Astronomers discover largest planet orbiting two suns

Astronomers using the U.S. Kepler Space Telescope said Monday they have found the largest planet yet discovered that orbits a pair of binary stars.

The new planet, Kepler-1647b, is 3,700 light-years away and about 4.4 billion years old, roughly the same age as the Earth, according to a team led by researchers from the U.S. space agency NASA's Goddard Space Flight Center and San Diego State University (SDSU).

The planet has a mass and radius nearly identical to that of Jupiter, making it the largest transiting circumbinary planet ever found.

The stars it orbits are similar to the Sun, with one slightly larger than our home star and the other slightly smaller.

Planets orbiting two stars are called circumbinary planets, or sometimes "Tatooine" planets, after Luke Skywalker's homeland in "Star Wars."

Using NASA's Kepler telescope, astronomers looked for slight dips in brightness that hint a planet might be transiting in front of a star, blocking some of the star's light.

"But finding circumbinary planets is much harder than finding planets around single stars," said coauthor William Welsh of the SDSU. "The transits are not regularly spaced in time and they can vary in duration and even depth."

The new planet was first noticed in 2011, but it took the team years to confirm it's indeed a circumbinary planet, with the help of a network of amateur astronomers in the KELT Follow-Up Network, which consists of small and mid-size telescopes used for confirming transiting planets.

"It's a bit curious that this biggest planet took so long to confirm, since it is easier to find big planets than small ones," said SDSU astronomer Jerome Orosz, another coauthor on the study. "It took so long to confirm because its orbital period is so long."

The planet takes 1,107 days -- just over three years -- to orbit its host stars, the longest period of any confirmed transiting exoplanet found so far.

The planet is also much further away from its stars than any other circumbinary planet, breaking with the tendency for circumbinary planets to have close-in orbits.

Interestingly, Kepler-1647b's orbit puts the planet within the so-called habitable zone, but like Jupiter, it's a gas giant, making the planet unlikely to host life.

Yet if the planet has large moons, they could potentially be suitable for life.

"Habitability aside, Kepler-1647b is important because it is the tip of the iceberg of a theoretically predicted population of large, long-period circumbinary planets," Welsh said.

The discovery was announced in San Diego at a meeting of the American Astronomical Society and has been accepted for publication in the Astrophysical Journal. Enditem