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Kepler-47: Our First Binary Star 2-Planet System
artist's impression of Kepler-47 system
An artists depiction of the Kepler-47 system. Credit: NASA/JPL-Caltech/T. Pyle
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Almost one year ago, 2011 September 15, the discovery of Kepler-16b was our first binary star (two stars orbiting each other) with a planet orbiting both stars (circumbinary). Now Kepler mission has discovered Kepler-47b and 47c, the first transiting circumbinary system — multiple planets orbiting two suns. To compound the excitement of the discovery, one of those planets is in the binary system's habitable zone (where liquid water may exist)!

Kepler-47 system diagram
Kepler-47 system diagram. Credit: NASA/JPL-Caltech/T. Pyle
"The presence of a full-fledged circumbinary planetary system orbiting Kepler-47 is an amazing discovery," Greg Laughlin, professor of Astrophysics and Planetary Science, University of California, Santa Cruz, Calif. "These planets are very difficult to form using the currently accepted paradigm, and I believe that theorists, myself included, will be going back to the drawing board to try to improve our understanding of how planets are assembled in dusty circumbinary disks."

"Unlike our Sun, many stars are part of multiple-star systems where two or more stars orbit one another. The question has always been: do they have planets and planetary systems? This Kepler discovery proves that they do," said William Borucki, Kepler mission principal investigator at NASA's Ames Research Center, Moffett Field, Calif. "In our search for habitable planets, we have found more opportunities for life to exist."

Photo of Jerry Orosz
Jerome A. Orosz
Jerome Orosz, associate professor of astronomy at San Diego State University and lead author of the discovery paper published in Science, explained that "In contrast to a single planet orbiting a single star, the planet in a circumbinary system must transit a 'moving target.' As a consequence, time intervals between the transits and their durations can vary substantially, sometimes short, other times long. That was the telltale sign that these planets are in circumbinary orbits."

While the inner planet, Kepler-47b, orbits in less than 50 days and must be a sweltering world, the outer planet, Kepler-47c, orbits every 303 days, putting it in the "habitable zone," where liquid water might exist. But Kepler-47c is slightly larger than Neptune, and hence in the realm of gaseous giant planets, difficult to imagine as suitable for life. That does not preclude the chance that it has large a moon with a solid surface and liquid water lakes or seas. Kepler-16b was likened to Tatooine, Luke Skywalker's home planet in the movie Star Wars—a world with a double sunset. Kepler-47c suggests a different possible scene: our hero standing on a moon, gazing at a double sunset, with a Neptune-class planet rising behind her.

The research team used data from the Kepler space telescope, which measures dips in the brightness of more than 150,000 stars, to search for transiting planets. Using ground-based telescopes at the McDonald Observatory at the University of Texas at Austin, they made crucial spectroscopic observations to determine characteristics of the stars in the binary system which is 4,900 light-years from Earth. They are orbiting each other very fast, eclipsing each other every 7.5 days. One star is similar to the Sun in size, but only 84 percent as bright. The second star is a red dwarf star only one-third the size of the Sun and less than one percent as bright.

photo of William Wlesh
William F. Welsh
According to William Welsh, a co-author of the discovery paper, the number of planets discovered in multiple star systems is growing—about 70 to date. In binary star systems, planets are of two types: P-type: a planet that orbits both stars (circumbinary, like Kepler-47b and c)
S-type: a planet that orbits just one of the stars (referred to as circumstellar)

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