About this world
HD 40307 g is a sub-Neptune orbiting HD 40307, an orange dwarf (type K2.5V) 42.2 light-years from Earth. It is about 2.6 times the size of Earth (its size estimated from its mass), with at least 7.1 Earth masses (a minimum, measured by radial velocity). It completes an orbit every 198 days at 0.6 AU from its star, on a strongly elongated orbit (e = 0.29).
Its equilibrium temperature is about 255 K (−18 °C), which places it in the conservative 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 2013 by radial velocity with La Silla Observatory. The star wobbles toward and away from us, Doppler-shifting its spectrum. HD 40307 has 5 known planets; the others are HD 40307 b, HD 40307 c, HD 40307 d and HD 40307 f.