LOS ANGELES — NASA scientists announced Thursday that the Kepler mission had confirmed finding three planets, slightly larger than our own Earth, orbiting in their stars' so-called habitable zones — that "Goldilocks" region where temperatures are not too hot and not too cold.
Researchers don't know for sure, but the planets' sizes and proximity to their stars mean that they could be rocky and could have liquid water, two attributes thought necessary for a planet to harbor life. What is certain, the scientists said during a news conference Thursday, is that the discoveries mark yet another step forward in the space agency's quest to find an Earth-sized planet in a star's habitable zone.
"We're not quite there, but we're pushing toward it," said Thomas Barclay, a Kepler scientist at the Bay Area Environmental Research Institute in Sonoma.
"These are our best candidates for planets that might be habitable," said William Borucki, Kepler science principal investigator at NASA's Ames Research Center in Moffett Field, Calif., and a longtime advocate for the mission.
Since its launch in 2009, the Kepler spacecraft has kept its telescope trained on a patch of of the Milky Way containing more than 150,000 stars, recording the tiny dips in light that result when a planet's orbit carries it between its star and the craft. Scientists on the ground (occasionally with the help of volunteer citizen scientists) then analyze the light curves in various ways to confirm that the dips do — or don't — correspond to distant planets.
The newly confirmed super-Earths orbit two different stars.
One planet of interest, Kepler-69c, is about 70 percent larger than Earth and orbits a star that is similar to our sun and is 2,000 light-years away, Barclay said Thursday. It lies on the inner edge of its star's habitable zone, and could be "more of a super-Venus than a super-Earth," he added — in other words, very hot. Its discovery was published Thursday in the Astrophysical Journal.
The other two planets, known as Kepler-62e and Kepler-62f, also are somewhat larger than Earth (60 percent and 40 percent larger, respectively) and are somewhat closer by, at 1,200 light-years away. Kepler-62e orbits the star Kepler-62 in 122 days and could be rocky or a "water world" unlike anything in our solar system, Borucki said.
Kepler-62f orbits Kepler-62 in 267 days and is thought to be rocky. The Kepler 62 planets were described in a paper published online Thursday by the journal Science.
There's a lot that the Kepler team doesn't know about the newly confirmed worlds, the scientists said. Because scientists have not been able to measure the masses of the planets they can't determine their composition in any direct manner, and instead must make inferences about their makeup based on previous observations of other worlds, including our own.
Because the Kepler-62e, Kepler-62f and Kepler 69 are so far away, scientists don't expect to study their atmospheres up close. But in the future, the researchers said, NASA's Transiting Exoplanet Survey Satellite mission should locate small planets closer to Earth, and its James Webb Space Telescope should be able to investigate those potentially habitable worlds in some detail, potentially learning exactly what makes a planet habitable and how life thrives.
"This is not just an academic exercise anymore," said Barclay. "We're finding planets to test our hypotheses."