About this world
GJ 3998 c is a sub-Neptune orbiting GJ 3998, a cool red dwarf (type M1) 59.2 light-years from Earth. It is about 2.5 times the size of Earth (its size estimated from its mass), with at least 6.8 Earth masses (a minimum, measured by radial velocity). It completes an orbit every 13.7 days at 0.09 AU from its star, closer than Mercury is to the Sun.
Its equilibrium temperature is about 423 K (150 °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 2016 by radial velocity with Roque de los Muchachos Observatory. The star wobbles toward and away from us, Doppler-shifting its spectrum. GJ 3998 has 3 known planets; the others are GJ 3998 b and GJ 3998 d.