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Tatooine-like planet: Astronomers spot pair of planets orbiting twin suns (+video)

Tatooine-like planet: In a cosmic first, astronomers have discovered two alien planets orbiting two stars, not unlike Luke Skywalker's home planet in the Star Wars films.

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One of the stars is much like our sun, and the other is about a third its size and 175 times fainter. The inner and outer planets are respectively 3 and 4.6 times Earth's diameter — the smaller planet is the smallest circumbinary planet seen yet. [Tatooine-Like System Found, 2 Planets, 2 Stars (Video)]

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The inner world completes an orbit every 49.5 days, while the outer one takes 303.2 days, giving it the largest known orbit for any transiting exoplanet. The stars themselves whirl around each other every 7.5 days.

The scientists published their findings online Aug. 28 in the journal Science. They will also detail their results Aug. 29 at the General Assembly of the International Astronomical Union in Beijing.

Planet in the habitable zone?

Intriguingly, the outer planet lurks in the system's habitable zone, where a rocky planet like Earth is the right temperature to have liquid water on its surface.

"We've learned that circumbinary planets can be like the planets in our own solar system, but with two suns," said study co-author Joshua Carter at the Harvard-Smithsonian Center for Astrophysics.

Although the outer planet is probably a gas giant slightly larger than Uranus and therefore not suitable for life as we know it, these findings do show that circumbinary planets can and do exist in habitable zones.

"The thing I find most exciting is the potential for habitability in a circumbinary system," said study co-author William Welsh at San Diego State University. "Kepler-47c is not likely to harbor life, but if it had large moons, those would be very interesting worlds."

Weird weather on Tatooine planets

Circumbinary planets might undergo relatively wild climate swings.

On Earth, the sun is a relatively stable source of light, with the solar insolation or energy we receive only varying by 0.1 percent or so. "As a result, we don't need to worry about what the sun is doing, at least over time-scales of years to decades," Orosz said. "For a planet around a binary system, there can be changes in the insolation of several percent on the time-scale of days to weeks. In addition, if the planet's rotation axis is tilted, then that also has an effect. Therefore the seasons are rapid and complicated."

"Also, let's say for the sake of discussion that the planet's rotation period is 24 hours, just like the Earth," he added. "Because you have two suns instead of one, you can have more than 12 hours of daylight, depending on the positions of the stars at sunrise or sunset."

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