About this world
K2-18 c is a sub-Neptune orbiting K2-18, a cool red dwarf (type M2.5V) 124 light-years from Earth. It is about 2.6 times the size of Earth (its size estimated from its mass), with at least 7.5 Earth masses (a minimum, measured by radial velocity). It completes an orbit every 8.96 days at 0.06 AU from its star, closer than Mercury is to the Sun, on a strongly elongated orbit (e = 0.47).
Its equilibrium temperature is about 363 K (90 °C): too cold for liquid water on its surface. At its size it almost certainly keeps a deep envelope of hydrogen and helium.
It was discovered in 2017 by radial velocity with La Silla Observatory. The star wobbles toward and away from us, Doppler-shifting its spectrum. K2-18 has 2 known planets; the others are K2-18 b.