"The quest for a twin Earth is heating up. Using NASA's Kepler spacecraft, 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."
"NASA's Kepler mission Monday announced the discovery of 461 new planet candidates. Four of the potential new planets are less than twice the size of Earth and orbit in their sun's "habitable zone," the region in the planetary system where liquid water might exist on the surface of a planet. Based on observations conducted from May 2009 to March 2011, the findings show a steady increase in the number of smaller-size planet candidates and the number of stars with more than one candidate."
Earth-Size Planets Are Common in Our Galaxy, University of California Berkeley
"An analysis of the first three years of data from NASA's Kepler mission, which already has discovered thousands of potential exoplanets, contains good news for those searching for habitable worlds outside our solar system. It shows that 17 percent of all Sun-like stars have planets one to two times the diameter of Earth orbiting close to their host stars, according to a team of astronomers from the University of California, Berkeley, and the University of Hawaii at Manoa."