One in five Sun-like stars may have Earth-sized planets: study

One in five Sun-like stars may have planets about the size of Earth and a surface temperature conducive to life, U.S. researchers said Monday.

Many of the extrasolar planets discovered so far are gas giants orbiting close to their parent star. Rocky Earth-like planets are hard to spot, leading to uncertainty about how many Earth-like planets may exist.

Researchers from University of California, Berkeley, and University of Hawaii, Manoa, used data from the Kepler space telescope, now crippled with its four-year mission at an end, to search for planets with radii one to two times that of Earth and receiving between one and four times the stellar irradiation received on Earth. Stellar irradiation may determine whether water could exist as a liquid on these planets' surfaces.

Researchers have previously combed through more than 42,000 stars that are like the Sun or slightly cooler and smaller, and discovered 600 likely planets, including 10 with radii and stellar irradiation similar to those of Earth.

Accounting for missed planets, the researchers concluded that about 22 percent of all Sun-like stars in the galaxy may be orbited by Earth-size planets receiving Earth-like levels of stellar energy, within the so-called habitable zone.

"What this means is, when you look up at the thousands of stars in the night sky, the nearest sun-like star with an Earth-size planet in its habitable zone is probably only 12 light years away and can be seen with the naked eye. That is amazing," UC Berkeley graduate student Erik Petigura, who led the analysis of the Kepler data, said in a statement.

"With this result we've come home, in a sense, by showing that planets like our Earth are relatively common throughout the Milky Way galaxy," said Andrew Howard, a former UC Berkeley post- doctoral fellow who is now on the faculty of the Institute for Astronomy at the University of Hawaii.

The researchers cautioned that Earth-size planets in Earth-size orbits are not necessarily hospitable to life, even if they orbit in the habitable zone of a star where the temperature is not too hot and not too cold.

The findings were published on-line in the U.S. journal Proceedings of the National Academy of Sciences.