Kepler discovers its 10th twin-sunned Tatooine

The discovery of a planet orbiting a binary star in the habitable zone is the tenth of its kind, indicating such planets are more common than previously thought.

Illustration of Kepler-453b and its twin stars.© Mark Garlick. Used with permission

Four years ago, when the Kepler mission helped locate Kepler-16b, the news was incredible. For the first time, we had confirmed a planet orbiting two stars, an idea that seemed at home on the twin-sunned "Star Wars" planet Tatooine.

Now, researchers have announced the discovery of the 10th such system in what seems a relatively short space of time. Kepler-453b, of the system Kepler-453, is what is known as a circumbinary planet, orbiting two stars which are themselves locked into orbit with each other.

The research was published in The Astrophysical Journal.

The discovery of Kepler-453b was an extraordinarily lucky one. Because it orbits two stars, the planet's tilt is wobbly and erratic. Exoplanets are already hard to see, usually located by what is known as the "transit method", which allows astronomers to detect when a planet is passing in between us and a star when that star's light dims. Because of Kepler 453b's wobble only 9 percent of its transits are detectable.

The next time Kepler-453b will be visible to astronomers won't be until the year 2066.

"If we had observed this planet earlier or later than we did, we would have seen nothing and assumed there was no planet there," said Stephen Kane, assistant professor of physics and astronomy at San Francisco State University, and a member of the team that discovered Kepler-453b. "That suggests that there are a lot more of these kinds of planets than we are thinking, and we're just looking at the wrong time."

Because the precession (that orbital wobble) is caused by the binary star, this means that there may be a lot of binary stars out there with planets. Jerome Orosz, a professor of astronomy of San Diego State University and paper co-author, said the number could be over 100.

"The low probability for witnessing transits means that for every system like Kepler-453 we see, there are likely to be 11 times as many that we don't see," he explained.

Kepler-453b is located in the habitable zone of binary system Kepler-453, which means it is at a distance from the stars where the temperature is right for liquid water: not so close to suns as to evaporate into steam, and not so far as to freeze into ice. It is the third Kepler circumbinary planet to be located in the habitable zone, but alas, it is not Earth-like. Kepler-453b is a gas giant, with a mass estimated to be under 16 times that of the Earth.

For comparison, Neptune has a mass 17 times that of the Earth, but Kepler-453b is bigger. It blocked 0.5 percent of the light of Kepler-453 in transit, which allowed the team to calculate that its radius is 6.2 times that of the Earth -- some 60 percent bigger than Neptune.

But life could still exist in Kepler-453's system, Kane said.

"[Kepler-453b] could have moons that are rocky, which means you could have life on the moons in this system," he explained.

The two stars are very different from one another. The larger is about 94 percent the size of the sun, while the smaller is about 20 percent the size of the sun and much cooler, emitting less than 1 percent of the energy emitted by the larger star. The two stars orbit each other every 27 Earth days, and Kepler-453b orbits them both every 240 Earth days.

"The systems tend to be very compact and they come in a wide range of configurations. The diversity and complexity of these circumbinary systems are wonderful," said study lead author William Welsh, a professor of astronomy at San Diego State University. "Each new circumbinary planet is a gem, revealing something unexpected and challenging."

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