Billions of Earthlike Planets Crowd Milky Way?

Tens of billions of Earthlike worlds are strewn across the Milky Way, many of them circling stars very much like our own sun, astronomers said today.

Earlier research suggested that rocky planets might be much more abundant around small stars than sunlike ones. (Also see "New 'Super Earth' Found at Right Distance for Life.")

But a fresh analysis of data from NASA's Kepler mission, which launched in 2009, suggests this is not the case, according to new research presented at the annual meeting of the American Astronomical Society in Long Beach, California.

"We found that the occurrence of small planets around large stars was underestimated," said astronomer Francois Fressin, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

A Starry Night ... Full of Planets

To find planets, Kepler stares at a patch of sky in the constellation Cygnus, made up of about 150,000 stars. The space telescope detects potential alien worlds by watching for telltale dips in starlight created when planets pass in front of, or "transit," their parent stars.

Using their own independent software for analyzing Kepler's potential planet detections, Fressin and his colleagues estimate that about 17 percent, or one in six, of all the sunlike stars in the Milky Way host a rocky planet that orbits closer than the distance at which Mercury orbits our own sun.

Since the Milky Way is home to about a hundred billion stars, that means there are at least 17 billion rocky worlds out there. (See Milky Way pictures.)

When the team expanded their search to Earth-size orbits or larger, they found that half of all sunlike stars may host rocky planets.

"Every time you look up on a starry night, [nearly] each star you're looking at has a planetary system," Fressin said.

A Hundred Billion Planets?

Rocky planets are just a fraction of the total number of planets in our Milky Way, however.

A study of the number of potential worlds orbiting M-dwarfs—faint stars smaller than our sun that make up the vast majority of the stellar population—suggests our galaxy may be home to at least a hundred billion planets overall. (See "Four White Dwarfs Found Eating Earthlike Planets.")

"Based on our calculations, which are very complementary to those of [Fressin] ... we are showing that there is about one planet per star, and that gives us a total of about a hundred billion planets throughout our galaxy," said Caltech planetary astronomer John Johnson.

"The vast majority of those planets are orbiting stars that are very much different from our sun."