Astronomers at the International Astronomical Union meeting announced the discovery of the first transiting circumbinary multi-planet system: two planets orbiting around a pair of stars. The discovery shows that planetary systems can form and survive even in the chaotic environment around a binary star. And such planets can exist in the habitable zone of their stars.

"Each planet transits over the primary star, giving unambiguous evidence that the planets are real," said Jerome Orosz, Associate Professor of Astronomy at San Diego State University and lead author of the study which is published in the journal Science.

The system, known as Kepler-47, contains a pair of stars whirling around each other every 7.5 days. One star is similar to the Sun while the other is a diminutive star only one third the size and 175 times fainter.

The inner planet is only 3x larger in diameter than the Earth, making it the smallest known transiting circumbinary planet. It orbits the stellar pair every 49 days.

The outer planet is slightly larger than Uranus and orbits every 303 days, making it the longest-period transiting planet currently known. More importantly, its orbit puts it in the "habitable zone", the region around a star where a terrestrial planet could have liquid water on its surface.

While the planet is probably a gas-giant planet and thus not suitable for life, its discovery establishes that circumbinary planets can, and do, exist in habitable zones.

Although much more difficult to detect than planets around single stars, the rich dynamics and wild climate changes make these circumbinary planets worth the effort to find. These two planets join the elite group of 4 previously known transiting circumbinary planets, Kepler-16, 34, 35 and 38.

The new planetary system is located roughly 5000 light-years away, in the constellation Cygnus. The planets are much too far away to see, so they were discovered by the drop in brightness they cause when they transit (eclipse) their host stars.

The loss of light caused by the silhouette is tiny, only 0.08% for planet b and 0.2% for planet c. By comparison, Venus blocked about 0.1% of the Sun's surface during its recent transit.

Precise photometric data from NASA's Kepler space telescope allowed the transits and eclipses to be measured, which in turn provided the relative sizes of the objects.

Spectroscopic data from telescopes at McDonald Observatory in Texas enabled the absolute sizes to be determined. "Based on their radii, these probably have masses of approximately 8 and 20 times that of the Earth," Orosz said.

"Kepler-47 shows us that typical planetary architectures, with multiple planets in co-planar orbits, can form around two stars," said co-author Joshua Carter, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics.

"We've learned that circumbinary planets can be like the planets in our own Solar System, but with two suns."

The work was presented at the International Astronomical Union meeting by Dr. William Welsh, Professor of Astronomy at San Diego State University, on behalf of the Kepler Science Team.

"The thing I find most exciting," said Welsh, "is the potential for habitability in a circumbinary system. Kepler-47c is not likely to harbor life, but if it had large moons, those would be very interesting worlds."

Funding for this work was provided in part by NASA and the National Science Foundation. "Kepler-47: A Transiting Circumbinary Multi-planet System" by J. A. Orosz, et al. is published on-line in Science Express here.