Free Astronomy Magazine July-August 2025

JULY-AUGUST 2025 that an abiotic origin cannot be ruled out. DMS was produced in the laboratory by simulating the atmos- phere of the primordial Earth with- out the need for living organisms being present (N.W. Reed et al., 2024), and it has also been discov- ered in an interstellar molecular cloud thanks to two radio telescopes (M. Sanz-Novo et al., 2025), an envi- ronment not really suitable for life, however one wants to imagine it. These recent works cast doubt on the reliability of DMS (and DMDS) as a strong biosignature. K2-18 b is not well enough known to establish whether the DMS perhaps detected in its atmosphere was produced by life forms. It is also not known whether in an atmosphere of that type, rich in hydrogen and meth- ane, the lifetime of that molecule is comparable to that recorded in the Earth’s atmosphere. Ultimately, what is a valid biosignature on Earth may not be so elsewhere. As if the above were not enough, we add that if the origin of the DMS on K2-18 b was biotic, we should si- multaneously have detected other compounds and by-products deriv- ing from the decomposition of the raw materials used to produce it, such as H 2 S (hydrogen sulfide), but there is no trace of that chemical corollary. If it were true, it would be incredible There are many good reasons not to believe in the biotic origin of the DMS on K2-18 b, but however small the chances that microalgae or sim- ilar organisms actually exist on that planet, those chances are not zero percent. We can, as one possibility, consider this chance of life in the context of the “mediocrity prin- ciple,” which proposes that the properties of a randomly-selected sample are likely to represent prop- erties of the larger set of samples. Having “neighbors” just 124 light years away might reasonably lead to the conclusion that the Milky Way is overflowing with life. Within a radius of 124 years around the Earth, there are about 10,000 stars (some say half, some say double). Since we do not live in a particularly privileged region, we can assume that if there are at least two planets for every 10,000 stars on which life thrives, then in the entire Milky Way, which includes about 200 billion stars (some say half, some say dou- ble), life should exist on about 40 million planets, or on about 10 mil- lion if we adopt the most unfavor- able parameters for the projection. This would not be an overly opti- mistic scenario because we know that the vast majority of stars have a planetary system, and we also know that sub-Neptunes like K2-18 b are the most numerous exoplanets discovered so far in the galaxy. So the Milky Way is teeming with life? Sixty-five years of SETI research tells us that reality can hardly be this in terms of intelligent life. The ob- servations with the JWST only let us now begin to develop models to ex- plain the reality of exoplanetary at- mospheres and conditions. Confi- dent assessment of merely the plau- sible existence of life on an exo- planet based on these observations will only come with many years of hypotheses, model development, and critical scientific review. !