About this world
Kepler-138 d is a super-Earth orbiting Kepler-138, a cool red dwarf (type M1V) 218 light-years from Earth. It is about 1.5 times the size of Earth and holds 2.1 Earth masses. It completes an orbit every 23.1 days at 0.13 AU from its star, closer than Mercury is to the Sun.
Its equilibrium temperature is about 345 K (72 °C): too cold for liquid water on its surface. By the "cosmic shoreline", its star has probably stripped its atmosphere away, leaving bare rock. Its atmosphere has already been studied in one spectroscopic observation.
It was discovered in 2014 by transit with Kepler. A planet crosses its star and dims it by a tiny, periodic fraction. Kepler-138 has 4 known planets; the others are Kepler-138 b, Kepler-138 c and Kepler-138 e.