Count of Earth-like planets in galaxy could reach billions

The number of habitable Earth-size planets in our galaxy may be as high as 40 billion, scientists analyzing data from NASA's Kepler space telescope say. A calculation based on a three-year study data from the Kepler spacecraft by Erik Petigura, a graduate student at the University of California, Berkeley, suggests around 20 percent of sun-like stars in the galaxy host an Earth-sized planet orbiting in the Goldilocks zone, where temperatures are neither too hot nor too cold and should be compatible with liquid water. "It seems that the universe produces plentiful real estate for life that somehow resembles life on Earth," Petigura told The New York Times. Kepler was launched in 2009 to perform a cosmic census, looking for dips in the brightness of distant stars when possible planets pass in front of them. In four years of operations until a mechanical malfunction in May inhibited the accuracy of its cosmic targeting, Kepler recorded several thousand candidate planet sightings, 1,000 of which have been confirmed by subsequent observations. Although many of the planets are Earth-size, it has yet to be determined whether they are rocky like the Earth, or balls of ice or gas, the researchers said. But recent observations suggest at least a significant percentage of them may be rocky worlds with a density similar to Earth, they said. "Nature," Petigura said, "knows how to make rocky Earth-size planets."