Pale blue dot

A new Earth has been discovered! This is what it would look like, if we could see it (this is a picture of Earth taken by Voyager). Beware though, this is not the first time this star has yielded an Earth-like planet. Subsequent estimates of Gliese 581c suggested it'd be more like Venus than Earth. Minor note: a radio message has been sent from Earth to Gliese 581c. The earliest possible response will come in 2049.

Anyway:

Unlike the previously discovered planets, Gliese 581g lies squarely in the region of space were life can thrive. "We had planets on both sides of the habitable zone — one too hot and one too cold — and now we have one in the middle that's just right," Vogt said.

One side of the planet is always facing the star, much as one side of the moon constantly faces Earth. This means that the far side of the planet is constantly in darkness. The most habitable region of the planet would be the line between the light and dark regions.

"Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude," [Steven] Vogt [astronomer at the University of California] said.

The interesting thing, regarding the Rare Earth debate is:

The number of systems with potentially habitable planets is probably on the order of 10 or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that's a large number. There could be tens of billions of these systems in our galaxy.

One of the factors in the likely rarity of Earth is our unusual moon. Amongst other things the Moon stabilises our climate by keeping Earth's orbit and angle of rotation in relative check. Gliese 581g is tidally locked with its star (as the Moon is with the Earth). Gliese 581 is an M-dwarf star, very common in the known universe not to mention long-living. If planets such as Gliese 581g are also common this massively cuts the odds for life in the universe.

The downside of this is, of course, before long someone'll be selling real-estate futures in Gliese 581g. There'll be a tremendous housing boom, followed by a crash, after which David Cameron will sell the planet off to Balfour Beatty and News International. We're all in this together, even the aliens.