In another step toward finding Earth-like planets that may hold life, Nasa said today that the Kepler space telescope has confirmed its first-ever planet in a habitable zone outside our solar system.
French astronomers earlier this year confirmed the first rocky exoplanet to meet key requirements for sustaining life. But Kepler-22b, initially glimpsed in 2009, is the first the US space agency has been able to confirm.
Confirmation means that astronomers have seen it crossing in front of its star three times. But it doesn't mean that astronomers know whether life actually exists there, simply that the conditions are right.
Such planets have the right distance from their star to support water, plus a suitable temperature and atmosphere to support life.
"We have now got good planet confirmation with Kepler-22b," said Bill Borucki, Kepler principal investigator at Nasa Ames Research Center.
"We are certain that it is in the habitable zone and if it has a surface, it ought to have a nice temperature," he told reporters.
Spinning around its star some 600 light years away, Kepler-22b is 2.4 times the size of the Earth, putting it in class known as "super-Earths," and orbits its Sun-like star every 290 days.
Its near-surface temperature is presumed to be about 72 degrees Fahrenheit (22 Celsius). Scientists do not know, however, whether the planet is rocky, gaseous or liquid.
The planet's first "transit," or star crossover, was captured shortly after Nasa launched its Kepler spacecraft in March 2009.
Nasa also announced that Kepler has uncovered 1,094 more potential planets, twice the number it previously had been tracking, according to research being presented at a conference in California this week.
Kepler is Nasa's first mission in search of Earth-like planets orbiting suns similar to ours, and cost the US space agency about $600 million.
It is equipped with the largest camera ever sent into space - a 95-megapixel array of charge-coupled devices - and is expected to continue sending information back to Earth until at least November 2012.
Kepler is searching for planets as small as Earth, including those orbiting stars in a warm, habitable zone where liquid water could exist on the surface of the planet.
The latest confirmed exoplanet that could support life brings to three the total number confirmed by global astronomers.
In addition to French astronomers' confirmed finding of Gliese 581d in May, Swiss astronomers reported in August that another planet, HD 85512 b, about 36 light years away seemed to be in the habitable zone of its star.
However, those two planets are "orbiting stars smaller and cooler than our Sun," Nasa said in a statement, noting that Kepler-22b "is the smallest yet found to orbit in the middle of the habitable zone of a star similar to our Sun."
"The Europeans have also been very active, actively working on confirming our candidates," said Natalie Batalha, Kepler deputy science team lead at San Jose State University.
"They have already confirmed two that are published and they have got another batch that are on the preprint servers so those will be, I'm sure, in the published literature soon," she added.
"So we are just thrilled about this. We need all telescopes observing these candidates so we can confirm as many as possible."
A total of 48 exoplanets and exomoons are potential habitable candidates, among a total of 2,326 possibilities that Kepler has identified so far.
The top rankers are listed in an online catalog that indexes bodies outside our solar system, available here.