New work on Gliese 581’s interesting planetary system may prove dismaying for those hoping for a planet in the habitable zone. With two ‘super-Earths’ and a Neptune class world, this is a system that cries out for close analysis. The Geneva team that detected the super-Earths had calculated surface temperatures on Gliese 581 c at roughly 20 degrees C. What they left out was the likely greenhouse effect of the atmosphere.
For habitability — defined here as the presence of liquid water at the surface — is not dependent on the central star alone, but also on the properties of the planets circling it. Werner von Bloh (Potsdam Institute for Climate Impact Research) and team tackle the habitability question in terms of atmosphere. From their paper:
…habitability is linked to the photosynthetic activity of the planet, which in turn depends on the planetary atmospheric CO2 concentration, and is thus strongly inﬂuenced by the planetary dynamics. In principle, this leads to additional spatial and temporal limitations of habitability, as the stellar HZ (deﬁned for a speciﬁc type of planet) becomes narrower with time due to the persistent decrease of the planetary atmospheric CO2 concentration.
From this, the team studies habitable zone limitations for super-Earths, using a thermal evolution model that takes into account atmospheric carbon dioxide and varying ratios between water and land surfaces. The results: Both super-Earths in the Gliese 581 system are inside the tidal locking radius, keeping one face turned toward the star at all times. Gliese 581 c, the source of so much press speculation about terrestrial worlds, turns out to be far too hot to support life. Even scaling for stellar luminosity, it’s closer to its star than Venus is to ours.
But we’re not quite through. Interestingly, Gliese 581 d, a super-Earth of roughly eight Earth masses, could well have built up a dense atmosphere. With at least some of the climate models and carbon dioxide pressures assumed, the planet nudges inside the habitable zone and could be habitable for a period as long as 7.2 billion years. From the paper:
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. Williams & Pollard (2002) concluded that a planet with a suffciently dense atmosphere could harbour life even if its orbit is temporarily outside the HZ. In conclusion, one might expect that life may have originated on Gl 581d.
The authors go on to add this caveat: Complex life is unlikely due to what they describe as ‘rather adverse environmental conditions.’ But of course we won’t know until we can get a look at potential biomarkers in the atmospheres of both these super-Earths. That will have to await later space missions like ESA’s Darwin and whatever mission grows out of the Terrestrial Planet Finder research. Given uncertainties of funding and ongoing technological reassessment, we may not have a definitive answer on the Gliese system for quite some time.
Thanks to Andy for the pointer to this one, and congratulations as well. Read through the comments on our first story on Gliese 581 c and you’ll see that Andy came up with remarkably similar conclusions not long after the story broke. Nice work indeed! The paper is von Bloh et al., “The habitability of super-Earths in Gliese 581,” available online.