About this world
K2-9 b is a sub-Neptune orbiting K2-9, a cool red dwarf (type M2.5V) 271 light-years from Earth. It is about 2.3 times the size of Earth. It completes an orbit every 18.4 days at 0.091 AU from its star, closer than Mercury is to the Sun.
Its equilibrium temperature is about 314 K (41 °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.
It was discovered in 2015 by transit with K2. A planet crosses its star and dims it by a tiny, periodic fraction.