WASHINGTON — One out of four Sun-like stars in our Milky Way galaxy could host Earth-size planets, making life-bearing systems like ours possibly more common than previously thought, an astronomical study said Thursday. Dubbed the most extensive planetary census of its kind, the NASA-funded study at the University of California, Berkeley found that smaller planets rather than massive ones are prevalent in close orbit to the stars. “The data tell us that our galaxy, with its roughly 200 billion stars, has at least 46 billion Earth-size planets, and that’s not counting Earth-size planets that orbit farther away from their stars in the habitable zone,” said study co-author Geoff Marcy. The astronomers for five years used the W.M. Keck Observatory in Hawaii to search 166 sun-like stars near our solar system for planets ranging from three to 1,000 times the mass of Earth. The results showed more small planets than large ones. “We studied planets of many masses — like counting boulders, rocks and pebbles in a canyon — and found more rocks than boulders, and more pebbles than rocks,” said UC Berkeley’s Andrew Howard, lead author of the study that will be published in Friday’s issue of the journal Science. “Our ground-based technology can’t see the grains of sand, the Earth-size planets, but we can estimate their numbers,” he said. “Earth-size planets in our galaxy are like grains of sand sprinkled on a beach — they are everywhere,” he added. The study suggests that potentially habitable planets like Earth could also be common in the Milky Way. These smaller planes would orbit farther away from their stars, where temperatures could be favorable for life. A similar survey of our galaxy is being conducted by the National Aeronautics and Space Administration’s Kepler spacecraft, which is expected to find the first true Earth-like planets in the next few years. In the UC Berkeley survey, about 6.5 percent of stars held intermediate-mass planets, with 10 to 30 times the mass of Earth, similar to Neptune and Uranus.
Another 11.8 percent had the so-called “super-Earths,” weighing in at only three to 10 times the mass of Earth.
The astronomers extrapolated from these survey data to estimate that 23 percent of sun-like stars in our galaxy host even smaller planets, the Earth-sized ones.