One in five stars in Milky Way circled by Earth-like planets

Twenty per cent of Sun-like stars in our Milky Way galaxy have Earth-sized planets that could potentially host life, a new study has found.
Based on a statistical analysis of all the Kepler observations, astronomers at University of California Berkeley and University of Hawaii, Manoa now estimate that one in five stars like the Sun have planets about the size of Earth and a surface temperature conducive to life.
Given that about 20 per cent of stars are Sun-like, the researchers say, that amounts to several tens of billions of potentially habitable, Earth-size planets in the Milky Way Galaxy.
"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, from the Institute for Astronomy at the University of Hawaii.

The team cautioned that Earth-size planets in orbits about the size of Earth's are not necessarily hospitable to life, even if they reside in the habitable zone around a star where the temperature is not too hot and not too cold.
"Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive. Others may have rocky surfaces that could harbour liquid water suitable for living organisms," Marcy said.

"We don't know what range of planet types and their environments are suitable for life," said Marcy.
Erik Petigura, who led the analysis of the Kepler data and his colleagues are using the Keck telescopes in Hawaii to obtain spectra of as many stars as possible.

This will help them determine each star's true brightness and calculate the diameter of each transiting planet, with an emphasis on Earth-diameter planets.
Researchers focused on the 42,000 stars that are like the Sun or slightly cooler and smaller, and found 603 candidate planets orbiting them.
Only 10 of these were Earth-size, that is, one to two times the diameter of Earth and orbiting their star at a distance where they are heated to lukewarm temperatures suitable for life.
The study appears in the journal Proceedings of the National Academy of Sciences.