Gliese 581d

Gliese 581d /ˈɡliːzə/ (often shortened to Gl 581d or GJ 581d) is a doubtful, and frequently disputed, exoplanet candidate orbiting within the Gliese 581 system, approximately 20.4 light-years away in the Libra constellation. It was the third planet claimed in the system and the fourth (in a 4-planet model) or fifth (in a disproven 5- or 6-planet model) in order from the star. Multiple subsequent studies found that the planetary signal in fact originates from stellar activity, and thus the planet does not exist, but this remains disputed.

Though significantly more massive than Earth (at a minimum mass of 6.98 Earth masses), this super-Earth was the first exoplanet of relatively low mass regarded as orbiting within the habitable zone of its parent star. Assuming its existence, computer climate simulations have confirmed the possibility of the existence of surface water and these factors combine to a relatively high measure of planetary habitability.