About this world
Kepler-391 c is a sub-Neptune orbiting Kepler-391, an orange dwarf (type K2III-I) 2.84 thousand light-years from Earth. It is about 2.9 times the size of Earth and holds 450 Earth masses. It completes an orbit every 20.5 days at 0.16 AU from its star, closer than Mercury is to the Sun, on a strongly elongated orbit (e = 0.27).
Its equilibrium temperature is about 662 K (389 °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 2014 by transit with Kepler. A planet crosses its star and dims it by a tiny, periodic fraction. Kepler-391 has 2 known planets; the others are Kepler-391 b.