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A planet that orbits two stars

This artist's concept illustrates Kepler-16b, the first planet known to definitively orbit two stars - what's called a circumbinary planet. The planet, which can be seen in the foreground, was discovered by NASA's Kepler mission. Credit: NASA/JPL-Caltech/T. Pyle

WASHINGTON: A planet that orbits two stars, like the world portrayed in Star Wars, has been discovered by scientists using data from NASA's Kepler space telescope.

Unlike Tatooine, the fictional desert-like home of Luke Skywalker, the planet, described today in the journal Science is a freezing cold, inhospitable world roughly the size of Saturn.

While this particular planet, dubbed Kepler-16b, isn't thought to harbour life, it demonstrates the diversity of planets in our galaxy.

"Given that most stars in our galaxy are part of a binary system, this means the opportunities for life are much broader than if planets form only around single stars," said Kepler's principal investigator and co-author William Borucki.

"This milestone discovery confirms a theory that scientists have had for decades but could not prove until now," he added.

Science fiiction realised

Circumbinary planets, those that orbit around both members of a stellar pairing, are not uncommon in science fiction and have been speculated to exist for some time. But until now, evidence confirming their existence had proven elusive.

"This discovery is stunning," said co-author Alan Boss, an astronomer at the Carnegie Institution of Washington in DC. "Once again, what used to be science fiction has turned into reality."

The planet is roughly 200-light years away from Earth and orbits its parent stars in a near perfect circle over 229 days - similar to the 225-day orbit of Venus. Both stars are smaller than our Sun - 20% and 69% as massive as our Sun respectively.

And while a spectacular double sunset would likely be visible from its surface, the temperature on the gaseous planet - billed somewhere between -73 and -101 degrees Celsius - is less than ideal for viewing.

Eclipses aid detection

Kepler measures dips in the brightness of more than 150,000 stars to search for transiting planets.

The team, led by astronomer Laurance Doyle from the California-based SETI Institute, detected the new planet by observing transits, where the brightness of a parent star dims from the planet crossing in front of it.

The planet was found in the Kepler-16 system, a pair of orbiting stars that eclipse each other from our vantage point on Earth.

When the smaller star partially blocks the larger star, a primary eclipse occurs, and a secondary eclipse occurs when the smaller star is completely blocked out by the larger star.

Astronomers further observed that the brightness of the system dipped even when the stars were not eclipsing one another, hinting at a third body.

The additional dimming in brightness events, called the tertiary and quaternary eclipses, reappeared at irregular intervals of time, indicating the stars were in different positions in their orbit each time the third body passed.

This showed the third body was circling, not just one, but both stars, in a wide circumbinary orbit.

Gravitational tug indicates mass

The gravitational tug on the stars, measured by changes in their eclipse times, was a good indicator of the mass of the third body.

Only a very slight gravitational pull was detected, one that only could be caused by a small mass, which the researchers determined to be comparable to Saturn

"Most of what we know about the sizes of stars comes from such eclipsing binary systems, and most of what we know about the size of planets comes from transits," said Doyle

"Kepler-16 combines the best of both worlds, with stellar eclipses and planetary transits in one system.