As many as one in five Sun-like stars may have a planet the size of Earth, and the nearest could be in systems visible to the naked eye, US astronomers said Monday.

The research is based on a new analysis of findings from NASA's Kepler space observatory, and appears in the Proceedings of the National Academy of Sciences.

Ten newly discovered planets are close to Earth-sized and in the habitable zone of their stars, meaning they orbit at a distance that is not too hot or too cold to support life, Kepler scientists told reporters.

There are about two dozen planets in total that may be a suitable distance from their suns so that their oceans would neither boil nor freeze, said Bill Borucki, Kepler science principal investigator.

If they have oceans, that is.

The Kepler mission stops short of delivering on key details in the hunt for life on other planets, such as the whether these planets have an atmosphere, oxygen or liquid water to support life.

Still, astronomers say the latest Kepler findings are a key milestone in the search for extraterrestrial life, and that more advanced technology in the future may answer more questions.

"We have lots of missions to consider in the future. I think some of those, which are already pushing technology, are likely to be done by our children or grandchildren," said Borucki.

The Kepler mission launched in 2009 on a search for planets outside the solar system that may orbit at a distance from their host stars that could allow life to exist.

A total of 3,538 planet candidates have been found so far, with astronomers observing them as distant spots of light called transits, or passages in front of their stars that cause dimming.

Astronomers have found 833 new planet candidates using the first three years of a total of four years of Kepler data.

After analyzing the first two years of data, the team had found a total of 351 Earth-sized planets. Now, they have found 647.

Just 104 are in the habitable zone, however, and of those, about 10 are believed to be potentially rocky like Earth, scientists told reporters.

Astronomers also made some calculations to project how many of the 100 billion stars in our galaxy have potentially habitable planets.

"You can think of it like we are taking a census of extra-solar planets but not everyone is answering the door," said Erik Petigura, a University of California, Berkeley graduate student.

Some transits might not be visible, or they may be lost in the brightness of the universe.

Based on this statistical analysis, astronomers estimate that 22 percent of stars like the Sun have planets about the size of Earth with a surface temperature conducive to life, Petigura.

That means the nearest Sun-like star with an Earth-size planet in its habitable zone could be "12 light years away and can be seen with the naked eye," he said.

The Kepler science consortium brings together about 500 scientists from around the world.

The space observatory itself is now hobbled by the loss of two of its orienting wheels, and is no longer operating at full capacity, NASA announced in August.