Study: Billions of Earth-size planets in Milky Way

Not all of the planets are potentially habitable, and scientists have yet to find a twin Earth — one that's not only the right size but also located in the "Goldilocks zone," where water might exist in liquid form.

LONG BEACH, Calif. — Astronomers hunting for Earth-like planets now have many places to look. A new estimate released Monday suggested the Milky Way galaxy is home to at least 17 billion planets similar in size to our planet.

This doesn't mean all are potentially habitable, but the sheer number of Earth-size planets is a welcome starting point in the search for worlds like our own.

Scientists have yet to find a twin Earth — one that's not only the right size but also located in the so-called Goldilocks zone, a place that's not too hot and not too cold where water might exist in liquid form.

Two independent groups came up with the new estimate after a fresh analysis of data gathered by NASA's Kepler spacecraft, which was launched in 2009 to track down other Earths. The craft spots a planet when it passes in front of its star.

One team led by Francois Fressin of the Harvard-Smithsonian Center for Astrophysics estimated at least one in six stars has an Earth-size planet orbiting it. Using a different method, researchers at the University of California, Berkeley and University of Hawaii determined 17 percent of stars host planets that are one to two times the diameter of Earth.

The findings were presented at the American Astronomical Society in Long Beach, Calif.

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