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Earth-Like Worlds Very Common

One in five sunlike stars harbors an Earth-size world that orbits in a "habitable zone" friendly to oceans and perhaps life, a new study suggests.

The findings, detailed in this week's issue of the Proceedings of the National Academy of Sciences, are based on a statistical analysis of observations made by NASA's now-crippled Kepler space telescope. (Related: "New Role for Disabled Kepler? Finding Exotic Alien Worlds.")

The astronomers estimate that 22 percent of sunlike stars may be orbited by small, rocky planets that reside within so-called habitable zones, where they receive Earth-like levels of sunlight.

"Our results show that small planets, with sizes similar to our Earth, are very common,” said study leader Geoffrey Marcy, an astronomer at the University of California, Berkeley.

What's New?

Launched in 2009, Kepler was tasked with finding alien planets, or "exoplanets," by looking for the telltale dimming of light that occurs when a world passes in front of, or "transits," its parent star.

For four years, until its malfunction this summer, Kepler stared at 150,000 stars situated in a patch of sky in the direction of the constellations Cygnus and Lyra.

Out of that group, Marcy and his colleagues Andrew Howard and Erik Petigura, focused on 42,000 stars that are like our sun or slightly cooler and smaller.

The team found 603 potential planets, or "planet candidates," orbiting these stars. Ten 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 thought amenable for life.

The team next introduced 40,000 fake planets into the Kepler data as a test for their analysis software.

The fake planets ranged from Earth-size to ten times the size of Earth. They also had a range of orbital distances: Some were located within the habitable zone, while others were either too close or too far from their star.

Similarly, the team accounted for missed planets in their analysis, as well as the fact that only a small fraction of planets follow orbits that allow their crossings in front of their host star to be seen from Earth.

Overall, the team estimated that 22 percent of all sunlike stars in the galaxy have Earth-size planets in their habitable zones.