Gliese 581 continues to occupy the attention, and understandably so. At least three planets orbit this M-dwarf, one of which sprang into the public consciousness with the announcement that it might be in its star’s habitable zone. But both Gl 581c and d are interesting from the habitability standpoint, even if subsequent discussions have pointed out just how problematic it is to make such judgments on insufficient data.

Ponder how tricky the call can be. For being in a circumstellar habitable zone only means that a terrestrial-size planet can have liquid water on its surface. A new paper by Franck Selsis (Centre de Recherche Astrophysique de Lyon), James Kasting (Pennsylvania State) and colleagues wades right into this morass, pointing out how many other factors could make such a planet remain uninhabitable:

  • Water may not be available
  • A high impact rate may prevent the emergence of life
  • The thus far unknown minimum ingredients for life’s formation may not be present
  • Gravity may be too low to retain a dense atmosphere
  • The planet may have formed an atmosphere that keeps the surface pressure too high for water to remain liquid

And so on. The point being that the habitable zone is no guarantee of life. For that matter, we have a long way to go in understanding how to put all these factors together. The Selsis/Kasting paper, already under discussion in various messages here, sees Gl 581c as definitely problematic but not completely out of the life-forming picture. While it’s unlikely to have liquid water, the uncertainties involved in its cloud properties and cloud cover leave the question at least slightly open. From the paper:

…Gl 581c would be habitable only if clouds with the highest re?ectivity covered most of the daytime hemisphere. A 50% cloud cover is not sufficient to prevent a runaway greenhouse effect on Gl 581c, which receives 30% more energy than Venus today. This problem is exacerbated by the fact that Venus has a much higher albedo than the expected value for a habitable planet at the orbital distance of Gl 581c. The composition of the atmosphere of Gl 581c depends on the mass of the initial water reservoir on the planet, and on the efficiency of the gravitational escape of H.

The paper goes on to describe possible scenarios, including one involving water forming “…a mantle of hot and high-pressure ice underneath a ?uid envelope of supercritical H2O,” as well as a Venus-like scenario leaving a CO2 rich atmosphere with surface temperatures as high as 1000 K. But we’re not through — change the models around and things get much iffier. The authors note, for example, how much depends on the carbonate-silicate cycle, which stabilizes surface temperature and the amount of CO2 in the atmosphere. Even at 1 AU, Earth itself would not be close enough to the Sun to maintain water above the freezing point without enough atmospheric CO2.

What happens if we do away with the greenhouse effect on Gl 581c, or take albedo to the extreme? How about geochemical processes and their effect on whatever atmosphere exists there? Dig into this paper for the speculative details. With so many questions, it’s clear how much we need by way of further data before we can make any calls. The authors put it this way:

Because of the uncertainties in the precise location of the HZ boundaries, planets at the edge of what is thought to be the HZ are crucial targets for future observatories able to characterize their atmosphere. At the moment, our theory of habitability is only con?rmed by the divergent fates of Venus and the Earth. We will have to confront our models with actual observations to better understand what makes a planet habitable. The current diversity of exoplanets (planets around pulsars, hot Jupiters, hot Neptunes, super-Earths…) has already taught us that Nature has a lot more imagination when building a variety of worlds than we expected from our former models inspired by the Solar System.

Amen to that. And as for Gl 581d, it’s conceivable that we’re looking at an early Mars scenario, a situation thought to have involved plentiful water at the surface. Add a greenhouse effect from CO2 ice clouds and planet d might just be the best bet for habitability in this odd and fascinating system. But here again, we don’t know enough about the geochemical processes involved that could stabilize such an atmosphere. A mission like Darwin or whatever emerges from the US equivalent could tell us much about the atmospheres of both worlds, searching in particular for water vapor bands in the atmosphere of Gl 581d, and on Gl 581c distinguishing between a CO2 atmosphere and an H2O rich alternative.

Prime targets for follow-up work with next generation space technology? You bet. The paper is Selsis et al., “Habitable planets around the star Gl 581?” accepted for publication in Astronomy and Astrophysics, available online in preprint form and, as one of our readers has already noted, as absorbing and thorough a take on these distant worlds as we’ve yet seen.