Gliese 581 is back in the news with a flourish. Astronomy & Astrophysics is publishing two independent studies of the system asserting that at least one of the inner planets is indeed located within the habitable zone of that star. Gliese 581 c and d are noteworthy every time they’re mentioned. Of five and eight Earth masses respectively, they are the first exoplanets ever considered serious candidates for habitability.
M dwarfs have inherent problems in terms of habitability, not the least of which is the tidal lock that planets in the HZ of such stars presumably experience, keeping one side perpetually dark. But models of atmospheric circulation exist that overcome that obstacle, and the intense magnetic activity of early M star life (producing dangerous flares) is no longer considered a necessary disqualifier for all forms of life. It’s going to be a while before we have any definitive answers, but current thinking is that habitable M dwarf planets are very much in the picture, with the inner two Gliese 581 worlds due for a look from future missions like Darwin.
So how big is the actual habitable zone around Gl 581? Franck Selsis (CRAL and LAB, France) and team look at the HZ in terms of liquid water at the surface, coping with the difficult problem of cloud modeling that makes precise determinations so difficult. Werner von Bloh (PIK, Germany) and team examine a more truncated HZ where photosynthesis could be possible. Interestingly, von Bloh’s work depends on the age of the planet, looking at the balance between sources of atmospheric CO2, released through ridges and volcanoes, and CO2 sinks, which consume the gas through weathering processes. Too old a planet might not release enough CO2 to keep the surface above the freezing point of water.
Here’s von Bloh’s conclusion, after ruling out Gliese 581c for habitability as being too close to its star. The team has looked at various climate models for Gl 581d, finding two of them promising:
A planet with eight Earth masses has more volatiles than an Earth size planet to build up such a dense atmosphere. This prevents the atmosphere from freezing out due to tidal locking. In case of an eccentric orbit of Gl 581d (e = 0.2), the planet is habitable for the entire luminosity range considered in this study, even if the maximum CO2 pressure is assumed as low as 5 bar. In conclusion, one might expect that life may have originated on Gl 581d. The appearance of complex life, however, is unlikely due to the rather adverse environmental conditions. To get an ultimate answer to the profound question of life on Gl 581d, we have to await future space missions such as the TPF/Darwin. They will allow for the ﬁrst time to attempt the detection of biomarkers… in the atmospheres of the two super-Earths around Gl 581.
And this from the Selsis paper:
Darwin/TPF-I and TPF-C could eventually reveal what the actual properties of the atmosphere of Gl 581c and Gl 581d are. From their thermal light curves we could infer if a thick atmosphere is making the climate more or less uniform on both the day and night hemispheres of these planets, despite a (nearly?) synchronized rotation… Visible and mid-IR water vapor bands could be searched in the atmosphere of Gl 581d to conﬁrm its habitability. Mid-IR spectra of this planet could also reveal other greenhouse gases at work. Spectral observations of Gl 581c could potentially distinguish between a Venus-like atmosphere dominated by CO2 or an H2O-rich atmosphere. The detection of O2 on this planet would generate a fascinating debate about its possible origin: as either a leftover of H2O photolysis and H escape or a biological release. There is certainly no doubt that Gl 581c and Gl 581d are prime targets for exoplanet characterization missions.
Image (click to enlarge): Illustration of the habitable zone (HZ) boundaries as obtained by the two teams. The upper part of the figure shows the HZ of the Sun (at its present age). The red curve shows only the most extreme outer limit of the HZ. The actual outer boundary is indeed located somewhere between 1.7 and 2.4 AU. The green limits show the boundaries of the photosynthetic zone as computed with the model by von Bloh et al. The middle part of the figure shows the limits of the HZ of Gliese 581 computed with the atmospheric models from Selsis et al. The lower part illustrates the boundaries of the photosynthetic zone computed with the geophysical models from von Bloh et al. The boundaries are shown for several possible ages (5, 7, and 9 Gyr-old) of the Gliese 581 planetary system. Following the latest estimation, Gliese 581 would be 7 Gyr-old. The purple bars surrounding planets Gliese 581 c and d illustrate the variable distance to the star caused by the eccentricity of the orbits. Credit: Astronomy & Astrophysics.
Note that the Selsis paper does not rule out GL 581c’s possible habitability, although noting that the world receives thirty percent more energy from its star than Venus does from the Sun. Uncertainties in cloud properties make the final call problematic. Like von Bloh’s team, however, Selsis leans toward the possibility of CO2 ice clouds creating much better conditions for life on GL 581d, with the potential for greenhouse gases to maintain habitable conditions over long periods of time.
The papers are von Bloh et al., “The habitability of super-Earths in Gliese 581” (abstract) and Selsis et al., “Habitable planets around the star Gliese 581?” (abstract), appearing in Astronomy & Astrophysics Vol 476-3 (2007), pp. 1365-1387. Note that a third paper, to be published in 2008 in the same journal, will examine the dynamical stability of the Gl 581 planetary system. That paper, by Hervé Beust (LAOG France) and team, will conclude that the GL 581 system appears dynamically stable, thus rendering the climate on both these worlds potentially stable as well.