National Geographic News
An illustration of Kepler-35.
Planet Kepler-35b and its two suns (artist's conception).

Illustration courtesy Lynette Cook, Nature

Andrew Fazekas

for National Geographic News

Published January 11, 2012

Hot on the heels of a report that says worlds with two suns can be Earthlike, a new study suggests our galaxy alone holds millions of "Tatooines"—nicknamed after the Star Wars planet with two suns.

The finding suggests that double-sun worlds are common across the universe and may represent an entirely new class of worlds, called circumbinary planets, experts announced today at a meeting of the American Astronomical Society in Austin, Texas.

The conclusions are based on the detection of two Saturn-size, gas giant planets—Kepler-34b and Kepler-35b—each orbiting its own pair of stars that are held close to each other by gravity. Until now only one circumbinary planet, Kepler-16b, had been documented.

All three discoveries were made using NASA's Earth-orbiting Kepler space telescope, which has detected more than 750 exoplanets—planets outside our solar system—by watching for dips in brightness of starlight as planets cross in front of their parent stars, as seen from Earth. (Related: "Fifty New Planets Found—Largest Haul Yet.")

The two new planets mean that Kepler-16b isn't a freak of nature, said William Welsh, lead author of the new study, published online today by the journal Nature.

Millions of Two-Sun Planets in Our Galaxy

Taken together, the three double-sun planets afforded Welsh's team the unique opportunity to calculate how likely such planets might be in our Milky Way galaxy.

The team based their calculation on several key factors:

  • Of the 750 known exoplanets 3 have two suns.
  • Within Kepler's field of view, there are roughly seven times more planets than can be detected by the telescope. Because Kepler's method requires a world to cross in front of its star, as seen from Earth, most planets' transits go undetected.
  • The latest studies allow the team to estimate how many binary stars are in the Milky Way.

Their conclusion? "We can estimate a lower limit of millions of such planets in our galaxy," said Welsh, an astronomer at San Diego State University in California.

"The fact that circumbinary planets are not rare flukes means that nature likes to form planets, and can and does form planets around binary stars, even though these are very dynamic and chaotic environments," Welsh said.

"And more planets mean more chances that some will be Earthlike," and therefore more likely to be friendly to life as we know it.

(Also see "New Planet May Be Among Most Earthlike—Weather Permitting.")

New Targets for Search for Life

The team's simulations, which span about ten million years, suggest the three double-sun planets' orbits—between 0.6 and 1.1 times the distance from the sun to Earth—should be stable.

"If the planets were just 25 percent closer to their stars, the quickly changing gravitational forces of the stars would cause chaos and destabilize the orbits, and the planets would be flung out into deep space," Welsh said.

No matter how stable they are, though, the new planets aren't hot spots for life, he said.

All three double-sun worlds, after all, are gas giants and are outside their stars' habitable zones—the regions around stars where planets would get just enough heat, namely to host liquid water, essential for life. (Related: "Earthlike Planet Found Orbiting at Right Distance for Life.")

But Kepler-16b, Kepler-34b, and Kepler-35b nevertheless increase the odds of finding life outside beyond Earth.

"The search for Earthlike planets," Welsh said, "can now include the binary star systems, which previously were thought to be unlikely places for planets."

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