Planet-hunters say they've detected the first world that's absolutely known to circle two stars, like Luke Skywalker's home planet Tatooine in the fictional "Star Wars" saga.
"Once again, what used to be science fiction has turned into reality," said Carnegie Institution astronomer Alan Boss, a member of the team for NASA's Kepler mission and a co-author of a paper on the discovery in the journal Science.
To mark the occasion, NASA invited John Knoll of Industrial Light and Magic, the special-effects company behind the "Star Wars" movies, to sit in on today's announcement. "When I was a kid, I didn't think it was going to be possible to make discoveries like this," Knoll told journalists.
Tatooine serves as the setting for the first movie in the series, released in 1977 and now subtitled "A New Hope." The saga's main character, Luke Skywalker, could watch a double-sunset as he toiled in the desert on his uncle's moisture farm, aided by his trusty robots C-3PO and R2-D2.
Luke probably couldn't stand on the surface of Kepler-16b, which orbits a red and an orange star in the constellation Cygnus, 200 light-years from Earth. It certainly wouldn't be a desert. The planet is most like Saturn in our own solar system — too cold for life as we know it, most likely with a thick, gassy atmosphere. "This one's just outside the habitable zone," the paper's lead author, SETI Institute astronomer Laurance Doyle, told me.
But if Han Solo were to park the Millennium Falcon on one of Kepler-16b's hypothetical moons, there'd be plenty of double-sunsets. In fact, because the two suns orbit each other, each sunset would bring a different configuration, with the small red sun occasionally crossing over the larger orange one. "You might get two eclipses every 41 days," Doyle said.
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Luke Skywalker surveys a double sunset on the planet Tatooine in "Star Wars: A New Hope."
How the Tatooine planet was found
It's the complex crossings of the suns and the planet that tipped off Doyle and his colleagues to Kepler-16b's existence. NASA's Kepler space telescope, launched in 2009, stares at a 105-square-degree patch of sky in the constellations Cygnus and Lyra, looking for the telltale signs of something dark moving across a star. Kepler watches for periodic dips in the light coming from 155,000 stars. When those dips are detected, scientists use sophisticated software to figure out if the pattern could be caused by a planet.
One of the big challenges is that such dips can also be caused by one of the companions in a double-star system crossing over the other one. This is what's known as an eclipsing binary. The Kepler team has found hundreds of eclipsing binaries, including Kepler 16 — but scientists saw something extra in Kepler 16's pattern of dimming and brightening. "We saw extra dips in the light curve," Doyle recalled.
In the Science paper and 12 pages of supporting material, the Kepler scientists describe the painstaking process used to figure out what was behind those extra dips. They analyzed the pattern of the dips, as well as the varying lengths of time it took for objects to cross over each other (a method known as transit timing variation, or TTV). That resulted in a gravitational model demonstrating that the pattern could only be caused by a planet and two suns passing across each other repeatedly, as seen from Earth's point of view.
R. Hurt (SSC) / JPL-Caltech / NASA
It's theoretically possible for the Kepler-16 system's two suns to line up directly behind the planet Kepler-16b, as shown here.
The team found that Kepler-16b is almost exactly a third as massive as Jupiter, and three-quarters as wide — which makes it comparable to Saturn. It's somewhat denser than Saturn, but not quite as dense as water — which suggests it's half-gassy (with a helium-hydrogen atmosphere) and half-heavy (with an icy-rocky core).
Both of the two suns are smaller and dimmer than our own sun, and they orbit each other once every 41 days. The Kepler-16b planet is in a nearly circular orbit around both stars. It takes 229 days to make one circuit at a distance of 65 million miles — which is similar to the parameters for Venus' 225-day orbit. Because the twin suns are dimmer, Kepler-16b is colder than Venus, with an estimated surface (or cloud-top) temperature of -100 to -150 degrees Fahrenheit (170 to 200 Kelvin).
"You better have your long underwear," Boss joked.
Doyle said it was lucky that Kepler happened to be watching now. The orbital characteristics are such that the planet-sun transits won't be visible from Earth starting in the 2014 time frame. "In 2018, the primary transits will stop for 24 years. And in 2014, the secondary transits will stop for 45 years. Delay Kepler, and a lot wouldn't have happened," he said.
