UC Berkeley and University of Hawaii astronomers analyzed all four years of Kepler data in search of Earth-size planets in the habitable zones of sun-like stars, and then rigorously tested how many planets they may have missed.
A major question is whether planets suitable for biochemistry are common or rare in the universe. Small rocky planets with liquid water enjoy key ingredients for biology. Astronomers now used the National Aeronautics and Space Administration Kepler telescope to survey 42,000 Sun-like stars for periodic dimmings that occur when a planet crosses in front of its host star. They found 603 planets, 10 of which are Earth size and orbit in the habitable zone, where conditions permit surface liquid water. They measured the detectability of these planets by injecting synthetic planet-caused dimmings into Kepler brightness measurements. They find that 22% of Sun-like stars harbor Earth-size planets orbiting in their habitable zones. The nearest such planet may be within 12 light-years.
"It's been nearly 20 years since the discovery of the first extrasolar planet around a normal star. Since then we have learned that most stars have planets of some size and that Earth-size planets are relatively common in close-in orbits that are too hot for life," said Andrew Howard, a former UC Berkeley post-doctoral fellow who is now on the faculty of the Institute for Astronomy at the University of Hawaii. "With this result we've come home, in a sense, by showing that planets like our Earth are relatively common throughout the Milky Way galaxy."