One out of every five sun-like stars in the Milky Way galaxy has a planet about the size of Earth that is properly positioned for water, a key ingredient for life, a study showed.

The analysis, based on three years of data collected by NASA's now-idled Kepler space telescope, indicates the galaxy is home to 10 billion potentially habitable worlds.

The number grows exponentially if the headcount also includes planets circling cooler red dwarf stars, the most common type of star in the galaxy.

Study leader Erik Petigura, an astronomy graduate student at the University of California at Berkeley, said during a conference call with reporters, that planets seemed to be the rule and not the exception.

Petigura wrote his own software program to analyze Kepler's results and found 10 planets one- to two times the diameter of Earth circling parent stars at the right distances for liquid surface water.

The telescope worked by finding slight dips in the amount of light coming from about 150,000 target stars in the constellation Cygnus.

Some light dips were due to orbiting planets passing in front of their parent stars, relative to Kepler's line of sight.

Extrapolating from 34 months of Kepler observations, Petigura and colleagues found that 22 percent of sun-like stars in the galaxy should have planets roughly the size of Earth suitably positioned for water.

A positioning system problem sidelined Kepler in May. Scientists are developing alternative missions for the telescope. More than a year of data already collected by Kepler still has to be analyzed.

So far, the telescope, which was launched in 2009, has found 3,538 candidate planets, 647 of which are about the size of Earth, said astronomer Jason Rowe, with the SETI Institute in Mountain View, Calif.

Of the 3,538 candidates, 104 are at the right distance from their parent stars for water.

In total, the list of planets beyond the solar system found by Kepler and other telescopes now numbers more than 1,000.

The research was published in this week's Proceedings of the National Academy of Sciences and presented on Monday at a Kepler science conference at NASA's Ames Research Center in Moffett Field, California.