Tatooine for real? Scientists confirm planet orbiting two suns.

Tatooine is the mythical home of Luke Skywalker. Now, scientists have identified a planet in a binary system 200 light-years away, although the planet is more like Saturn in mass and makeup.

Lucasfilm Ltd./Reuters
Natalie Portman portrays Padmé Amidala and Hayden Christensen (r.) plays Anakin Skywalker as they are joined by R2-D2 and C-3PO on the desert planet Tatooine in the film 'Star Wars Episode II Attack of the Clones.'

Cue the "Star Wars" theme, but look no farther than 200 light-years away in our own galaxy for the first confirmed planet orbiting two suns – in effect, a frigid, oversize Tatooine.

On Thursday, a team of scientists with NASA's Kepler planet-hunting mission announced the discovery. Unlike Luke Skywalker's mythical home, however, this planet is more like Saturn in mass and makeup – a thick gaseous atmosphere surrounding a rocky core.

The planet's core has 40 to 60 times the mass of Earth, the Kepler scientists estimate. Temperatures range between an Antarctica-like minus 100 and minus 150 degrees F.

The discovery suggests that Kepler's burgeoning data archive may be hiding many more planets than the mission's architects first envisioned, including Earth-size planets in a system's so-called habitable zone, Kepler's ultimate quarry.

The reason: Kepler scientists use a spacecraft to measure the telltale dimming and brightening that takes place as a planet passes across the face of the star it orbits. Single stars with transiting planets represent the easiest targets for this approach.

But so-called eclipsing binary stars can present similar dimming and brightening, and so they can trigger a false detection. As scientists identify these systems in the field of 156,000 stars Kepler is watching, eclipsing binaries in effect become castoffs in Kepler's hunt for extrasolar planets.

With this latest discovery, Kepler has shown that the spacecraft has the ability to tease out the presence of planets orbiting two suns. In the process, Kepler has settled a scientific debate over whether planets could form and remain in stable orbits around binary stars.

"They nailed it," says Alan Boss, a member of the Kepler team, although not part of the group that opted to hunt for planets among the binary stars that Kepler measured.

Indeed, billions of stars in the galaxy are in binary systems, says Ray Jayawardhana, a scientist at the University of Toronto who studies solar-system evolution.

Researchers have identified plenty of planets orbiting single stars, and they've spotted newly formed binary stars with disks of dust orbiting them – disks that in single-star systems represent the raw material for planets, he continues.

"This discovery shows that at least in some cases – when two stars are quite close to each other – planets can form out of those disks and survive," he says, adding that the new Kepler observations "are exquisite."

For Laurance Doyle, a physicist with the SETI Institute in Mountain View, Calif., and the member of the Kepler team who led the hunt, dumpster diving in Kepler's refuse bin had a lot to recommend it.

Dr. Doyle says as he has studied the potential for finding planets in binary systems in general, simulations indicated that if such a system had a planet, the object would orbit the pair of stars along the same plane that the stars occupied as they orbited each other.

Thus, with eclipsing binaries, any planet would be orbiting edge-on – the orientation needed to use Kepler's transit method to detect it. At the same time, however, a planet in such a system would have to be fairly far from the pair of stars it orbits to be detected, he adds.

In this case, the main star's smaller sibling orbits roughly 20 million miles away, and the planet orbits some 65 million miles away.

The two suns that the planet orbits are oddballs, Doyle notes. The largest star, Kepler 16-A, has about 69 percent of the sun's mass, is some 700 degrees cooler, and displays the slow rotation rate of a stellar geezer. But its intense sunspot and flare activity would suggest a young star.

And its companion? Kepler 16-B tips the cosmic scales at about 20 percent of the sun's mass and burns a few thousand degrees cooler than the sun. Yet for its low temperature, it is significantly more massive and has a larger radius than theory suggests it should be, Doyle says.

Thus, even though the planet, tagged Kepler 16-b, is in an orbit comparable to that of Venus, it receives about as much light from the two stars combined as Mars does from the sun.

As for habitable zones, this system has one, but it falls well inside Kepler 16-b's orbit. An Earth-mass planet might be stable in the zone's outermost reaches, Doyle says. Any deeper into the zone, however, and the gravitation variations from the orbiting stars would be strong enough to either eject the planet from the system or pull it to its doom, into one of the stars.

Still, if Kepler 16-b has moons – something Doyle and colleagues are searching for – and if one were habitable, residents would be privy to stunning sunrises and sunsets, with the largest orb glowing orange, the other red, and their own orbital dance ensuring no two sunrises and sunsets are ever the same.

Indeed, the Kepler 16 system could become a new playground for astrobiologists, now that the Kepler team has shown unambiguously that planets form and survive in a binary-star system.

The detection also has an element of serendipity. The system's orbital plane shifts position with time. Between 2018 and 2042, the team calculates, Kepler 16-b's transit across the largest star will vanish, while transits across the smaller star will disappear for about 35 years beginning in 2014.

For Greg Laughlin, an astronomer at the University of California at Santa Cruz, the discovery of this circumbinary planet could be a tantalizing foretaste of what may lie a mere 4.24 light-years from Earth in the Alpha Centauri system, a binary pair.

"We can't help but imagine what kinds of planets are orbiting the pair of those stars and those two stars individually," he says.

Doyle, who chuckles at the sci-fi connection between Kepler 16-b and Tatooine, notes that if there is an underlying theme to this discovery, it's that while science fiction may be engaging, it's hard to beat the real universe.

"There's so much pretend stuff," he says. "Doing the real thing is a lot more fun."

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