Gliese 581 d ( /ˈɡliːzə/) or Gl 581 d is a proposed extrasolar planet orbiting the star Gliese 581 approximately 20 light-years away in the constellation of Libra. It was the third planet claimed in the system and (assuming a six-planet model) the fifth in order from the star.
Though not confirmed to be a terrestrial planet and significantly more massive than Earth (at 3.1 Earth masses), the Super-Earth is the first exoplanet of terrestrial mass proposed to orbit 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. It would have an Earth Similarity Index of 0.74.
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