The results of a new analysis of Kepler data 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)

The quest to determine if planets like Earth are rare or common is taking another stride forward on the journey. Using NASA's Kepler spacecraft, managed by NASA Ames Research Center, astronomers are beginning to find Earth-sized planets orbiting distant stars. 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.



Francois Fressin, of the Harvard-Smithsonian Center for Astrophysics (CfA), presented the analysis today in a press conference 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.



The research team found that 50 percent of all stars have a planet of Earth-size or larger in a close orbit. By adding larger planets detected in wider orbits up to the orbital distance of the Earth, this number increases to 70 percent.



Extrapolating from Kepler's currently ongoing observations and results from other detection techniques, scientists have determined that nearly all sun-like stars have planets.



Planets closer to their stars are easier to find because they transit more frequently. As more data are gathered, planets in larger orbits will be detected. In particular, Kepler's extended mission will enable the detection of Earth-sized planets at greater distances, including Earth-like orbits in the "habitable zone," the region in a planetary system where liquid water might exist on the surface of an orbiting planet.



Kepler is the first NASA mission capable of finding Earth-size planets orbiting in or near the habitable zone of the host star. NASA's Ames Research Center, Moffett Field, Calif., is the home organization of the science principal investigator, and is responsible for the ground system development, mission operations, and science data analysis.



NASA's Jet Propulsion Laboratory in Pasadena, Calif., managed Kepler mission development. Ball Aerospace and Technologies Corp. in Boulder, Colo., developed the Kepler flight system and supports mission operations with JPL at the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.



The Space Telescope Science Institute in Baltimore archives, hosts and distributes the Kepler science data. Kepler is NASA's 10th Discovery Mission and is funded by NASA's Science Mission Directorate at the agency's headquarters.



To read more about the discovery, see the Harvard-Smithsonian Center for Astrophysics press release.



Read more about the Kepler Mission.