About this world
K2-18 b is a sub-Neptune orbiting K2-18, a cool red dwarf (type M2.5V) 124 light-years from Earth. It is about 2.4 times the size of Earth and holds 8.9 Earth masses. It completes an orbit every 32.9 days at 0.14 AU from its star, closer than Mercury is to the Sun.
Its equilibrium temperature is about 284 K (11 °C), which places it in the optimistic habitable zone: the band where liquid water could exist on a rocky surface. At its size it almost certainly keeps a deep envelope of hydrogen and helium. Its atmosphere has already been studied: 10 spectroscopic observations, all of them with the James Webb Space Telescope.
It was discovered in 2015 by transit with K2. A planet crosses its star and dims it by a tiny, periodic fraction. K2-18 has 2 known planets; the others are K2-18 c.