Scientists and their Kepler spacecraft have discovered for the first time a planet in distant space that is much like Earth, circling a sun-like star and lying in a region neither too hot nor too cold for an atmosphere that could support some form of life.
The temperature on that planet, the scientists say, probably is a comfortable 72 degrees, rain or shine.
The planet, whose discovery was announced Monday at NASA's Ames Research Center in Mountain View, was first detected by NASA's Kepler spacecraft two years ago, shortly after it began surveying 155,000 stars in the constellations Cygnus and Lyra.
Dubbed Kepler 22b, the planet exists in what astronomers call the habitable zone of a solar system at whose center is a star only a little smaller and fainter than Earth's sun. It lies about 600 light-years away and is 2.4 times Earth's size, although its mass is still unknown, said William Borucki, the leader of the Kepler science team.
Kepler 22b circles its sun every 290 days, compared with Earth's year of 365 days, the Kepler team has determined.
Listening for life
The planet's discovery is intriguing enough that astronomers at the SETI Institute in Mountain View are about to tune their array of 42 radio telescopes, located in a valley near Redding, to listen for possible signals that could - just conceivably - indicate there's a civilization on the planet.
"This is a superb opportunity for SETI observations," said Jill Tarter, director of the SETI Institute's search for signs of extraterrestrial life.
Whether Kepler's planets harbor civilizations remains, of course, unknown, she said, but "we won't know unless we look."
Science fiction? Not at all.
The Kepler spacecraft has now discovered 2,326 "candidate" planets in habitable zones, and of these, 207 are roughly the size of Earth. The rest are the size of Neptune or Jupiter, or even larger, according to Natalie Batalha an astrophysicist at San Jose State
University and deputy leader of the Kepler group.
"The tremendous growth in the number of Earth-size candidates tells us that we're really homing in on the planets Kepler was designed to detect - those that are not only Earth-size, but are also potentially habitable," she said.
Habitable regions, which astronomers like to call "Goldilocks zones," are areas around stars where temperatures would be just right for supporting life.
"Candidate" planets are considered true planets when astronomers can confirm that they aren't a pair of "binary" stars orbiting each other, or some other false signal.
The spacecraft's powerful and highly sensitive telescope detects planets by measuring the tiny dimming of the star's light as an object passes - or transits - in front of it. In the case of Kepler 22b, the spacecraft has seen the planet transiting its sun more than three times - a criterion for identifying it.
'Christmas planet'
The space-borne telescope first detected the planet passing in front of its sun nearly two years ago, just after the spacecraft became operational, Borucki said. The second crossing was seen just before the spacecraft went offline temporarily, and the planet's third transit was detected just before Christmas last year.
"It was a great gift," Borucki said. "We consider it sort of our Christmas planet."
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