Free Astronomy Magazine July-August 2025

10 JULY-AUGUST 2025 ASTRO PUBLISHING Arguments in favor of life on K2-18 b “Extraordinary claims require ex- traordinary evidence,” said Carl Sagan. This rule also applies to the claims of Madhusudhan and col- leagues. Let’s see if there is extraor- dinary evidence. The most concrete argument is linked to seeing the DMS (or DMDS) signal persist and even strengthen by changing spectrographs and ob- serving different wavelengths dur- ing the observations. It is unlikely that a spurious signal would reap- pear after more than a year with dif- ferent investigation conditions. A second promising aspect has to do with a recent photochemical model- ing of K2-18 b (S.-M. Tsai et al., 2024), which suggests that signifi- cant quantities of DMS and DMDS can accumulate in the atmosphere, up to levels several tens of times higher than those on Earth. Such high values would seem consistent with the data extrapolated by Mad- husudhan’s team. A third argument in favor derives from the limited knowledge we have about the possible abiotic gen- esis of DMS. As far as we know, there do not appear to be alterna- tive processes to the biotic one capa- ble of producing the abundance and concentration of DMS that re- searchers claim to have recognized in the atmosphere of K2-18 b. It is true that a recent analysis of the data collected by the Rosetta probe on comet 67P/Churyumov–Gerasi- menko has highlighted the presence of DMS in the gas jets, suggesting the possibility of a potential abiotic source for that biosignature. How- ever, the quantity and current fre- quency of cometary impacts nec- essary to account for the presumed levels of DMS on K2-18 b make this an untenable solution. The Cam- bridge researchers’ interpretation is also supported by the fact that if K2-18 b were to be a hycean, this planetary model would be most con- sistent with the scenario proposed by the team. Arguments against life on K2-18 b It is almost impossible to list in a few lines all the arguments that should scale down the optimism of Mad- husudhan and colleagues. We will limit ourselves to mentioning the less questionable ones. The first concerns the Webb Space Telescope, which for several analysts (such as, for example, the well- known planetary scientist Sara Sea- ger of MIT) simply does not include all of the technology necessary to in- controvertibly demonstrate the exis- tence of life on planets other than Earth. In principle, Webb can detect biosignatures through transmission spectroscopy, but the extrapolation of that data is complicated by our poor collective knowledge of exoat- mospheres and by the pollution of the signal due to starlight. Each spectrum represents a highly medi- ated signal of chemical and physical atmospheric processes that manifest themselves in three dimensions but which, upon observation, are con- centrated in a single point source that includes only star and planet. A second critical aspect concerns the very nature of K2-18 b. The idea that it could be a hycean was put forward when the clear trace of methane detected by Hubble was mistakenly attributed to water va- por. Today, we know that water is not present in any form in the upper atmosphere; if it is on the surface, and therefore K2-18 b is a hycean, the existence of surface water in any amount is a discovery that remains to be made. Furthermore, even if an ocean existed beneath the thick at- T he K2-18 system, with planet “b” in the foreground and planet “c” transiting in front of the disk of the red dwarf. [ESO/M. Kornmesser/N. Risinger (skysurvey.org )] mospheric blanket, its waters might be subjected to conditions so ex- treme as to be uninhabitable. Another, even more robust argu- ment against the possibility of life on K2-18 b has to do with its star. As already mentioned, it is a red dwarf, a class of stars characterized by very violent magnetic activity, capable of generating powerful flares even sev- eral times over the course of an Earth day. The flares launch intense flows of radiation into space in the form of a stellar wind, potentially shattering the atmospheres of the unfortunate planets. Given the small masses of these stars, their orbiting planets always find themselves within very short distance of these intense radiation flows. The most ac- tive red dwarfs are the youngest ones, with ages of a few billion years: K2-18 is about 3 billion years old, compared to a “life expectancy” of trillions of years. It is therefore fully active, and the phenomena that affect it can contaminate the spectra collected by telescopes, as happens, for example, with the TRAPPIST-1 system. Furthermore, even leaving aside the devastating effects of stellar activity, we know that red dwarf planets are often gravitationally forced to always turn the same hemisphere towards the star. This is not an optimal condition for life as we know it (but if it pro- duces DMS, it can’t be very different from what we know.) A further aspect of the issue that clashes with the conclusions of Mad- husudhan’s team is the presumed universal validity of DMS as a biosig- nature. While it is true that on K2-18 b it can hardly have been conferred by countless comets, it is equally true