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One out of four Sun-like stars in the Milky Way galaxy could host Earth-size planets, making life-bearing systems like ours possibly more common than previously thought, say US astronomers.

Dubbed the most extensive planetary census of its kind, the NASA-funded study at the University of California, 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," says study co-author Geoff Marcy.

The five-year study, published in today's issue of Science , 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 researchers found 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.

"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," says study lead-author Andrew Howard.

"Our ground-based technology can't see the grains of sand, the Earth-size planets, but we can estimate their numbers," he says.

"Earth-size planets in our galaxy are like grains of sand sprinkled on a beach they are everywhere," he adds.

The study suggests that potentially habitable planets like Earth could also be common in the Milky Way.

These smaller planets would orbit farther away from their stars, where temperatures could be favourable for life.

A similar survey of our galaxy being conducted by NASA's Kepler spacecraft, is expected to find the first true Earth-like planets in the next few years.

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