About this world
K2-21 c is a sub-Neptune orbiting K2-21, a cool red dwarf (type M0.0) 273 light-years from Earth. It is about 2.5 times the size of Earth. It completes an orbit every 15.5 days at 0.11 AU from its star, closer than Mercury is to the Sun, on a strongly elongated orbit (e = 0.21).
Its equilibrium temperature is about 420 K (147 °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 2015 by transit with K2. A planet crosses its star and dims it by a tiny, periodic fraction. K2-21 has 2 known planets; the others are K2-21 b.