A blogger who identifies himself as a "college professor teaching physics and astronomy" calls attention to how Gliese 581c may not be "habitable" in the ways we expect them to be. He notes that "we really don't know how planet form around red dwarf stars... We can't say much of anything about the composition of the planet. so, how can we call it Earth-like?" He also points that with a mass 5 times Earth's, and a radius 1.5 times Earth's, acceleration due to gravity would be roughly twice as on Earth, which "would make the planet not very comfortable to life forms like us."
On the matter of the temperature and climate, he notes:
But, there is a more serious concern... This planet, Gliese 581c, is indeed close: about 11 million kilometers from the star. It takes the planet only about 12.9 days to orbit the star. At that distance, it may well be that the planet is tidally locked to the star. That would mean that one side of the planet perpetually faces the star, much like one side of the Moon perpetually faces Earth. In that case, habitability becomes problematic. The side facing the star will be baked. The star will never set. The temperature will rise higher and higher. My back-of-the-envelope calculations suggest a sub-stellar temperature possibly up to 200° C (possibly even much hotter), but certainly well above the boiling point of water. The nighttime temperature would be always well below freezing. Only a tiny zone near the terminator would have any hope of habitability, but even that would be tenuous, and it would be likely wracked by storms of unbelievable intensity... The other possibility might be a coupling with the much larger planet that orbits just a little closer to the star than Gliese 581c. If it were to couple to that planet, then it might rotate fast enough that some hope of habitability might exist. Of course, it still would likely have nothing approximating an Earth climate.
http://astroprofspage.com/archives/889
He concludes though, that planet 581c does "bring hope of finding others that are even more of match for what we expect a habitable planet to be like."
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