My friend Adam Crowl, a polymath if there ever was one, is working hard on Project Icarus and keeping an eye on the exoplanet situation. When you’re working on a starship design, no matter how theoretical, a major issue is the choice of targets, and the study of Kepler planets we looked at yesterday caught Adam’s eye some time ago. We’re not finding as many planets in the habitable zone thus far in the Kepler hunt as we might hope to, given that the ideal would be a habitable world somewhere within reach of near-future technologies of the kind that Icarus represents.

Sure, Kepler’s target stars are much further away in most cases, but the mission is giving us a useful statistical sampling from which we can generalize. Working with the data from Lisa Kaltenegger and Dimitri Sasselov’s paper, Adam thus takes a back-of-the-envelope stab at the galactic population of terrestrial worlds, knowing that Kepler is far from through, as we’re moving into the domain of planets with longer orbital periods. But if you had to go with what we have now, Adam estimates a population of ‘Earths’ somewhere in the range of 150 to 200 million in the Milky Way, assuming 100 billion stars for the galaxy, a figure that may be on the low side.

200 million habitable planets is a huge number even so, but if the total number goes down, so does the chance of finding one of these within, say, 100 light years of our Sun. We’re still shooting in the dark here, but we can only hope that the three ongoing searches of Centauri A and B will yield something interesting, or that we may find other exoplanet candidate stars hosting rocky worlds nearby. On that score, it’s intriguing to see that Gliese 581 d is back in the news, in the form of a new paper by Robin Wordsworth (Institut Pierre Simon Laplace, Paris) and colleagues.

Continuing Saga of a Nearby Red Dwarf

GJ 581 has been a major player in the exoplanet story since the 2007 announcement from the Geneva team of the potential habitability of GJ 581 c. Gliese 581 itself is an M-class red dwarf, smaller and cooler than our Sun, and a variety of studies have suggested that a habitable climate might be possible even on a world that, like GJ 581 c, is tidally locked to a star like this one, presenting the same face to the star throughout its orbit. Subsequent work, though, makes a strong case that GJ 581 c is more like Venus than anything else, while GJ 581 d, further out, may just make it inside the habitable zone, depending on conditions in its atmosphere.

Wordsworth and team are obviously aware of the discussion about the controversial detection of another planet, dubbed GJ 581 g, which appeared to be in the middle of the habitable zone. That one was announced in September of 2010, but its existence remains unconfirmed and in some cases strongly disputed. But GJ 581 d is a known quantity at least in some respects — it’s a ‘super-Earth’ with a minimum mass between 5.6 and 7.1 times that of Earth, and the question is whether a sufficiently dense atmosphere could give the planet a strong enough greenhouse effect for surface habitability, even though it receives 35 percent less energy from its star than does Mars.

The new paper uses global climate model simulations starting with the assumption that GJ 581 d’s climate is dominated by the greenhouse effects of CO2 and H2O, which is the case for the rocky planets with atmospheres in the Solar System. The model is put to work on a planet that may have a permanent night side, a place where volatiles like CO2 and H20 can freeze out on the surface. Even a relatively dense CO2 atmosphere can collapse under these conditions, so the simulations are designed to evaluate whether liquid water at the surface is possible.

The simulations used a global climate model developed specifically for exoplanet studies, assuming CO2 as the main atmospheric gas and drawing on these methods (from the paper):

To assess the influence of water on the climate independently, we considered two classes of initial condition: a rocky planet with no water, and an ocean planet, where the surface is treated as an infinite water source. CO2 was taken as the primary constituent of the atmosphere and H2O was allowed to vary freely, with surface ice / liquid and cloud formation (including radiative effects) taken into account for either gas when necessary. Restricting the composition of the atmosphere to two species in this way allows us to determine conservative conditions for habitability, as it neglects the warming due to other greenhouse gases like CH4 or buffer gases like N2 or Ar…

Greenhouse Warming and its Consequences

The researchers then performed simulations with 5, 10, 20 and 30 bar atmospheric pressure and varied the orbital resonances for both rocky and ocean planets. CO2 turns out to have a strong warming effect in these simulations, which showed that for pressures below about 10 bar, the atmosphere was unstable and began to condense on the poles and dark side of the planet. But denser atmospheres allowed enough heat transport and greenhouse warming to bring temperatures at the surface above the melting point of water, even as CO2 ice clouds frequently formed in the middle atmosphere, similar to the ice clouds we see on Mars.

So is it likely that CO2 partial pressures over 10 bar occur on GJ 581 d? We know nothing about the geophysics of this world, but if a mechanism exists there like the carbonate-silicate cycle we see on Earth, the atmospheric CO2 should stabilize at levels that allow oceans to exist. And of course there are other scenarios, as the authors point out: The planet could have a thick envelope of hydrogen and helium, like the atmospheres of Uranus and Neptune, or it could have no atmosphere at all — remember, GJ 581 is a red dwarf, a star whose frequent flare activity in its youth could have removed the early atmosphere. At this point, we just don’t know.

But the beauty of GJ 581 is that at 20 light years, it’s relatively near to the Earth, unlike most of the stars Kepler is now studying. That means that in the future, we’ll be able to discover which atmospheric scenario applies here by direct spectroscopic observations. The paper goes through the indicators that would help us distinguish between these scenarios, and it’s clear that this is a call we’ll one day be able to make as the technology comes online.

Thus the title of the paper by Wordsworth et al., “Gliese 581d is the first discovered terrestrial-mass exoplanet in the habitable zone,” accepted at Astrophysical Journal Letters (preprint). The caveat is obvious: The fact that a planet is in the habitable zone in some scenarios does not mean that it is actually habitable, but the new paper does tell us that this is a world that merits intense scrutiny, because oceans here are not beyond the realm of plausibility. If that’s the case, Project Icarus may be able to find at least one astrobiologically interesting planet within range of its vehicle after all.