Kepler-22b, a planet 600 light-years away, is the smallest yet found to orbit in the middle of the habitable zone. The planet is about 2.4 times the radius of Earth and has a rotation period of 290 days around a Sun-like star, which a bit smaller and cooler. Scientists are yet to find out whether Kepler-22b is rocky, gaseous or liquid in composition.
"This is a major milestone on the road to finding Earth's twin," said Douglas Hudgins, Kepler program scientist at NASA Headquarters in Washington
Launched on March 6, 2009, the Kepler mission simultaneously observes more than 156,000 stars and detects planets by looking for tiny dips in the brightness of a star when a planet crosses in front of it.
The project's chief scientist William Borucki dubbed Kepler-22b the "Christmas Planet," as the decisive transit, which confirmed the discovery, was made on December 22. "It's a great gift," he said. "We could've easily missed it altogether."
The planet is right in the middle of the habitable zone, the region around a star where temperatures are neither too high nor too low for liquid water to exist. The previously discovered planets where life could exist were located at the edge of the zone, and temperatures on their surface were more extreme.
"If this planet has a surface, it would have a very nice temperature of some 70 degrees Fahrenheit [21 degrees Celsius]," Borucki said. "There's a good chance it could be rocky."
The planet would probably contain huge amounts of compressed ice and a global ocean.
"We have no planets like this in our own solar system," the scientist added.
Of the 54 habitable zone planet candidates reported in February 2011, Kepler-22b is the first to be confirmed.
The Kepler team announced the discovery of 1,094 new candidates, bringing the total number of planet candidates to 2,326, a 89 percent since the previous release. 207 of those planets are approximately Earth-size and 680 are super Earth-size.
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