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.

In 2019, the presence of water vapour in K2-18b's atmosphere was reported, drawing scientific attention to this system. In 2023, the JWST detected carbon dioxide and methane in the atmosphere of K2-18b. 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.

In 2025, the atmosphere of K2-18b was reported to contain dimethyl sulfide (DMS), a chemical that could serve as a biosignature on exoplanets, in quantities 20 times higher than on Earth. As the molecule is short-lived, the concentration is highly suggestive that DMS is being replenished. Ethan Siegel criticised this statement for its bold claims and flawed analysis, and other scientists pointed to lab experiments that can produce DMS without life.