Kepler-16b, which orbits two stars like Tatooine in the science fiction film, was detected by Nasa's Kepler spacecraft

A giant and frigid planet that enjoys double sunsets as it circles two stars has been spotted in the northern skies.

Astronomers said the planet, named Kepler-16b, marked the first unambiguous discovery of a world in orbit around two stars – an oddity made famous by the planet Tatooine in Star Wars.

The planet, which lies about 200 light years from Earth and has a similar mass to Saturn, was observed by Nasa's Kepler spacecraft as it passed in front of its twin suns, causing light from the stars to dim.

The unusual arrangement means that an observer on the planet's surface, where temperatures range from -70C to -100C, would see two small suns rise and set together, and on occasion appear as one as they passed in front of each other.

Alan Boss, a researcher at the Carnegie Institution for Science in Washington DC and a member of the team that discovered Kepler-16b, said: "You would never get constant daylight because the two stars are so close together.

"They would come together in an eclipse every 20.5 days and then move apart again. As their separation increased, they would go down at different times, and that could make cocktail hour hard."

Models of planetary formation have shown it is possible for worlds to form around twin stars, but astronomers have only set eyes on them in science fiction. Even in Hollywood, the inhabitants of planets warmed by two suns might not appreciate their heavenly situation. Stuck on the arid desert world of Tatooine, Luke Skywalker moaned: "If there's a bright centre to the universe, you're on the planet that it's farthest from."

Boss was more upbeat about Kepler-16b, though it is unlikely to be habitable. "This discovery is stunning. Once again, what used to be science fiction has turned into reality," he said.

The Kepler spacecraft was launched in March 2009 to spot Earth-like worlds among the 155,000 stars in the constellations of Cygnus and Lyra which are visible from the northern hemisphere. Planets reveal themselves when they move across the face of their sun, or suns, as this blocks some of the starlight that reaches Kepler's sensitive, wide-angle camera.

Researchers led by Laurance Doyle, at the Carl Sagan Centre for the Study of Life in the Universe at the Seti Institute in California, spotted the planet after noticing unusual signals in the data collected by the Kepler spacecraft.

Images captured by Kepler's camera showed two stars orbiting each other and producing eclipses as they moved in front of one another. Both stars were small in comparison with our own sun, at about 69 and 20% of the sun's mass.

On closer inspection the footage revealed further eclipses that could not be explained by the movement of the two stars, or an additional third star. Instead, a subtle drop in light from the stars, which amounted to a dimming of only 1.7%, was attibuted to an orbiting planet.

The astronomers turned next to a ground based-telescope, the Whipple Observatory in Arizona. With this, they monitored the shifting velocity of the heaviest star as it moved around in its orbit.

Those observations gave Doyle's team the details they needed to reconstruct the likely orbits of the stars and its planet. They showed that the two suns orbit each other every 41 days at a distance of about 21 million miles. The planet completes a circular orbit around both stars every 229 days at a distance of 65 million miles. Details of the discovery are reported in the US journal, Science.

Josh Carter, a co-author on the study, at the Harvard-Smithsonian Centre for Astrophysics, said: "Kepler-16b is the first confirmed, unambiguous example of a circumbinary planet – a planet orbiting not one, but two stars. Once again, we're finding that our solar system is only one example of the variety of planetary systems nature can create."