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Planets discovered with double Sun-like stars
Posted: 11 January 2012

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Following the recent discovery of exoplanet Kepler-16b orbiting two stars, two further planets both orbiting stellar pairs have been unveiled by the acclaimed spacecraft.

Kepler-16 was announced in September 2011 (read our news story here) and from a sample of 750 systems using 671 days of Kepler data, William Welsh of San Diego State University and colleagues have found two further circumbinary gas planets, Kepler-34 b and Kepler-35 b. The results are reported in the 11 January issue of the journal Nature.

Artist impression of a planet orbiting two stars. The discovery of Kepler-34 and Kepler-35 establishes a new class of "circumbinary planets", and suggests many millions of such systems exist in our Galaxy. Image: Illustration by Lynette Cook.

Detecting these planetary systems presents its own unique challenges. For a transit-detecting spacecraft like Kepler, it looks for the characteristic dip in brightness of the host star as a planet passes in front of it, a phenomenon that repeats on a regular basis. This enables scientists to plot a light-curve, that is, a graph of the brightness of the star over time. "Now imagine a light curve of an eclipsing binary star, like Kepler 34," explains co-author Jerome Orosz. "Every 27.8 days, you see a large dip in the brightness of nearly 50 percent due to primary eclipses (the dimmer star passing in front of the brighter, primary star), and in between those you see 40 percent dips due to the secondary eclipses (the primary star passing in front of the dimmer, secondary star). These eclipses last several hours, during which it is very hard to see dips that would be due to planets. And because of the geometries involved, the transits of the planets in a circumbinary system don't have to be strictly periodic."

Orosz explains that for the case of two stars orbiting each other, the eclipses should occur like clockwork, but if you add a third body – such as a planet – this can perturb the stars and change their orbits slightly.  "This in turn causes the eclipse times (as seen from Earth) to change slightly, and the graph of the eclipse time versus the cycle count number is not exactly straight."

The discovery of the Kepler-34 and 35 planets came from identifying the graphs where the eclipse times deviated from a perfect line. Kepler-34 b, which is 22 percent the mass of Jupiter, orbits two Sun-like stars every 289 days, while Kepler-35 b is a planet with 13 percent of Jupiter's mass that orbits a pair of smaller stars every 131 days. The stars of Kepler-34 are separated by 0.22 astronomical units (AU, the Earth-Sun separation) and have an orbital period of 28 days, while those of Kepler-35 are separated by 0.17 AU, and have an orbital period of 21 days.

The planets experience large variations in sunlight as a result of their host stars' orbital motion, leading to complex climate cycles that are unique to circumbinary planets. "You would see swings in the received sunlight that are not quite periodic with time," Orosz tells Astronomy Now. "For example in Kepler-34b, the change in solar radiation varies from about twice that of the Earth to about four times over a time scale of six months. In the case of Kepler-35b, the insolation changes from about three to five times the Earth equivalent over a time scale of about five months. Also, you would observe eclipses of the stars – two per orbital cycle – if you lived on either planet, so every 14 days or so for Kepler-34b your insolation would drop by a factor of two for a few hours during the eclipse."

Kepler-34b and -35b lie interior to the habitable zone of their stars – the region around a star where liquid water is stable – while Kepler-16b lies just outside the habitable zone of its star. "The distance between the planet and each star is always changing [for circumbinary planets], thus, at any given time, the effective habitability zone also changes," says Orosz. "The stellar orbits are somewhat elliptical, so the separations between the two stars are changing. In addition, the orbit of the planet is not quite circular in the case of Kepler-35, so the distance it is from the system's centre of mass changes. Things get complicated very quickly!"

Orosz adds that to find a circumbinary planet in the habitable zone, you would want a binary star with a short period (a few days) orbit that is circular, with a planet also on a near-circular orbit.

Although many Sun-like stars in the Galaxy reside in binary systems, the prevalence of planets orbiting in such systems is unclear but, as Welsh et al report, "A simple argument suggests that circumbinary giant planets are not extremely rare, as three such objects have been seen in our sample of 750 systems." Indeed, the scientists estimate that approximately one percent of close binary stars have nearby giant planets with closely aligned orbits like those presenting themselves already, meaning that several million cases must exist across the whole Galaxy.