This prevents the atmosphere from freezing out due to tidal locking. The team has looked at various climate models for Gl 581d, finding two of them promising:Ī planet with eight Earth masses has more volatiles than an Earth size planet to build up such a dense atmosphere. Here’s von Bloh’s conclusion, after ruling out Gliese 581c for habitability as being too close to its star. Too old a planet might not release enough CO 2 to keep the surface above the freezing point of water. Interestingly, von Bloh’s work depends on the age of the planet, looking at the balance between sources of atmospheric CO 2, released through ridges and volcanoes, and CO 2 sinks, which consume the gas through weathering processes. Werner von Bloh (PIK, Germany) and team examine a more truncated HZ where photosynthesis could be possible. So how big is the actual habitable zone around Gl 581? Franck Selsis (CRAL and LAB, France) and team look at the HZ in terms of liquid water at the surface, coping with the difficult problem of cloud modeling that makes precise determinations so difficult. It’s going to be a while before we have any definitive answers, but current thinking is that habitable M dwarf planets are very much in the picture, with the inner two Gliese 581 worlds due for a look from future missions like Darwin. But models of atmospheric circulation exist that overcome that obstacle, and the intense magnetic activity of early M star life (producing dangerous flares) is no longer considered a necessary disqualifier for all forms of life.

M dwarfs have inherent problems in terms of habitability, not the least of which is the tidal lock that planets in the HZ of such stars presumably experience, keeping one side perpetually dark. Of five and eight Earth masses respectively, they are the first exoplanets ever considered serious candidates for habitability. Gliese 581 c and d are noteworthy every time they’re mentioned. Astronomy & Astrophysics is publishing two independent studies of the system asserting that at least one of the inner planets is indeed located within the habitable zone of that star. Gliese 581 is back in the news with a flourish.

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