Earthlings will surely thrill at finding their planetary double: our calculation suggests the discovery could happen next year.
In 2010, one new exoplanet appeared every four days or so; by the end of the year, the total topped 500. But in September, a truly exceptional find punctuated this steady drumbeat of discovery: the first alien planet that could host life on its surface.
Gliese 581 g, spotted by a team led by Steven Vogt of the University of California, Santa Cruz, inhabits a "Goldilocks" zone around its host star, a band just warm enough to boast liquid water. At 3.1 to 4.3 times the mass of Earth, it is also small enough that it should be made mostly of rock. Although a second team of astronomers failed to find signs of Gliese 581 g in their data, if its existence is confirmed, it will be the most habitable exoplanet yet found.
An even bigger prize awaits, however: a planet with the size and temperature of our own. We're unlikely to be exceptional, so such a doppelgänger must be out there. Will it be found in 2011? See "Prediction: Get ready for Earth's doppelgänger".
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