About this world
GJ 581 b is a Neptune-like planet orbiting GJ 581, a cool red dwarf (type M3.0V) 20.5 light-years from Earth. It is about 4.8 times the size of Earth (its size estimated from its mass) and holds 21 Earth masses. It completes an orbit every 5.37 days at 0.04 AU from its star, closer than Mercury is to the Sun.
Its equilibrium temperature is about 464 K (191 °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 2005 by radial velocity with La Silla Observatory. The star wobbles toward and away from us, Doppler-shifting its spectrum. GJ 581 has 3 known planets; the others are GJ 581 e and GJ 581 c.