STEVE INSKEEP, HOST:
And now, let's turn our gaze from star-crossed lovers to the heavens above. A study examines our long-held speculation that there might be other planets out there, something like Earth. The study estimates how many such worlds there could be in this galaxy orbiting stars like our Sun. The answer is billions and billions. Well, maybe not actually that many, but quite a lot.
Here's NPR's Nell Greenfieldboyce.
NELL GREENFIELDBOYCE, BYLINE: Back in 2009, NASA launched the Kepler Space Telescope to hunt for planets beyond our solar system. Geoffrey Marcy is at the University of California, Berkeley, he says the big question was this: Is Earth a cosmic freak or just one of many potentially habitable worlds.
GEOFFREY MARCY: For me, personally, as a child, I remember looking up at the night sky and wondering if any of those twinkling lights had Earth-like planets around them.
GREENFIELDBOYCE: Kepler stared at around 150,000 stars, looking for a tell-tale dimming in brightness that might indicate a passing planet.
MARCY: Every minute for four years - while you were sleeping, working, playing, watching television - every moment of your life, Kepler was taking snapshot after snapshot.
GREENFIELDBOYCE: Researchers have seen dimming events that suggest thousands of possible planets, from big gassy ones a bit like Jupiter, to smaller ones that might be rocky like Earth. Earlier this year, the Kepler telescope stopped working. But scientists are still combing through all the data.
Marcy and two colleagues, Erik Petigura and Andrew Howard, have now looked a bunch of stars like our Sun - 42,000 of them. They searched for orbiting planets about the size of Earth in the so-called Goldilocks Zone around the star, where conditions are not too hot and not too cold, which means there could be liquid water and the possibility of life.
They also did an analysis to figure out how many planets their detection system would likely miss. And the results are published in the Proceedings of the National Academy of Sciences. They calculate that 22 percent of stars like the Sun have an Earth-like planet in the habitable zone. When he talked about these findings at a press briefing, Marcy said he felt a little tingly.
MARCY: We're learning that Earth-sized planets having the temperature of a cup of tea are common around Sun-like stars.
GREENFIELDBOYCE: This is the first time anyone has come up with a number to answer the big question that the Kepler telescope set out to tackle, so it's getting a lot of attention.
Natalie Batalha is Kepler mission scientist at NASA Ames Research Center.
NATALIE BATALHA: My office was crowded with people actively talking about the details of the work. The mood was very festive and the dialogue was very productive.
GREENFIELDBOYCE: Still, this study is unlikely to be the last word on how many Earths might be orbiting other suns. David Charbonneau is an astronomer at Harvard University.
DAVID CHARBONNEAU: You're going to see dozens of studies along these lines in the years to come, as we keep pushing on our analysis of the Kepler data.
GREENFIELDBOYCE: But it does seem that Earth-like planets may be pretty common.
Nell Greenfieldboyce, NPR News.
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