NASA's Kepler mission has identified 10 possible Earth-size planets in habitable zones around stars (the region where liquid water could exist), and yesterday it confirmed that one of those, Kepler-22b, is indeed a planet.

Kepler-22b is the smallest planet yet found in another star's habitable zone, but its radius is still 2.4 times the size that of Earth's. It is unknown whether the planet is primarily rocky, gaseous, or liquid. The planet's star belongs to the same class as our sun -- G-type -- but it is slightly smaller and cooler. Above, an artist's idea of what Kepler-22b might look like, some 600 light years from Earth.

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Image: NASA.