Billions and billions of Earth-sized planets call Milky Way home

The galaxy contains billions of potentially habitable Earth-sized planets, according to even the most conservative estimate drawing on data from NASA’s Kepler space telescope.

Although a mechanical failure recently put the telescope out of commission (SN: 6/15/13, p. 10), Kepler’s census of planets orbiting roughly 170,000 stars is enabling astronomers to predict how common planets similar to Earth are across the galaxy.

The authors of a study published November 4 in the Proceedings of the National Academy of Sciences conclude that between 14 and 30 percent of stars similar in mass and temperature to the sun host a possibly habitable planet. Such a planet has a diameter as large as Earth’s is but no more than twice it; the planet also orbits in a star’s habitable zone, a temperate region where liquid water could exist at the surface. The estimate comes from identifying, and then extrapolating from, suitable worlds around more than 42,000 stars.

The estimate is pretty rough: If applied to the solar system, the researchers’ definition of habitable zone would include the orbits of Venus and Mars, planets that are certainly not Earthlike (though they may have been in the past). Using tighter constraints, the researchers estimate that between 4 and 8 percent of sunlike stars host an Earth-sized world that takes 200 to 400 days to orbit.

Still, even 4 percent would yield a galactic population of more than a billion potentially habitable Earth-sized planets.