Scientists find 1st planetary system with 2 suns

BEIJING - A team of astronomers has found two planets orbiting around a pair of stars roughly 5,000 light years away from the Earth, declaring it the first multi-planet system centering a binary star.

Named as Kepler-47, the transiting circumbinary multi-planet system, as it is academically described, has made the scientists believe that planetary systems can form and survive even in the chaotic environment around a binary star, according to a study published in the journal Science on Wednesday.

"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 of the United States and lead author of the study.

The discovery, also announced at the 28th General Assembly of the International Astronomical Union held in Beijing from August 20 to 31, demonstrates that such circumbinary planets - planets that orbit two stars instead of one -- can exist in the habitable zone of their stars.

The system, in the constellation of Cygnus (the Swan), 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, Kepler-47b, is only three times 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, Kepler-47c, is about 4.5 times the size of the Earth - slightly larger than Uranus - and orbits the stars every 303 days, which makes it the longest-period transiting planet currently known.

Most importantly, the outer planet orbits in places well within what astronomers refer to as the "habitable zone," the region around a star where a terrestrial planet 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 harbor life," says William Welsh, co-author of the study from San Diego State University.

The planets were discovered using NASA's Kepler space telescope. They are much too far away to see, so they were found by the drop in brightness when they transit (eclipse) their host stars.

Precise photometric data from 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 size, the inner and outer planets probably have masses of approximately eight and 20 times that of the Earth, Orosz said.

Before their discovery, binary star systems were found only with one planet.

"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."

"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," Jerome Orosz said.