About one in five stars similar to our sun -- 22 percent, plus or minus 8 percent -- harbor Earth-sized planets in Earth-sized orbits.
But even worlds in habitable zones may not be conducive to life, as they may have no atmosphere, little atmosphere or thick atmospheres, making surface conditions inhospitable.
Still, these Earth-like exoplanets are now known to be numerous enough to sustain hope that habitable worlds may be nearby.
As close as 12 light-years away, in fact, according to University of California, Berkeley, and University of Hawaii, Manoa astronomers Erik Petigura, Andrew Howard, and Geoff Marcy, who analyzed Kepler's data and presented their findings at NASA's Ames Research Center for the second Kepler Conference.
"If the stars in the Kepler field are representative of stars in the solar neighborhood, … then the nearest (Earth-size) planet is expected to orbit a star that is less than 12 light-years from Earth and can be seen by the unaided eye," the researchers wrote in their paper. "Future instrumentation to image and take spectra of these Earths need only observe a few dozen nearby stars to detect a sample of Earth-size planets residing in the habitable zones of their host stars."
Nearly 400 scientists from 30 different countries attended the conference, indicating the importance of this new estimate. Extrapolated out across the Milky Way, the estimate can figure in the billions or tens of billions.
"The primary goal of the Kepler mission was to answer the question, When you look up in the night sky, what fraction of the stars that you see have Earth-size planets at lukewarm temperatures so that water would not be frozen into ice or vaporized into steam, but remain a liquid, because liquid water is now understood to be the prerequisite for life," Marcy said.
"Until now, no one knew exactly how common potentially habitable planets were around Sun-like stars in the galaxy."
William Borucki, Kepler science principal investigator at Ames, called the findings "the opening of a new era of astronomy."
"For NASA, this number -- that every fifth star has a planet somewhat like Earth -- is really important, because successor missions to Kepler will try to take an actual picture of a planet, and the size of the telescope they have to build depends on how close the nearest Earth-size planets are," Howard said. "An abundance of planets orbiting nearby stars simplifies such follow-up missions."
The Kepler spacecraft, launched in 2009, studied its small corner of space for four years, and found some 3,500 planet candidates.