Looking back and looking ahead
The Kepler team says Kepler-16b is the first confirmed, unambiguous example of a planet orbiting two stars. Several years ago, astronomers wondered whether binary-star systems, which make up more than half of our Milky Way's stellar population, would be too unstable to harbor planets for long. Since then, theoretical models have shown that double-sunset planets could be far more common than previously thought.
There have been a number ofl tentative reports of double-star planets. Last year, astronomers reported detecting a "Tatooine planet" that orbited one of the stars in a binary-star system. That research team used a different analysis method known as astrometry.
Boss said the case for Kepler-16b was more solid, not only because it orbited two stars in a close-in binary system, but also because Kepler's transit observations were "rock-hard solid."
"With astrometric observations, you're always a bit uncertain if it's real," Boss said.
Beyond the "Star Wars" angle, Kepler-16b is significant because it shows once again that a wide variety of star systems can foster planets, and perhaps habitable planets at that. "This is an example of another planetary system, a completely different type that no one's ever seen before," Doyle said. "That's why people are making a big deal out of this."
William Borucki, an astronomer at NASA's Ames Research Center who serves as the Kepler mission's principal investigator, said the research "confirms a new class of planetary systems that could harbor life."
"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," Borucki said in a NASA news release. "This milestone discovery confirms a theory that scientists have had for decades but could not prove until now."
Doyle said Kepler-16b almost certainly will not be the last double-sunset planet discovered by the $600 million Kepler mission. When the numbers all added up, "I didn't feel like it's the end of 20 years of searching ... it felt like the beginning of something" he said. "I predict that in the next couple of months, we're going to have some more."
But time's running out for Kepler. Boss noted that the current mission plan calls for the telescope to be "out of business one year from now." That would be a shame, Boss said, because it looks as if it will take longer than expected for Kepler to get the data to identify Earthlike planets in Earthlike orbits around sunlike stars — which is the mission's prime objective. The reason for that is that the readings from alien suns are unusually noisy. "It turns out that most stars are not as quiet as the sun," Boss said.
Kepler's scientists are already talking about seeking an extension of the mission. That could be a challenge in this era of tightening budgets, but Boss argues that it could be a long time before NASA gets another opportunity to launch a planet-hunting mission.
"Kepler has become, in essence, our only Terrestrial Planet Finder," Boss said. "This is it, for the foreseeable future."
Extra credit: Doyle says that anyone with a good telescope (8-inch mirror or larger) and a CCD camera could record a Kepler 16 planetary transit next June 28 from China and other parts of northeastern Asia. The light from the star system would be seen to dip by about 1.7 percent, if observers train their telescopes on the stars at just the right time. "They'll be able to measure the next transit since the discovery of the planet," Doyle said.
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This report was last updated at 5:20 p.m. ET.
In addition to Doyle and Boss, the authors of "Kepler-16: A Transiting Circumbinary Planet" include Joshua A. Carter, Daniel C. Fabrycky, Robert W. Slawson, Steve B. Howell, Joshua N. Winn, Jerome A. Orosz, Andrej Prsa, William F. Welsh, Samuel N. Quinn, David Latham, Guillermo Torres, Lars A. Buchhave, Geoffrey W. Marcy, Jonathan J. Fortney, Avi Shporer, Eric B. Ford, Jack J. Lissauer, Darin Ragozzine, Michael Rucker, Natalie Batalha, Jon M. Jenkins, William J. Borucki, David Koch, Christopher K. Middour, Jennifer R. Hall, Sean McCauliff, Michael N. Fanelli, Elisa V. Quintana, Matthew J. Holman, Douglas A. Caldwell, Martin Still, Robert P. Stefanik, Warren R. Brown, Gilbert A. Esquerdo, Sumin Tang, Gabor Furesz, John C. Geary, Perry Berlind, Michael L. Calkins, Donald R. Short, Jason H. Steffen, Dimitar Sasselov, Edward W. Dunham, William D. Cochran, Michael R. Haas, Derek Buzasi and Debra Fischer.
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First published September 15 2011, 10:59 AM