1. Gliese 581g
Topping the list is a rocky world that may not even exist. The exoplanet Gliese 581g, which is 20 light-years away and perhaps two to three times as massive as Earth, was discovered in September 2010, but its existence is a point of contention among critics, because of what was then believed to be a non-circular orbit. Still, newer findings suggest that Gliese 581g is indeed real, and is "probably nice and Earthy warm," says Rebecca Boyle at Popular Science, inhabiting a "Goldilocks zone" that gives it roughly the same surface temperature as our planet, thanks to a desirable proximity to its own red dwarf star.
2. Gliese 667Cc
Discovered in February 2012, this massive exoplanet is at least 22 light-years away and orbits a red dwarf in the constellation Scorpius. The "super Earth" is 4.5 times larger than our planet, and completes an orbit every 28 days. Unlike our planet with its single sun, however, Gliese 667Cc resides in a triple-star system, "so the planet's night sky would probably be a sight to behold," says Mike Wall at Space.com.
This exoplanet was spotted by NASA's planet-hunting Kepler space telescope, which has detected more than 2,300 potentially inhabitable worlds since it was first used in March 2009. Kepler-22b was the first confirmed to officially exist in the "habitable zone," or the set of conditions that a potential Earth-clone would need to meet to be deemed livable. It boasts a "balmy temperature" of 72 degrees, which would theoretically support the existence of liquid water.
4. HD 85512b
This alien world 35 light-years away has an estimated surface temperature of 77 degrees. HD 85512b is about 3.6 times "as massive as our planet," says Space.com's Wall, but beyond that little else is known. Follow-up work is needed to understand exactly how the exoplanet orbits its star, says Discovery News. "Is it a rocky, truly Earth-like world? Does it have an atmosphere? If it does, is the atmosphere thick like Earth's; as suffocating as Venus'; or, as tenuous as Mars'?" The answers are all unknown.
5. Gliese 581
The largest exoplanet on the list is roughly seven times as massive as Earth, and orbits the same sun as its Gliese siblings, says Space.com's Wall. Discovered in 2007, Gliese 581 is probably too cold to be habitable, although recent studies suggest it could in fact be warmed by greenhouse gases, which gives it some potential to harbor life.
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