The oxygen and methane in the Earth’s atmosphere, which are indicators of biological activity, albeit inconclusive, would be too faint to be detected by an alien JWST from 120 light-years ago. However, from an orbit near the Earth, the JWST has indeed detected possible biosignatures such as methane and carbon dioxide between wavelengths of 2-6 micrometres in several exoplanets hundreds of light-years away.

One such planet atmosphere is K2-18b, a planet slightly smaller than Neptune (sub-Neptune, two-and-a-half times the size of Earth), around an M-dwarf star K2-18, 124 light-years (1 million trillion km) away from us — a distance far beyond what any human could hope to travel in a lifetime. Nevertheless, K2-18b is one of the most-studied planets among the 6,000 or so planets around other stars that have so far been identified throughout the universe.

A group from University College London found evidence of water in the atmosphere of K2-18b as early as 2019 from Hubble Space Telescope observations. In 2023, a team of astronomers led by Nikku Madhusudhan, a professor at the University of Cambridge, discovered methane and carbon dioxide from Hubble observations of this planet.

Last week, Madhusudhan and his group declared that they have found “signs of life”, based on new JWST observations, on planet K2-18b. This is based on hints of dimethyl sulphide (DMS), a pungent compound, in the atmosphere of K2-18b. Their observations also suggest that the related chemical dimethyl disulphide (DMDS) might also be present. These molecules are interesting for the search of life because on Earth they are produced by living organisms such as bacteria and marine phytoplankton.

These latest results are only “three-sigma”, which means that, assuming normal statistics, there is a 0.3% chance that the detection of the gases is not correct. This sounds small, but it is not enough to convince the scientific community. Even if the detection of dimethyl sulphide on K2-18b were to eventually prove out to the entire field’s satisfaction, its presence is not necessarily a bankable sign of life. Last year, the chemical, clearly not of biological origin, was observed in the dead, icy spray of a comet 67P/Churyumov-Gerasimenko. Whatever dimethyl sulfide’s organic origins might be on Earth, the universe, in all its creativity, is clearly capable of making it without the help of organisms.

We don’t have much detailed information about K2-18b. We don’t even know for certain what kind of a planet it is. Madhusudhan pictures a “hycean” world — wrapped in hydrogen, and its surface hidden beneath a global ocean. Another group, studying the same data record, sketches a very different scene — that of a magma ocean hostile surface, overwhelmingly hot.

To begin with, there are questions about whether K2-18b even has water — or a surface that could support life. Modelling studies of it and similar planets suggest that they are probably barren. Even though this planet lies in the “habitable zone” of the dwarf star, one must point out that the vicinity of this class of stars is not likely to be very hospitable, due to frequent flares of high-energy near-ultraviolet radiation from the surface of M-dwarf class stars.

Saturn, Jupiter, and Neptune’s atmospheres are made of hydrogen and helium, with traces of methane. These are gaseous planets with no hospitable environment. Venus and Mars have atmospheres of mostly carbon dioxide, and some moons like Titan in our solar system have copious amounts of methane in their atmosphere. The origin of these gases is not thought to be from biological activity.

The detection of trace gases in the atmospheres of invisible planets a million trillion km away is indeed supremely difficult and these are remarkable discoveries. However, media headlines announcing “We-may-not-be-alone” based on these revelations are unusually hyperbolic.

As Carl Sagan used to say (rephrasing a quote from Laplace), “Extraordinary claims require extraordinary evidence”. I have no doubt that we will one day find life on another planet. My fear is that by then, we would have cried wolf so often that members of the jaded public might cease to care by then.

Somak Raychaudhury is vice-chancellor and professor of Physics, Ashoka University. The views expressed are personal