Gliese 581g is the first planet other than Earth to exist within its star's habitable zone, meaning it could support human life. Now we've got an idea of what it looks like...and it's seriously spooky.
The problem with Gliese 581g, which discoverer Steve Vogt unofficially named Zarmina after his wife, is that's it's too close to its sun. Of course, it has to be, because the star itself is a dim red dwarf, and so the planet needs to be very close to be within the habitable zone.
But at a distance from its star that is only about a third that of Mercury and the Sun, the odds are that the star's gravity has made Zarmina tidally locked. That means one side would always face its star, while the other side would always be facing away. Naturally, that means one side is going to be very hot and the other side very cold. A thin strip between the two regions might just support life, but it doesn't seem like a particularly hopeful scenario.
University of Chicago researcher Raymond Pierrehumbert outlines a number of possible climates the planet could support, and he found one that's particularly intriguing. For the sake of argument, let's assume Zarmina has water on its surface and carbon dioxide in its atmosphere. The greenhouse effect created by the gas would trap heat above the part of the planet that faces the star. This in turn would melt the water directly face the star, while keeping the rest of the planet trapped under ice.
Zarmina would then have a circular region of open ocean created by the glare of its star, while the rest of the planet would remain an icy white block. As you can see in this artist's conception that compares Zarmina's size with that of Earth, the planet would look like a gigantic eyeball. Like we said, that is going to look seriously eerie for any far future Earth explorers.
There are a couple snags to this idea, though. For the greenhouse effect to kick in properly, the atmosphere would need to be about 20% carbon dioxide. That's a lot more than what we find on Earth, but it is still within the known limits of what can happen during planet formation.
But then there's the more basic question of whether Gliese 581g actually exists. We discussed some of the skepticism surrounding the planet back in October, but since then the controversy has heated up again, in part stoked by some rather sensationalist reporting. For a really excellent overview of the latest surrounding the planet's existence, check out this post by Phil Plait over at Bad Astronomy.