Kepler, NASA’s planet hunting mission, has bagged its first ‘Goldilocks’ planet, one that sits in the habitable zone of its star.

Kepler 22-b is not an Earth twin: at 2.4 times Earth’s radius, it could be more like a gassy Neptune than a rocky planet (artist’s impression at right). But it sits in a comfortably warm orbit from its host star at temperatures similar to those experienced by Earth.

The team got a third blip on its light curves just before Christmas in 2010 — enough to make Kepler 22-b a candidate — and then spent another full year trying to confirm the planet, with follow-up observations from other telescopes in space and on the ground.

“We consider this our sort of Christmas planet,” says principal investigator Bill Borucki, of NASA’s Ames Research Center in Moffett Field, California, who spoke to scientists today at the first Kepler science conference.

The Kepler team also was preparing to unload another instalment of its catalogue, which now includes 2,326 planet candidates. Among these candidates are 207 Earth-sized planets (in orbits of any period), and 48 planets (of any size) in the habitable zone. Natalie Batalha, the Kepler deputy science team lead at Ames, says that in the current catalogue, there is just one Earth-size candidate in the habitable zone, and that this candidate’s host star has some uncertainties that could push it out of its sweet spot.

Kepler 22-b is not the first discovery in the habitable zone. There have been two discoveries from HARPS, a European instrument team on a ground-based telescope. But neither of those planets orbits a solar-type star. Kepler 22-b, on the other hand, has a 290-day orbit around a star very similar to the Sun. “It’s almost a solar twin,” says Batalha. “I find it very compelling.”

Image credit: NASA/Ames/JPL-Caltech