Largest Ever “Tatooine” World Was Discovered by Astronomers

June 14, 2016 | Joanne Kennell

Two stars behind a planet
Photo credit: Lynette Cook

It is also in the “habitable zone.”

What do Luke Skywalker’s home planet of Tatooine and a newly discovered planet in the constellation Cygnus have in common? Two suns, of course.

Although the double-star system, Kepler-1647, is too faint to see with the naked eye, a team of astronomers, led by NASA’s Goddard Space Flight Center, identified the new planet, a gas giant with the same mass and radius as Jupiter, using the Kepler Space Telescope.

The binary system, located 3,700 light-years away, consists of two stars that are similar to our sun, except that one is slightly larger and the other is slightly smaller. At an estimated 4.4 billion years old, the planet, named Kepler-1647b, is roughly the same age as Earth. But that is all the two planets have in common.

Planets that orbit two stars, like Kepler-1647b, are known as circumbinary planets, or “Tatooine” planets, named after Luke Skywalker’s home world in Star Wars. To spot an exoplanet, astronomers rely on Kepler data to search for slight drops in the star’s brightness. This can signal that a planet is passing in front of the star, blocking a teeny-tiny amount of its light.

But finding circumbinary planets (CPBs) using this method is rather difficult. "CBPs are harder to detect because their transits are not strictly periodic," Veselin Kostov, a NASA Goddard postdoctoral fellow and lead author of the study, told The Washington Post. "A planet around a single star is like a clock."

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From outside our solar system, Earth for example, would transit in front of the sun once every 365 days. But it is not so simple with CBPs. "In contrast, when a circumbinary planet transits, the consecutive events can be early or late by many days. Sometimes, the transits of a CBP can even cease altogether, only to reappear decades, even centuries later," Kostov continued.

Kepler-1647b was really tough to spot. Although it is the largest circumbinary planet ever discovered (seen below), it also has the longest orbit of any CBP. The planet takes 1,107 days — just over 3 years — to orbit its host stars.

Comparison of the relative sizes of several Kepler circumbinary planets

Comparison of the relative sizes of several Kepler circumbinary planets. Kepler-1647b is the largest discovered so far. Photo credit: Lynette Cook

Interestingly, Kepler-1647b is also a lot further away from its stars than any other circumbinary planet. In fact, it’s bizarre orbit puts the planet in the “habitable zone” — where liquid water could be on the planet’s surface. However, Kepler-1647b is a gas giant like Jupiter, so it likely doesn’t host life. On the other hand, if the planet has large, rocky moons, that could be an entirely different story.

"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," said San Diego State University astronomer William Welsh, one of the paper's co-authors, in a NASA press release.

The research has been accepted for publication in the Astrophysical Journal.

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