Habitable planets around the star Gl 581?

TL;DR

This paper evaluates the habitability of exoplanets Gl 581c and d around star Gl 581, using atmospheric models and habitable zone boundaries, suggesting planet d might be potentially habitable due to CO2-ice cloud effects.

Contribution

It provides simplified formulas for habitable zone limits and assesses the habitability of Gl 581c and d considering atmospheric uncertainties.

Findings

Gl 581c is near but outside the conservative habitable zone.

Gl 581d's conditions are similar to early Mars, with potential habitability.

CO2-ice clouds could enable habitability on planet d.

Abstract

Radial velocity surveys are now able to detect terrestrial planets at habitable distance from M-type stars. Recently, two planets with minimum masses below 10 Earth masses were reported in a triple system around the M-type star Gliese 581. Using results from atmospheric models and constraints from the evolution of Venus and Mars, we assess the habitability of planets Gl 581c and Gl 581d and we discuss the uncertainties affecting the habitable zone (HZ) boundaries determination. We provide simplified formulae to estimate the HZ limits that may be used to evaluate the astrobiological potential of terrestrial exoplanets that will hopefully be discovered in the near future. Planets Gl 581c and 'd' are near, but outside, what can be considered as the conservative HZ. Planet 'c' receives 30% more energy from its star than Venus from the Sun, with an increased radiative forcing caused by the spectral energy distribution of Gl 581. Its habitability cannot however be positively ruled out by theoretical models due to uncertainties affecting cloud properties. Irradiation conditions of planet 'd' are comparable with those of early Mars. Thanks to the warming effect of CO2-ice clouds planet 'd' might be a better candidate for the first exoplanet known to be potentially habitable. A mixture of various greenhouse gases could also maintain habitable conditions on this planet.