NASA's astroboffins spot the largest ever Tatooine planet

Kepler 1647-b bigger than Jupiter, circles two stars... just like Tatooine

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A team of astronomers led by NASA have discovered the largest circumbinary planet on record, according to the American Astronomical Society.

The research has been accepted for publication in the Astrophysical Journal with Veselin Kostov, a NASA Goddard postdoctoral fellow, as lead author.

Circumbinary planets are sometimes called “Tatooine” planets, named after the planet that is home to Luke Skywalker in Star Wars, and has two stars in its sky - since they orbit around a twin set of stars.

With a radius that is slightly larger than Jupiter, Kepler 1647-b is the largest circumbinary planet found to date. It also has the longest orbit of any circumbinary planet, Kepler 1647-b takes 1,107 days - just under three years - to run a lap around its stars.

Despite its bulk, however, it took astronomers three years to confirm its planetary status due to its long orbital period, and was only spotted by the telescope on board the Kepler spacecraft twice over its mission's lifetime.

"It's a bit curious that this biggest planet took so long to confirm," said Jerome Orosz, co-author on the study and astronomer at San Diego State University.

Laurance Doyle, co-author of the paper and a research scientist at the Search for Extraterrestrial Intelligence (SETI) institute, was the first to notice a foreign object cruising across the binary star system back in 2011. But it was not considered a circumbinary planet until 2014.

When Kepler 1647-b transits, it travels along both stars and blocks out some of the brightness from the binary system. Astronomers detect this as a dip in the stars’ lightcurve.

“When one sees an extra dip in the light curve of the eclipsing binary [system], one knows (assuming no instrumental error) that it is either: 1) a background eclipsing binary star, 2) a third star grazing the system, or 3) a circumbinary planet.

“One can look for and identify a background eclipsing binary star, for example, by high resolution imaging. One can see the tugging on the eclipsing binary by a third star because of its mass. If there is no tug showing up (in the changes in times of eclipses of the eclipsing binary) then it is a circumbinary planet,” Doyle told The Register.

Once a candidate planet is found, researchers run a computer programme to model the object to determine if it is truly a planet. Circumbinary planets can be tricky to model as the binary stars are circling each other whilst the planet is transiting, so no two transits are the same.

Circumbinary systems are amongst the most precisely determined systems in space, since the eclipsing binary light curve can be used to determine properties in stars and planets. Kepler 1647-b is 3,700 light-years away and is approximately 4.4 billion years old. The stars have an 11 day orbital period - considerably shorter than the planetary orbit. One star is slightly bigger than the Sun, whilst the other is slightly smaller.

Although Kepler 1647-b was found in the stars’ ‘habitable zone’, as a gas giant with no solid surface, it is unlikely that it can support life. The prospect hasn’t been ruled out, however, as astronomers think that if it has a large enough gravitational pull - enough to attract large moons - it may be suitable for life.

Interestingly, about 40 per cent of circumbinary planets are found in habitable zones, said Doyle, who added, “I’d like to find out why.”

Kostov, said he hoped to "use the results from Kepler to predict circumbinary planet discoveries from TESS (a new NASA exoplanet mission to fly next year), then go and find them." ®


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