Atmospheric studies of habitability in the Gliese 581 system

¹ Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt, Rutherfordstr. 2, 12489 Berlin, Germany
e-mail: philip.vonparis@dlr.de
² Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany

Received: 3 November 2010
Accepted: 18 April 2011

Abstract

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 other planet candidates have been claimed, one of them in the habitable zone.

Aims. In view of the ongoing search for planets around M stars that is expected to result in numerous detections of potentially habitable super-Earths, we take the GL 581 system as an example for investigating 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 performed detailed atmospheric simulations for a much larger subset of potential planetary and atmospheric scenarios than previously considered.

Methods. A 1D radiative-convective atmosphere model was 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 CO₂ 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 atmospheric scenarios considered here, at an orbital distance of 0.117 AU. For an orbital distance of 0.146 AU, CO₂ 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 CO₂ concentrations of 5% and 95% were found to be habitable, so an additional super-Earth planet in the GL 581 system in the previously determined dynamical stability range would be considered a potentially habitable planet.