K2-18b

K2-18b, extrasolar planet for which, on the basis of a possible detection of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS), the possible presence of life was claimed in 2025.

K2-18b was discovered in 2015 by the Kepler satellite during its K2 mission phase, when the satellite operated with only two of its four gyroscopes. The planet circles the red dwarf star K2-18, which has a spectral type M2.5 V and is 124 light-years from Earth. K2-18b orbits its star every 32.94 days at a distance of 21.4 million km (13.3 million miles). The planet has a mass 8.63 times that of Earth and a radius 2.61 times that of Earth. Its density is therefore 2.67 grams per cubic centimeter (g/cm³), between that of Earth (5.51 g/cm³) and Neptune (1.64 g/cm³). K2-18b is a possible Hycean world, a planet with most of its mass as a watery ocean surrounded by a hydrogen-helium atmosphere. (The word Hycean is a portmanteau of hydrogen and ocean.) K2-18b also orbits in its star’s habitable zone, the region around a star in which an Earth-like planet can possess liquid water on its surface and possibly support life.

Indian British astronomer Nikku Madhusudhan and his collaborators used the James Webb Space Telescope (JWST) to observe K2-18b’s atmosphere as the planet passed in front of its star and in 2023 published results showing the presence of methane (CH₄) and carbon dioxide (CO₂), as well as a possible detection of DMS (C₂H₆S). Subsequent observations published in 2025 by Madhusudhan and his collaborators showed that either DMS or DMDS (CH₃SSCH₃) or a DMS/DMDS mix was possibly present in the planet’s atmosphere.

On Earth, DMS is mostly produced by phytoplankton, small organisms found in the oceans. (DMS is the gas that gives beach areas their distinctive smell.) DMDS is produced by bacteria. Both molecules have very short lifetimes in Earth’s atmosphere and are only present because of their continual replenishment by phytoplankton. The claimed detections of DMS and DMDS would have had much higher concentrations in K2-18b’s atmosphere than on Earth, and Madhusudhan and his collaborators wrote that sustaining such high concentrations “would be implausible without a significant biogenic flux.”

Such a claim was, of course, controversial. For example, objections were raised that DMS has been found both in a comet and in the interstellar medium, showing it can be formed in lifeless environments. For their part, Madhusudhan and his collaborators admitted that the statistical significance of their detection of DMS and DMDS “is at the lower end of robustness typically required for scientific evidence” and that further work would be needed to show that the DMS/DMDS signal is an unambiguous sign of life.

Subsequent observations of K2-18b with the JWST published by astronomer Renyu Hu and collaborators in 2025 found that the planet was rich in water but that the possible detection of DMS was not statistically significant. Furthermore, their models of K2-18b’s atmosphere found that DMS could be formed there without life.