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In another step toward finding Earth-like planets that may hold life, NASA says its 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 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," Bill Borucki, Kepler principal investigator at NASA Ames Research Center, told reporters.
"We are certain that it is in the habitable zone, and if it has a surface it ought to have a nice temperature," he said.
Spinning around its star some 600 light years away, Kepler 22b is 2.4 times the size of the Earth and orbits its sun-like star every 290 days.
Scientists do not know, however, if 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 1000 more potential planets, twice the number it had previously 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.
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 the total number confirmed by global astronomers to three.
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.
According to an online catalogue that indexes bodies outside our solar system by the Planetary Habitability Laboratory (PHL) of the University of Puerto Rico at Arecibo, a total of 47 exoplanets and exomoons are potential habitable candidates.
The Habitable Exoplanets Catalogue (HEC), available online at http://phl.upr.edu, gives scientists "the ability to compare exoplanets from best to worst candidates for life," said principal investigator Abel Mendez.
© 2011 AFP
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Planet confirmed that could have water
- Kepler-22b is the first confirmed planet in the "habitable zone" where liquid water can exist
- The planet is more than twice as big as Earth
- Scientists are now trying to establish its mass
- The Kepler mission has identified more than 2,000 planet candidates that could hold life
(CNN) -- Kepler-22b is the first confirmed planet in the "habitable zone," the area around a star where a planet could exist with liquid water on its surface, that has been discovered by NASA's Kepler mission.
The planet's radius is about 2.4 times that of the Earth. It is located about 600 light years away. Its orbital period is shorter than that of the Earth: a "year" on Kepler-22b is 290 days instead of 365.
There were two other planets confirmed this year by other projects in the habitable zone, but their stars are much cooler than our Sun, and their orbits are more like that of Venus or Mars, scientists say.
Kepler-22b is 15% closer to its star and we are to the Sun. But since Kepler-22b's star is dimmer, lower in temperature and smaller than our Sun, researchers' modeling suggests it is a similar temperature to the Earth, said Bill Borucki, Kepler principal investigator at NASA's Ames Research Center.
"If the greenhouse warming were similar on this planet and had a surface, its surface temperature would be something like 72 Fahrenheit, a very pleasant temperature here on Earth."
The warmer a planet, the more evaporation of water there would be, Borucki said. A planet can't have a surface temperature that's very hot without losing all of the surface water.
The Kepler mission reported in February that it had found 54 planet candidates in the habitable zone; Kepler-22b is the first of these to be confirmed, and those results will be published in The Astrophysical Journal. There are still 48 potential planets from that batch.
Researchers find planets by examining the brightness of stars as a function of time; brightness drops when a potential planet crosses the star. Three transits are required for a planet confirmation. The period of the transit of Kepler 22-b was 7.4 hours. It did not appear to give off its own light, indicating it is a planet and not a star.
Scientists do not yet have a measurement of the mass of Kepler 22-b, which would tell them more about the composition of the planet. This summer, when the planet's star will be high in the sky, ground-based telescopes can attempt to get its mass.
The planet is even more mysterious because its radius is between that of Earth and Uranus and Neptune, both of which have radii about four times the size of Earth's. So we don't know what a planet in this size range typically looks like.
"We have no planet like this in our solar system," he said.
Is life restricted to Earth, or could it exist somewhere like Kepler 22-b? It may be that the characteristics of Earth, with its particular position in the solar system, particular magnetic field strength and presence of larger planets like Jupiter make Earth very rare in having life (this is called the "Rare Earth Hypothesis.") But it's also possible that life in the universe is common, and we're only one example.
"As soon as we find an independent example of life somewhere else, we'll know it's ubiquitous throughout the universe. We're all looking for No. 2," said Jill Tarter of the SETI Institute.
The Kepler mission has now identified a total of 2,326 planet candidates.
You can read more about the Kepler discoveries at the mission website.
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