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A team of astronomers from the University of Hawaiʻi at Mānoa and the University of California, Berkeley has found that 17 percent of all sun-like stars have planets one to two times the diameter of Earth in close orbits. The finding, based on an analysis of the first three years of data from NASA’s Kepler mission, was announced on January 8 at the American Astronomical Society meeting in Long Beach, California.
While other studies had shown that planets around stars are common in our galaxy, until this study, it remained unclear if this is true for Earth-size planets.
The team consists of former UC Berkeley postdoctoral fellow Andrew Howard, now on the faculty of the UH Mānoa Institute for Astronomy, UC Berkeley graduate student Erik Petigura and UC Berkeley Professor Geoff Marcy.
To find planets, the Kepler space telescope repeatedly images 150,000 stars in a small region of the sky. It looks for a tiny dip in each star’s brightness that indicates a planet is passing in front of it, much like Venus passed between Earth and the sun last summer.
The team’s estimate includes only planets that circle their stars within a distance of about one-quarter Earth’s orbital radius—well within the orbit of Mercury—which is the current limit of Kepler’s detection capability. Further evidence suggests that the fraction of stars having planets the size of Earth or slightly bigger orbiting within Earth-like orbits may be as high as 50 percent.
“Kepler’s one goal is to answer a question that people have been asking since the days of Aristotle: What fraction of stars like the sun have an Earth-like planet?” said Howard. “We’re not there yet, but Kepler has found enough planets that we can make statistical estimates.”
The analysis indicates that perhaps 1 percent of stars have planets the size of Jupiter, while 10 percent have planets the size of Neptune. Marcy compared this to rocks on a beach— large boulders are rare, stones are more common and pebbles extremely abundant.
Unlike the beach, where sand grains and flecks are even more abundant, they now estimate that the abundance of planets stops rising at about twice Earth’s diameter and remains the same until the size of Earth, the limit of their analysis.
For more information, read the Institute for Astronomy’s news release.