Earth-Size Planets: Scientists Say 1 in 6 Stars May Have One

Kepler Telescope finds small planets probably common.

ByABC News
January 7, 2013, 11:20 AM

Jan. 7, 2013— -- The Kepler Space Telescope, an observatory launched by NASA in 2009 to find Earth-like planets, has provided data that suggests there are billions of them -- enough that one in six stars may have at least one orbiting it.

Out of roughly 100 billion stars in the Milky Way galaxy, a new analysis of Kepler data shows that around 17 percent of them have Earth-sized planets orbiting them, meaning there could be as many as 17 billion Earth-sized worlds.

Such planets, trillions of miles or more away, are too small for current technology to see, so Kepler watches thousands of stars -- and if one dims by a tiny amount regularly, that's a sign that perhaps a planet has crossed its face, blocking a little of its light. Researchers at the Harvard-Smithsonian Center for Astrophysics analyzed the entire data set from Kepler and concluded that virtually all sun-like stars have at least one planet.

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"This is the first time we've been able to say with any certainty how many stars out there have Earth-like planets," Christine Pulliam of with the Harvard-Smithsonian Center for Astrophysics told

Francois Fression of the Harvard-Smithsonian Center presented the statistical study today at a meeting of the American Astronomical Society in Long Beach, Calif. A paper detailing the research has been accepted for publication in The Astrophysical Journal.

Contradicting previous findings, researchers also found that the type of star does not make certain sizes of planets more or less common. The team grouped planets into five different sizes and 17 percent of them, roughly 1 in 6, have a planet 0.8-1.25 times the size of Earth in an orbit of 85 days or less.

"As far as planets go, Earth-sized planets are very small," Pulliam said. "Jupiter-like planets are easier to find; however, our study finds that the smaller planets are more common."

Using Kepler data, Christopher Burke, a scientist at the SETI Institute, said that 58 planets found so far are believed to be in their host stars' habitable zones -- where any water on them has a chance of being liquid. Planets orbiting close to their suns are likely to be infernos; planets in distant orbits will probably be too icy.

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Kepler's mission is to find and document Earth-sized planets at greater distances. The more planets discovered with Earth-like orbits in the habitable zone, the greater the chances of extraterrestrial life.