About this world
GJ 3998 d is a sub-Neptune orbiting GJ 3998, a cool red dwarf (type M1) 59.2 light-years from Earth. It is about 2.3 times the size of Earth (its size estimated from its mass), with at least 6.1 Earth masses (a minimum, measured by radial velocity). It completes an orbit every 41.8 days at 0.19 AU from its star, closer than Mercury is to the Sun.
Its equilibrium temperature is about 292 K (19 °C), which places it in the optimistic habitable zone: the band where liquid water could exist on a rocky surface. At its size it almost certainly keeps a deep envelope of hydrogen and helium.
It was discovered in 2025 by radial velocity with Multiple Observatories. 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 c.