K2-18b

K2-18b, also known as EPIC 201912552 b, is an exoplanet orbiting the red dwarf K2-18, located 124 light-years (38 pc) away from Earth. The planet is a sub-Neptune about 2.6 times the radius of Earth, with a 33-day orbit within the star's habitable zone; it receives approximately a similar amount of light as the Earth receives from the Sun. Initially discovered with the Kepler space telescope, it was later observed by the James Webb Space Telescope (JWST) in order to study the planet's atmosphere.

JWST discovered water vapour, carbon dioxide and methane in its atmosphere. JWST's data has been variously interpreted as indicating a water ocean planet with a hydrogen-rich atmosphere, and a gas-rich mini-Neptune. K2-18b has been studied as a potential habitable world that, temperature aside, more closely resembles an ice giant like Uranus or Neptune than Earth. It is the prototype for hycean planets, planets which have abundant water under a hydrogen envelope.

A controversial discovery of dimethyl sulfide (DMS) was reported in 2025, a chemical that could serve as a biosignature on exoplanets. It has not been widely accepted as proof of extraterrestrial life, however, as its presence could be explained by abiotic chemical processes and there are doubts that the observations actually show the presence of DMS instead of other compounds or measurement artifacts.