Are we alone in the universe? Are there other planets like Earth in the galaxy? Could they harbor life?
On Monday, mission scientists said the most sophisticated and comprehensive analysis of Kepler data yet added 219 exoplanet candidates to the list of thousands already discovered by the telescope.
And 10 of those are about the same size as Earth and orbiting stars similar to our sun.
That puts Kepler's planet-finding tally at a total of 4,034 exoplanet candidates, about 50 of which are Earth-sized and at the right distance from their host star that they could sustain liquid water on their surface.
So far, scientists have confirmed that 2,335 of those candidates are indeed exoplanets, including 30 Earth-sized planets, the researchers said.
"It's an important question for us, 'Are we alone?'" said Mario Perez, Kepler program scientist in the astrophysics division at NASA. "Kepler tells us today, although indirectly, that we are not."
To find planets outside the solar system, Kepler looks for tiny dips in the brightness of distant stars that indicates a small body has passed in front of them.
Using this method, Kepler looked at 200,000 stars in a patch of sky in the Cygnus constellation for a four-year period. Scientists said the space-based telescope found 34,000 signals that suggested a planet had crossed in front of a star, but the vast majority of those were due to noise in the data.
In this most recent analysis of the Kepler data, however, the researchers wanted to make sure the team's data processing methods were not dismissing signals that might look like noise but were in fact planet transits.
So they ran a test: They injected their own, simulated planet transits into the data, to see how often these signals would be rejected. They also simulated noise signals to see how often they were counted as a planet.
The result is the most complete catalog of exoplanets discovered by Kepler, said Susan Thompson, a Kepler research scientist at the SETI Institute in Mountain View, California who led the work.
"This survey catalog will serve as the foundation to answer one of most astronomy's most compelling questions — how many planets like Earth are in our galaxy?" she said.
But the scientists said there is still more work to do before that question can be answered.
Kepler looked at just 0.25% of the sky and there is no way it identified all the Earth-sized planets that orbit the stars in its field of view. For example, if Kepler had trained its sights at our own sun, there is just a 1-in-200 chance that it would have spotted us, researchers said.
Still, Thompson said the new work will allow scientists to determine what fraction of worlds in the galaxy are Earth-like planets around sun-like stars — eventually.
"That analysis is very complicated and a lot of pieces go into it," she said. "But now we have those pieces, and scientists will spend the next year figuring out how to get to the most accurate number, and the best way to do it."
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