Published online by Cambridge University Press: 13 November 2008
The unexpected diversity of exoplanets includes a growing number of super Earth-planets, i.e. exoplanets with masses smaller than 10 Earth masses and a similar chemical and mineralogicalcomposition like Earth. We present a thermal evolution model for a super-Earth to calculate the sourcesand sinks of atmospheric carbon dioxide. The photosynthesis-sustaining habitable zone (pHZ) isdetermined by the limits of biological productivity on the planetary surface. We apply the model tocalculate the habitability of super-Earths in Gliese 581. The super-Earth Gl 581c is clearly outside thepHZ while Gl 581d at the outer edge of the pHZ could at least harbor some primitive life forms.