K2-18b

K2-18b

Artist's impression of K2-18b (right) orbiting red dwarf K2-18 (left). The exoplanet K2-18c is shown between them.
Discovery
Discovery site	Kepler space telescope
Discovery date	2015
Detection method	Transit
Orbital characteristics[1]
Semi-major axis	0.15910+0.00046 −0.00047 au 21,380,000 km
Eccentricity	0.09+0.12 −0.09[2]
Orbital period (sidereal)	32.940045±0.000100 d
Star	K2-18
Physical characteristics
Mean radius	2.610±0.087 R🜨
Mass	8.63±1.35 M🜨
Mean density	2.67+0.52 −0.47 g/cm3
Surface gravity	12.43+2.17 −2.07 m/s2
Temperature	265 ± 5 K (−8 ± 5 °C)

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. This means it receives about a similar amount of starlight 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 attention to this system. In 2023, the JWST detected carbon dioxide and methane in the atmosphere of exoplanet K2-18b. JWST’s data has been variously interpreted as indicating a water ocean or a magma 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 a gas planet like Uranus or Neptune than Earth.