Planets that orbit two stars have traditionally been difficult to detect. Despite decades of suspicion, we didn't even spot our first one until 2011 and even now their irregular orbits make life tricky for those in the planet-hunting game. NASA has today confirmed the discovery of the largest of these circumbinary planets, the imaginatively named Kepler-1647 b, some 3,700 light years away.

When looking to confirm the existence of an exoplanet, scientists look through a telescope for changes in brightness as it passes in front of a star. This is hard enough when hunting for single-star planets, with loads of follow up observations required to verify its existence. The paths taken by circumbinary planets add yet another layer of complexity.

"Finding circumbinary planets is much harder than finding planets around single stars," says San Diego State University astronomer William Welsh, one of the paper's coauthors. "The transits are not regularly spaced in time and they can vary in duration and even depth."

Laurance Doyle, coauthor on the paper and astronomer at the SETI Institute, first noticed the transit of what came to be called Kepler-1647 b using the Kepler Space Telescope in 2011. It wasn't until now, following years of crowdsourced observations from the group of amateur astronomers called the KELT Follow-Up Network, that NASA was able to confirm its existence.

Around 4.4 billion years old, Kepler-1647 b has a mass and radius almost identical to that of Jupiter and circles stars similar in size to our Sun, one slightly bigger and one slightly smaller. Also like Jupiter, the exoplanet is a gas giant and is therefore not likely to host life. It does, however, sit in the habitable zone and the researchers say if it turns out to have large moons it is possible they could harbor life.

Kepler has detected more than 2,300 exoplanets since it entered service in March 2009. The total number of verified exoplanets sits at 3,200, but Kepler's latest scalp has the longest orbit of any of them, at 1,107 days. The team says this is what made it so difficult to detect in spite of its large size. The planet also orbits its stars at a much larger distance than any other circumbinary planet, which are referred to by some as Tatooine planets in a tip of the hat to Luke Skywalker's homeland.

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

The research will be published in the Astrophysical Journal, and can be read online here.

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