Exoplanets—planets in star systems other than our own—have been found in orbit around single stars, with a lone exception: Kepler-16b is circumbinary, having two host stars in close orbit. Now, researchers working with data from the Kepler space-based observatory have identified two more promising exoplanet candidates orbiting binary stars, known as Kepler-34b and Kepler-35b. While the planets most likely are gas giants, the host stars in Kepler-34b are more Sun-like than the Kepler-16 system. These observations indicate that giant exoplanets orbiting two stars may be fairly common, occurring in perhaps one percent of close binaries.

In some ways, our Sun is unusual: stars with similar masses are more commonly found in binary systems. Lower-mass stars are more often found on their own and, since they are also less luminous than the Sun, it has been easier for astronomers to detect planets orbiting them. Finding circumbinary planets is plagued by two major difficulties: both the gravitational pull by the exoplanet and the amount of light it blocks if it passes between us and its hosts are tiny in comparison with the effect the two stars have on each other.

Nevertheless, the instruments aboard the Kepler spacecraft are sensitive enough to detect the small dip in the light that comes from an exoplanet transiting a binary star. As with all transits, astronomers must be lucky: the planet's orbit must lie edge-on to us, or else the planet will never cross between its host star(s) and Earth. The Kepler team identified 750 eclipsing binary systems in which the stars are in close mutual orbit, and examined the data for smaller, secondary eclipses. Kepler-34 and Kepler-35 both showed clear indications that a planet is present; a third system (KOI-2939) could not be identified positively as containing a planet.

Kepler-34b is roughly 22 percent of the mass of Jupiter (69 times Earth's mass) and 76 percent of Jupiter's radius (8.6 times larger than the radius of Earth); Kepler-35b is about 13 percent of Jupiter's mass and 73 percent of its radius. Both these planets are less massive than Kepler-16b (which is 33 percent of Jupiter's mass), but all three worlds are giants rather than Earth-like. Unlike the fictional Tatooine from Star Wars, they will lack a solid surface on which to stand.

The host stars in Kepler-34 are both very slightly more massive than the Sun, and therefore will have nearly the same light output. Kepler-35's stars are about 89 percent and 81 percent the mass of the Sun, respectively, making them correspondingly redder and fainter. In both systems, the stars orbit each other rapidly, completing one cycle in 28 days (Kepler-34) and 21 days (Kepler-35).

Kepler-34b orbits its host stars in 289 days, while Kepler-35b has a year 131 days long. Both Kepler-34b and Kepler-35b—and any planet in a binary system—will experience large variations in the light they receive as their host stars eclipse each other, resulting in fluctuations in temperature at the tops of their atmospheres. Neither world lies within the habitable zone (the region where liquid water can exist on a surface) of its star system; since two stars are involved, the habitable zone is much farther out than in our Solar System.

Finding Earth-mass planets is a difficult proposition even in single-star systems, much less binary systems, so Luke Skywalker will need to wait a little longer before we find his home planet. Nevertheless, by establishing how common circumbinary exoplanets are in our galaxy, researchers have come closer to finding Tatooine-like worlds. With three probable exoplanets in a sample of 750 close binary systems, there may be millions of circumbinary exoplanets in our galaxy alone.

Nature, 2012. DOI: 10.1038/nature10768 (About DOIs).