About this world
K2-21 b is a super-Earth orbiting K2-21, a cool red dwarf (type M0.0) 273 light-years from Earth. It is about 1.8 times the size of Earth. It completes an orbit every 9.33 days at 0.076 AU from its star, closer than Mercury is to the Sun.
Its equilibrium temperature is about 500 K (227 °C): far too hot for liquid water on its surface. By the "cosmic shoreline", it has likely held on to an atmosphere.
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 c.