Outer orb sits in habitable zone of binary star system

By Nadia Drake

Web edition: August 28, 2012
Print edition: October 6, 2012; Vol.182 #7 (p. 12)

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Kepler-47, illustrated here, is the first transiting multiplanet system found around two stars. The outer planet is in the system’s life-friendly zone, though it is bigger than Uranus and likely incapable of hosting life.

Credit: NASA, JPL-Caltech, T. Pyle

BEIJING — And then there were two. The Kepler spacecraft has spied the first pair of planets passing in front of the binary star system they orbit. Adding spice is that the outer planet — a potential Neptune-like world — inhabits the life-friendly zone around the two stars.

“It receives about 88 percent the amount of energy the Earth receives from the sun,” said William Welsh of San Diego State University on August 29 at the International Astronomical Union meeting. “And it’s a multiple planet system. It’s hard enough to imagine how you get one planet in the binary; now we have two.”

The system, called Kepler-47, could have even more planets: A tantalizing but unconfirmed hint of an additional world lurks in the blinking starlight produced when the planetary companions pass between the two stars and Earth. The additional blink has been seen clearly just once, so more observing time would be needed to confirm a third planet. 

Kepler-47, which is also described online August 28 in Science, further demonstrates the hardiness of planets. "Circumbinary multiple-planet systems were fully to be expected, given that single planet versions have been found," says astronomer Greg Laughlin of the University of California, Santa Cruz. "But it's exciting nonetheless." Now there's evidence that more than one planet at a time can form and survive in the tumultuous environment around a binary star.

So far, scientists know that the outer planet, Kepler-47c, is roughly 4.6 times wider than Earth and that it goes around the stars every 303 days. The inner planet, Kepler-47b, is three times wider than Earth, and whips around the stars every 49 days. One of the stars is similar to the sun, and the other is much smaller and dimmer. The two stars orbit one another in roughly 7.5 days, a whirling duo some 5,000 light-years away in the constellation Cygnus.

Determining the boundaries of the habitable zone in binary systems isn’t as simple as for single stars, because the moving stars create a shifting region in which liquid water could survive on an orbiting planet. 

At the meeting, several scientists talked about ways to determine where the habitable zone lies in binary star systems — including those in which a planet orbits just one of the stars. In such a case, the second star can influence the amount of energy a planet receives, though indirectly. "The second star never comes that close to the planet; otherwise the system would be unstable," said Elke Pilat-Lohinger of the University of Vienna. Instead, the second star's gravity can yank or push on the planet, moving it around relative to its host and thus changing the habitable zone.

For planets circling two stars, habitable zones mostly depend on the average amount of energy the stellar pair produces, though the extreme temperatures a planet might encounter are also important, said Welsh.

Whether a planet in the life-friendly zone can host life ultimately depends on the planet itself — what it's made of, what its atmosphere is, and whether its clouds are insulating, said Nader Haghighipour of the University of Hawaii at Manoa. "It depends on your central star and also on your planet," he said.

So though Kepler-47c is clearly in the habitable zone, the planet itself is probably too big and gassy to host life. But that doesn't mean a large, Earth-sized moon couldn't serve as an exo-incubator. “It’s pure speculation,” Welsh said. “But being in the habitable zone, if it had a big moon around it, it’s in the right place to have the conditions you would need for life.”

There is no evidence of such a large moon, but smaller ones, more like Saturn’s moon Titan, could be present. “If this object had a moon the size of Titan, that could be very interesting,” Welsh said.