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New Celestial Census: 17 Billion Earth-sized Worlds Out There!

This artist's illustration represents the variety of planets being detected by NASA's Kepler spacecraft. A new analysis has determined the frequencies of planets of all sizes, from Earths up to gas giants. Key findings include the fact that one in six stars hosts an Earth-sized planet in an orbit of 85 days or less, and that almost all sun-like stars have a planetary system of some sort. Credit: C. Pulliam & D. Aguilar (CfA)

A new analysis examined the frequencies of planets of different sizes based on findings from NASA's Kepler spacecraft, correcting for both incompleteness and false positives. The results show that one in six stars has an Earth-sized planet in a tight orbit. About a fourth of all stars in the Milky Way have a super-Earth, and the same fraction have a mini-Neptune. Only about 3 percent of stars have a large Neptune, and only 5 percent a gas giant at the orbital distances studied. Credit: F. Fressin (CfA)

At least one in six stars has an Earth-sized planet – that’s the word from Francois Fressin of the Harvard-Smithsonian Center for Astrophysics (CfA).

Fressin and his colleagues have been sifting through data gleaned by NASA’s Kepler spacecraft. A new analysis of Kepler data shows that about 17 percent of stars have an Earth-sized planet in an orbit closer than Mercury.

Since the Milky Way has about 100 billion stars, there are at least 17 billion Earth-sized worlds out there.

Altogether, the researchers found that 50 percent of stars have a planet of Earth-size or larger in a close orbit. By adding larger planets, which have been detected in wider orbits up to the orbital distance of the Earth, this number reaches 70 percent.

“Earths and super-Earths aren’t picky. We’re finding them in all kinds of neighborhoods,” says co-author Guillermo Torres of the CfA. More good news.

The Kepler spacecraft’s extended mission should allow it to spot Earth-sized planets at greater distances, including Earth-like orbits in the habitable zone.

Fressin presented the analysis today in a press conference at a meeting of the American Astronomical Society in Long Beach, Calif.

By Leonard David

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