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Aims: The planetary system around the M star Gliese 581 consists of a hot Neptune (Gl 581b) and two super-Earths (Gl 581c and Gl 581d). The habitability of this system with respect to the super-Earths is investigated following a concept that studies the long-term possibility of photosynthetic biomass production on a dynamically active planet. Methods: A thermal evolution model for a super-Earth is used to calculate the sources and sinks of atmospheric carbon dioxide. The habitable zone is determined by the limits of biological productivity on the planetary surface. Models with different ratios of land / ocean coverage are investigated. Results: The super-Earth Gl 581c is clearly outside the habitable zone, since it is too close to the star. In contrast, Gl 581d is a tidally locked habitable super-Earth near the outer edge of the habitable zone. Despite the adverse conditions on this planet, at least some primitive forms of life may be able to exist on its surface.

Don. Edwards wrote:It sounds to me like Gliese 581D is a perfect candidate for being a Panthalassic world. Almost all its outer layers are liquid water. It posible that C is also such a world but much hotter. So in the future we may never walk its surface but a floating city might work, and who knows what may down in the depths.

Don. Edwards

drsimmo wrote:Gliese 581 d is not what I would call the most robust of results. That is, I think it is a marginal detection at best...