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Earth-like planets next door? Prospect could point to 9.6 billion more

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The results also "highlight just how quickly the field of extrasolar planets is blooming," he adds.

In 2000, astronomers had only detected 33 planets, all gas giants the size of Saturn or larger. They have now found 3,300, when Kepler's planet candidates are included. 

The new study, conducted by Harvard University graduate student Courtney Dressing and astronomer David Charbonneau at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., casts a new eye on the red dwarfs previously cataloged by the Kepler team. 

To begin with, the duo had to estimate the stars' masses, temperatures, and other factors that bear on where a habitable zone would fall and where planets would orbit. Using the Kepler observations and star traits based on a recent model well suited to characterizing low-mass stars, Ms. Dressing and Dr. Charbonneau found evidence for 95 planets orbiting 64 stars. Of those, 60 percent were smaller than Neptune, the smallest gas giant. 

"That's a remarkably large number," Dressing said during a press briefing on Wednesday announcing the results.

Among the planets, three appear to be Earth-mass planets orbiting within their star's habitable zones. The team then estimated the number of such planets that Kepler can't find. That yielded the estimate that 6 percent of red dwarfs host Earth-mass planets in their habitable zones. 

These stars, however, are between 300 to 600 light-years away. Applying the estimate to the sun's neighborhood, which hosts some 284 red dwarfs within about 33 light-years, the team estimated that the nearest habitable planet is likely to appear about 13 light-years from Earth. 

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