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.
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.
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 CO2 concentration as well as orbital
distance and planetary mass are varied.
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, 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. 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.Comment: 5 pages, 4 figures, accepted in Astronomy&Astrophysic