Context : The M-type star Gliese 581 is orbited by at least one terrestrial planet candidate in the habitable zone , i.e .
GL 581 d. Orbital simulations have shown that additional planets inside the habitable zone of GL 581 would be dynamically stable .
Recently , two further planet candidates have been claimed , one of them in the habitable zone .

Aims : In view of the ongoing search for planets around M stars which is expected to result in numerous detections of potentially habitable Super-Earths , we take the GL 581 system as an example to investigate such planets .
In contrast to previous studies of habitability in the GL 581 system , we use a consistent atmospheric model to assess surface conditions and habitability .
Furthermore , we perform detailed atmospheric simulations for a much larger subset of potential planetary and atmospheric scenarios than previously considered .

Methods : A 1D radiative-convective atmosphere model is used to calculate temperature and pressure profiles of model atmospheres , which we assumed to be composed of molecular nitrogen , water , and carbon dioxide .
In these calculations , key parameters such as surface pressure and CO _ { 2 } concentration as well as orbital distance and planetary mass are varied .

Results : Results imply that surface temperatures above freezing could be obtained , independent of the here considered atmospheric scenarios , at an orbital distance of 0.117 AU .
For an orbital distance of 0.146 AU , CO _ { 2 } concentrations as low as 10 times the present Earth ’ s value are sufficient to warm the surface above the freezing point of water .
At 0.175 AU , only scenarios with CO _ { 2 } concentrations of 5 % and 95 % were found to be habitable .
Hence , an additional Super-Earth planet in the GL 581 system in the previously determined dynamical stability range would be considered a potentially habitable planet .

Conclusions :