Exoplanets that orbit binary stars are known as circumbinary planets, or sometimes ‘Tatooine’ planets, after Luke Skywalker’s home world in Star Wars.
Using data from NASA’s Kepler/K2 mission, scientists search for slight dips in brightness that hint an exoplanet might be passing or transiting in front of its host star, blocking a tiny amount of the star’s light.
“But finding circumbinary planets is much harder than finding planets around single stars,” said co-author Dr. William Welsh of San Diego State University.
“The transits are not regularly spaced in time and they can vary in duration and even depth.”
The newfound planet, named Kepler-1647b, has a mass of 1.06 Jupiter masses and is slightly larger than Solar System’s largest gas giant (1.52 Jupiter radii), making it the largest transiting circumbinary planet ever found.
It revolves around Kepler-1647, an 11-day period eclipsing binary consisting of two stars, on a slightly inclined, mildly eccentric orbit.
The two stars, about 3,700 light-years from Earth in the constellation Cygnus, are approximately 4.4 billion years old and are similar to the Sun, with one slightly larger than our home star and the other slightly smaller.
Kepler-1647b circles them both, taking 1,107 days (just over 3 years) to complete an orbit — 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.
Despite having an orbital period three times longer than Earth’s, Kepler-1647b is in the habitable zone of its host pair of stars.
Like Jupiter, however, the planet is a gas giant, making the planet unlikely to host life.
Yet if Kepler-1647b 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,” Dr. Welsh said.