By Dan Satherley
Astronomers have discovered, for the first time, a solar system that has two suns and two planets.
In September last year, NASA's space-based Kepler telescope found its first lone planet orbiting a binary star system, and now it's found its first twin-sun system with more than one planet.
The findings were published in the latest edition of journal Science.
Named Kepler-47, the system is located about 5,000 light years away in the constellation of Cygnus. One of the stars is much like our own Sun, and the other is a third the size and 175 times less bright.
The inner planet circles the stars in around 49.5 Earth days, and the outer, in 303 days. The stars, on the other hand, whizz around each other in only 7.5 days.
Astronomers have calculated the outer planet, Kepler 47c, would lie in the "Goldilocks zone" - the distance from a star that liquid water is a possibility.
At least it would be if it wasn't likely to be a gas giant, with a diameter 4.6 times wider than Earth's.
"The thing I find most exciting is the potential for habitability in a circumbinary system," says astronomer William Welsh at San Diego State University.
"Kepler-47c is not likely to harbour life, but if it had large moons, those would be very interesting worlds."
MIT astrophysicist Sara Seager agrees.
"Even though [K-47c] is a big planet, certainly too big for life," she says. "It could have a moon that could support life."
The climate on any such moon could be unpredictable, however, as the interactions between planets and the twin stars would be much more complex than in our own, relatively basic solar system.
"We don't need to worry about what the sun is doing, at least over time-scales of years to decades," says lead author of the study, Jerome Orosz. In the Kepler-47 system, every 7.5 days the dimmer sun would eclipse the brighter, and seasons would be "rapid and complicated".
The discovery of binary stars with planets last year brought comparisons with Tattooine, the planet with two moons from the Star Wars movies that Luke Skywalker lives on.
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