About this world
K2-251 b is a sub-Neptune orbiting K2-251, a cool red dwarf 480 light-years from Earth. It is about 2.4 times the size of Earth. It completes an orbit every 9.3 days at 0.069 AU from its star, closer than Mercury is to the Sun.
Its equilibrium temperature is about 437 K (164 °C): far too hot for liquid water on its surface. At its size it almost certainly keeps a deep envelope of hydrogen and helium.
It was discovered in 2018 by transit with K2. A planet crosses its star and dims it by a tiny, periodic fraction.