About this world
K2-138 c is a sub-Neptune orbiting K2-138, a yellow, Sun-like star (type G8V) 661 light-years from Earth. It is about 2.3 times the size of Earth and holds 6.3 Earth masses. It completes an orbit every 3.56 days at 0.045 AU from its star, closer than Mercury is to the Sun.
Its equilibrium temperature is about 1,012 K (739 °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 2017 by transit with K2. A planet crosses its star and dims it by a tiny, periodic fraction. K2-138 has 6 known planets; the others are K2-138 b, K2-138 d, K2-138 e, K2-138 f and K2-138 g.