IN DECEMBER, a pair calling themselves “The Benevolent Fisted Rulers” offered up 4-hectare plots of Gliese 581 g, the most habitable exoplanet yet discovered, for sale on eBay.

Setting aside the ethics of exoplanetary land grabs, the move seems a touch premature. The alien world is 20 light years away and its very existence is not confirmed. Still, if the planet does exist, it is possible that it has some good exo-real estate.

Raymond Pierrehumbert at the University of Chicago examined the range of climates that Gliese 581 g might have and found one that would have a pool of water on one side, making it look like an eyeball. Even if further observations disprove the existence of Gliese 581 g, the work could help determine the habitability of exo-Earths still to be discovered.

First spotted in September via wobbles in the light emitted by its host star, Gliese 581 g is likely to be rocky. That, combined with the fact that it orbits the star at just the right, “Goldilocks” distance to provide the warmth needed for liquid water, made it the first planet discovered outside our solar system with the potential to host life.


But Gliese 581 g also differs from Earth. For a start, it orbits its dim, red dwarf star so closely that its year lasts just 37 days. This boosts the chance that its star’s gravitational tug has caused the planet to spin at the same rate that it moves around the star, so that one side of the planet always faces the star and the other is always dark. Pierrehumbert assumed that the planet is indeed locked in this configuration when he looked at the various climates it might support.

One option is that the planet has no atmosphere. Although all water on the surface would remain frozen, at least there would be no gas to transport heat away from the starlit side. This could keep temperatures there high enough to thin the ice, allowing light that could support life to reach liquid water beneath the surface.

The scenario most likely to support life, though, was one with an atmosphere containing carbon dioxide. A greenhouse effect would heat mainly the region directly facing the host star, producing an ice-covered planet with a large area of liquid water – the “eyeball Earth” (The Astrophysical Journal Letters, DOI: 10.1088/2041-8205/726/1/L8).

“The most habitable case is where you have enough CO 2 to actually have this open water swimming pool,” Pierrehumbert says. The pool’s diameter would be about a quarter of the planet’s circumference (see diagram). It would arise if the atmosphere contained about 20 per cent CO 2 – much higher than Earth but within the limits imposed by the carbon available to form planets.

NASA’s infrared James Webb Space Telescope, due to launch in 2014, should help resolve what Gliese 581 g actually looks like. That’s assuming the exoplanet exists: it was spotted once, but a second set of planet hunters failed to find evidence of it in their data.

In any case, the new work could aid studies of the habitability of planets orbiting red dwarfs that are yet to be discovered, says Manoj Joshi of the University of Reading in the UK. “Hopefully we’re getting to the point when we can shift from theorising what the climates might look like, to what we think they should look like”.