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If there's another planet in the universe exactly like Earth astronomers are most likely to find it in a patch of sky near the constellations Cygnus and Lyra. That's where NASA's planet-finding mission called Kepler is sifting through a field of about 100,000 stars, searching for a twin of Earth. Today, Kepler scientists reported on the three closest candidates so far. NPR's Joe Palca has that story.

JOE PALCA, BYLINE: 25 years ago, if you asked astronomers if there were planets around stars other than our sun, they'd probably say well, maybe, but they'd admit they were just speculating. Boy, have times changed. In the last two decades, using some innovative measurement techniques, astronomers have confirmed the existence of lots of these so-called exoplanets - 697 according to the Exoplanet Orbit Database.

PAUL BUTLER: Back in the good old days, you'd find one or two crappy, you know, Jupiter-like planets and you'd be on the cover of Time Magazine, but those days are long gone.

PALCA: Paul Butler is a planet-hunter at the Carnegie Institution for Science in Washington.

BUTLER: Most planets barely elicit a yawn these days.

PALCA: The Kepler mission is partly to blame for that. The spacecraft launched in 2009. It finds planets by looking for tiny dips in the light coming from a particular star. The dips come when a planet passes in front of that star. By measuring the interval between dips, astronomers can figure out how long it takes a planet to orbit its star. Kepler has been wildly successful; it's found more than 100 planets, but most of those have been those giant, gassy Jupiter-sized planets Butler was talking about. The three new planets being announced today are different.

BUTLER: Yeah, I think this is worth reporting.

PALCA: So here goes. One of the Earth-like planets orbits a star with the prosaic name Kepler-69.

THOMAS BARKLEY: Kepler-69 is a sun-like star.

PALCA: Thomas Barkley is one of the astronomers using Kepler. He says the outermost of the two planets found orbiting Kepler-69 is quite interesting.

BARKLEY: Around 70 percent bigger than Earth, so what we call supera-sized(ph), and it is in the habitable zone of its star.

PALCA: By habitable zone, Barkley means the planet's orbit is not so close to the sun that its surface would scorch and not so far that everything would freeze solid. In other words, just right.

BARKLEY: This represents the first supera-sized planet in the habitable zone of a star like our sun.

PALCA: The two other planets being announced today and being described online in the journal Science are also somewhat bigger than Earth, but they orbit a star somewhat dimmer than our sun. NASA Ames astronomer William Borucki is the principal investigator for Kepler. He says the mission's goal is to find out how many Earth twins are out there.

WILLIAM BORUCKI: If they're frequent, then there may be lots of life throughout the galaxy. They may just be waiting for us to call and say, hello, we'd like to join the club. Or, if we don't find any, the answer may be just the opposite. Maybe we're alone. There isn't anybody out there. There will never be a "Star Trek" because there's no place to go to.

PALCA: Now there's a sobering thought. Joe Palca, NPR News Washington.

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