Astronomers have found three planets orbiting far-off stars that are close to Earth-sized and in the "habitable zone": a distance from their suns that makes the planets' surfaces neither too hot nor too cold, but just right.

One of the three planets orbits a star with the prosaic name Kepler-69.

"Kepler-69 is a sun-like star," says Thomas Barclay, a research scientist at the Bay Area Environmental Research Institute who uses the Kepler space telescope, which is on a mission to search for Earth-like planets. It finds planets by looking for tiny dips in the light coming from a star. The dips come when a planet passes in front of a star. By measuring the interval between dips, astronomers can figure out how long it takes a planet to orbit its star.

The planet around Kepler-69 is "around 70 percent bigger than Earth, so what we call super-Earth-sized," says Barclay. "This represents the first super-Earth-sized planet in the habitable zone of a star like our sun."

Twenty five years ago, if you had asked astronomers if there were planets around other stars, they'd probably say maybe, but they'd admit they were just speculating.

Boy, have times changed. In the past two decades, using some innovative measurement techniques, astronomers have confirmed the existence of lots of planets — 697, in fact — according to the Exoplanet Orbit Database.

"Back in the good old days, you'd find one or two crappy, Jupiter-like planets, and you'd be on the cover of Time magazine. But those days are long gone," says Paul Butler, a planet hunter at the Carnegie Institution for Science. Most new planets barely elicit a yawn these days.

The Kepler mission is partly to blame for that. The spacecraft, which launched in 2009, has been wildly successful, having found more than 100 planets, most of which have been the nasty Jupiter-sized planets Butler talks about. But the three planets being announced today are different.

In addition to the one orbiting Kepler-69, there are two around Kepler-62 that are even closer to Earth-sized. Kepler-62 is a dimmer star than Kepler-69, so the planets' orbits must be closer to the star to keep them in the habitable zone. The planets around Kepler-62 are described in the online edition of the journal Science.

William Borucki, an astronomer with the NASA Ames Research Center and the principal investigator for Kepler, says the mission's goal is to find how many Earth twins are out there.

"If they're frequent, then there may be lots of life throughout the galaxy," says Borucki. "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."

And that's a sobering thought.

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