A team of astronomers, including University of Hawaii astronomer Nader Haghighipour, will announce on June 13 the discovery of an unusual new transiting circumbinary planet (orbiting two suns). This planet, detected using the Kepler spacecraft, is unusual because it is both the largest such planet found to date, and has the widest orbit.

Reminiscent of the fictional planet Tatooine in Star Wars, circumbinary planets orbit two stars and have two "suns" in their skies. The new planet, Kepler-1647b, is Jupiter-sized in radius, the largest of all currently known circumbinary planets, and has an orbital period of 3.0 years, the longest of any confirmed transiting planet.

Nearly half of all Sun-like stars are members of gravitationally-bound binary star systems. The most important subset of these systems are the eclipsing binary stars (stars that pass in front of each other, as seen from Earth), because they provide precise stellar masses and sizes. NASA's Kepler Mission has observed nearly 3,000 short-period (less than 1,000 days) eclipsing binaries. Among these binaries, only nine have been found to host circumbinary planets, including Kepler-1647.

The detection of Kepler-1647b is significant for two reasons. First, its large size and large orbit are very different from those of all other known circumbinary planets. All of the previously identified Kepler circumbinary planets are Saturn-sized or smaller.

Also, most of these planets tend to orbit close to their host binaries, near the so-called "critical instability radius," where if they orbited any closer, the planet's orbit would be dynamically unstable and the planet would be ejected from the system or crash into one of the stars.

This apparent lack of large circumbinary planets and their proximity to the critical stability limit have left astronomers puzzled. Theoretical models of the formation and dynamical evolution of these planets, developed by Haghighipour and his colleagues, have suggested that "circumbinary planets form the same way planets form around single stars, suggesting that these planets must exist in large sizes, with the larger ones residing far from their host binaries," Haghighipour said. Being a Jupiter-sized planet, the discovery of Kepler-1647b confirms these predictions.

The second significance of the discovery of Kepler-1647b is that this planet resides in the habitable zone of its host binary, a surprisingly common occurrence for circumbinary planets discovered by the Kepler Space Telescope.

Although Kepler-1647b's large size - similar to Jupiter - means the planet itself is not habitable, "a giant planet similar to those in our solar system may well host undetected large moons that could be habitable" Haghighipour said. According to Haghighipour, the orbit of Kepler-1647b will remain stable for at least tens of millions of years, increasing the possibility of life forming on its moons.

The Kepler-1647b system is 3,700 light-years from Earth, and lies in the direction of the constellation Cygnus. It is estimated to be approximately 4.4 billion years old, roughly the same age as Earth.

The discovery comes five years after the first transiting circumbinary planet, Kepler-16b, was detected. The long wait is because transiting circumbinary planets on long-period orbits (far from their binaries) are much more difficult to detect than close-in planets. Prior to Kepler-1647b, no long-period circumbinary planets had been found.

"The detection of Kepler-1647b has been very special to us because after several years of developing a theoretical framework for the formation and evolution of circumbinary planets, we have finally found the objects predicted by our models," said Haghighipour.

The team is led by Dr. Veselin Kostov, a researcher at the NASA Goddard Space Flight Center in Maryland. The work was recently accepted for publication in the Astrophysical Journal The announcement will be made at the annual meeting of the American Astronomical Society in San Diego.