鈥� or glances at any news site 鈥� you may have seen a story about hints of a sign of alien life on an exoplanet called K2-18b. The claim is controversial, but much of the coverage was positive. The problem, I think, is Star Trek, or any other science fiction you might care to mention. Thanks to the budgets that govern television, we鈥檙e used to aliens being obvious; they鈥檙e usually about 6 feet tall and bipedal, with something odd stuck to their heads 鈥� and when they show up, there鈥檚 no question they exist.
Real aliens may be more elusive. And indirect signs of extraterrestrial life even more so. Take the discovery of methane 鈥渂urps鈥� by NASA鈥檚 Curiosity rover coming from under the surface of Mars. On Earth, we know methane is produced by cows. But on Mars, the several rovers on the ground can鈥檛 even agree with the measurements we鈥檝e made from orbit as to whether the gas really exists in the Red Planet鈥檚 atmosphere, let alone whether the source is biological or the result of some odd geological process 鈥� low-level volcanism, perhaps.
If such debates rage when studying a planet we can send robots to, it鈥檚 probably not surprising that an attempt to study the atmosphere of a world 124 light years away is fraught with difficulty. We don鈥檛 even have an image of K2-18b; the planet betrays its presence only when it passes in front of its sun, dimming the light from that red dwarf star. The fact that we can say anything at all about what this distant world is like is a triumph.
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Prior work on K2-18b, by the same group that announced the recent findings, used our most advanced telescope, the James Webb Space Telescope (JWST), to show that K2-18b鈥檚 surprisingly methane-rich atmosphere is different from anything we have in our solar system. That alone makes the planet a valuable target for astronomers who crave places to test their models of how planets form and evolve. These new results add to previously disputed evidence that dimethyl sulphide (DMS), a molecular species that, on Earth, is produced mainly by plankton, may be part of the rich soup of chemicals this world harbours.
Whether DMS is there or not, this enigmatic world is not Earth-like 鈥� indeed, this is why it鈥檚 fascinating to study. The authors of the recent study on K2-18b suggest it has a water ocean buried deep under a hydrogen atmosphere; other work on the planet suggests a magma ocean, presumably inhospitable to life, might be a better fit.
The problem is that while we may have left Star Trek behind by imagining aliens that don鈥檛 fit our Earth-bound limitations, we also need to leave behind the idea that the chemistry of these planets will be Earth-like. Worlds as different from Earth as K2-18b, or even Mars and Venus, have their own chemical pathways and patterns. Any chemical that might be a biosignature will stand out only in its proper context 鈥� phosphine on Earth, for example, is produced by penguins and so it was big news when it was found on Venus in 2020. But it鈥檚 commonplace in the atmospheres of Jupiter and Saturn, where it is definitely not produced by aliens.
The thing I鈥檓 really excited about is that we finally have the ability to study the diversity of chemistry we expect to find among the huge variety of planets that we know exist in the Milky Way. Among such abundance, we need to be very cautious in translating our terrestrial experience to other worlds, lest we mislead ourselves. This is nothing new 鈥� in the 19th century, astronomers mistook dark lines on Mars鈥檚 slopes for irrigation canals from a lost civilisation.
The peer-reviewed paper that describes the new observations is suitably careful. In it, Nikku Madhusudhan at the University of Cambridge and his colleagues note, for example, that the detection 鈥渋s at the lower end of robustness required for scientific evidence鈥�. 聽But in an accompanying press release, he says, 鈥淭he signal came through strong and clear.鈥� The paper suggests a possible detection of a possible biosignature that might, possibly, be life. But the press release says: 鈥淎n ocean that is teeming with life is the scenario that best fits the data we have鈥�. As a result, instead of a discussion of a fascinating place, we鈥檙e having an argument about whether we鈥檝e found aliens.
The fun of this sort of science is that it is difficult. The observations are a marvel of engineering and scientific flair, the range of possibilities we can imagine for this world are huge and the breadth of science 鈥� from lab chemistry to theoretical atmospheric physics 鈥� being brought to bear is astounding. To me, it鈥檚 a shame that a claim of a major discovery is being driven by what, charitably, is a belief that the biggest headline is the best option. That approach obscures all of the real, fascinating, frustrating and ultimately rewarding mess of how sciences get done.
