A team of astronomers has found the first transiting circumbinary multi-planet system: two planets orbiting around a pair of stars. This discovery shows that planetary systems can form and survive even in the chaotic environment around a binary star. And that such planets can exist in the habitable zone of their stars. The results are being announced at the XXVIII General Assembly of the International Astronomical Union (IAU) in Beijing, China and published in the 30 August 2012 issue of the journal Science.
The system, known as Kepler-47, harbours the smallest known transiting circumbinary planets — planets orbiting a pair of stars — to date. The planets were discovered using NASA’s Kepler space telescope  by monitoring the faint drop in brightness produced when both planets transit (eclipse) their host stars .
“In contrast to a single planet orbiting a single star, planets whirling around a binary system transit a moving target,” explains Jerome Orosz (San Diego State University, USA), lead author of the study. ”The time intervals between the transits and their duration can vary substantially, from days to hours, and therefore the extremely precise and almost continuous observations with Kepler space telescope were fundamental.”
This planetary system is located roughly 5000 light-years away from Earth, in the constellation of Cygnus (The Swan). The pair of stars whirls around each other every 7.5 days. One star is similar to our Sun while the other is a diminutive star only one third the size and 175 times fainter.
Thanks to Kepler’s observations, astronomers were able to characterise the planetary system. The inner planet — Kepler-47b — is only three times larger in diameter than the Earth and orbits the stellar pair every 49 days. The outer planet — Kepler-47c — is about 4.5 times the size of the Earth — slightly larger than Uranus — and orbits the stars every 303 days. This makes the outer planet the longest-period transiting planet currently known.
More importantly, the outer planet's orbit places the planet well within what astronomers refer to as the habitable zone — the region around a star within a terrestrial planet that could have liquid water on its surface.
“While the outer planet is probably a gas giant planet and thus not suitable for life, large moons, if present, would be interesting worlds to investigate as they could potentially harbour life,” says William Welsh (San Diego State University, USA), co-author of the study.
Since both planets are rather small, they do not gravitationally disturb the stars or each other measurably. Hence their masses cannot be directly measured. However, astronomers can place upper limits to their masses, showing that these small objects are certainly planets and not brown dwarfs . Based on their size, the inner and outer planets probably have masses of approximately 8 and 20 times that of the Earth, respectively.
“Since about one third of all stars are either binary or multiple star systems, finding planets in binary star systems has very important implications not only for estimating the total numbers of planets that exist, but for how star–planet systems form as well,“ concludes Jerome Orosz.
 Kepler is a space observatory operated by NASA aimed to survey a portion of our region of the Milky Way galaxy to discover Earth-like planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets.
 The loss of light caused by the eclipse is tiny, only 0.08% for Kepler-47b and 0.2% for Kepler-47c. By comparison, Venus blocked about 0.1% of the Sun's surface during its recent transit on 5 June 2012.
 Brown dwarfs are objects between stars and planets in size. They are not massive enough to fuse hydrogen in their cores but are larger than giant planets such as Jupiter.
The IAU is an international astronomical organisation of about 10 000 professional astronomers from 90 countries. Its mission is to promote and safeguard the science of astronomy in all its aspects through international cooperation. The IAU also serves as the internationally recognised authority for assigning designations to celestial bodies and surface features on them.
Jerome A. Orosz
San Diego State University
San Diego, USA
Tel: +1 619 594 7118
William F. Welsh
San Diego State University
San Diego, USA
Lars Lindberg Christensen
IAU Press Officer / ESO ePOD
Garching bei München, Germany
Tel: +49 89 3200 6761
Cell: +49 173 3872 621