About this world
GJ 3138 d is a sub-Neptune orbiting GJ 3138, a cool red dwarf (type M0) 92.9 light-years from Earth. It is about 3.2 times the size of Earth (its size estimated from its mass), with at least 11 Earth masses (a minimum, measured by radial velocity). It completes an orbit every 258 days at 0.7 AU from its star, on a strongly elongated orbit (e = 0.32).
Its equilibrium temperature is about 152 K (−121 °C): too cold 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 radial velocity with La Silla Observatory. The star wobbles toward and away from us, Doppler-shifting its spectrum. GJ 3138 has 3 known planets; the others are GJ 3138 b and GJ 3138 c.