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.

© Lucasfilm Ltd. & TM. All Rights Reserved

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.

More about weird planets:

• Q&A with planet hunter Laurance Doyle
• 'Super-Earth' just might support life
• Planetary six-pack poses a puzzle
• Probe finds planetary 'missing link'
• Planets spotted in changing orbits
• 'Invisible' planet spotted with new technique
• Looking for alien Earths? Here they come
• Cosmic Log archive on the Kepler mission
• Search for Kepler planets on msnbc.com

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.

Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding me to your Google+ circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds. 

European astronomers have announced the discovery of more than 50 new planets beyond our solar system, including 16 that are just a notch above our own planet in mass. They say their record-breaking findings suggest that more than half of the stars like our sun possess planets, and that many of those worlds are less massive than Saturn.

The pick of the litter is a planet that's already been in the spotlight: HD 85512 b, a world at least 3.6 times as massive as Earth that's located 36 light-years away in the constellation Vela. HD 85512 b is the only one of the 16 super-Earths on today's list that is located in its star system's habitable zone. That's the area around a star where scientists believe water could exist in liquid form, which would make a rocky planet potentially livable.

HD 85512 b's status came to light a couple of weeks ago in a paper submitted to the journal Astronomy and Astrophysics, but the team behind the discovery provided more details about that super-Earth and the dozens of other worlds in papers presented today at the Extreme Solar Systems II conference in Wyoming.

The findings came from the team behind the High Accuracy Radial Velocity Planet Searcher, or HARPS, which is installed at the European Southern Observatory's 11.8-foot (3.6-meter) La Silla Observatory in Chile.

"The detection of HD 85512 b is far from the limit of HARPS, and demonstrates the possibility of discovering other super-Earths in the habitable zones around stars similar to the sun," University of Geneva astronomer Michel Mayor said in today's news release from the ESO.

Super-Earths, which range from Earth's mass to worlds 10 times more massive, are of particular interest to planet-hunters because it's thought that they could be even more conducive to the development of life than our own planet. When the search for extrasolar planets began more than 15 years ago, the telescopes used for the task could only detect giant planets like our own solar system's Jupiter. Since then, the techniques and tools used for the search have become much more sensitive.

HARPS, for example, can detect the slight gravitational wobble caused by planets as small as Earth, if they have incredibly close-in orbits. HARPS' observations of 376 sunlike stars has led the team to conclude not only that more than half of such stars are surrounded by planets (maybe as many as 70 or 80 percent), but also that about 40 percent of sunlike stars have at least one planet less massive than Saturn.

One of the team members, Lisa Kaltenegger of the Max Planck Institute for Astronomy and the Harvard Smithsonian Center for Astrophysics, told journalists today that the latest round of findings marked a new age in the search for habitable planets.

"We are actually entering an incredibly interesting time in our history," she said.

Keeping track of the habitables
ESO's Markus Kissler-Patig said the discovery of HD 85512 b could be one of the first entries in "a good catalog of habitables" marked for further study. Kissler-Patig is the project scientist for the ESO's European Extremely Large Telescope, or E-ELT, which is slated to be built over the next decade at a cost of 1 billion euros ($1.4 billion).

HD 85512 b "is in the zone where we can directly image it," Kissler-Patig said, and that means astronomers could theoretically analyze its atmosphere for the signatures of life, such as the presence of oxygen, methane and water vapor.

The HARPS team members were able to figure out the minimum mass and orbital characteristics of HD 85512 b, but they couldn't determine its density, composition or the nature of its atmosphere — which means astronomers will have to wait for the completion of E-ELT or similar high-resolution observing instruments to confirm that the world is truly habitable.

Francesco Pepe, a colleague of Mayor's at the University of Geneva, said that the HARPS team's discoveries include 10 worlds described in papers submitted to Astronomy and Astrophysics, including HD 85512 b, and 49 planets reported today at the Wyoming conference. Eight of the new planets were detected as part of the Swiss-led CORALIE search effort in Chile, he said. The ESO says this is the largest number of extrasolar planets reported at one time.

Pepe said the findings pointed up a fresh mystery for planet-hunters to ponder: the existence of a "planet desert" between low-mass worlds and gas giants. Relatively few planets have been found at a level around 30 times the mass of Earth. "It may point towards different formation mechanisms" for planets like Earth and Neptune vs. planets like Jupiter and Saturn.

HARPS isn't the only instrument engaged in the search for extrasolar planets: Two space telescopes, NASA's Kepler and the European Space Agency's Corot, are detecting planets by looking for the telltale dimming of their parent stars. Kepler and Corot can determine how big a planet is, but they can't tell how massive it is. In contrast, HARPS can determine the mass but not the size.

Unfortunately, Kepler can't be used to confirm HARPS' discoveries, nor can HARPS confirm Kepler's. The good news is that the William Herschel Telescope in the Canary Islands is being outfitted for a HARPS North instrument that will begin operation next year and facilitate the follow-up of Kepler detections. 

Today's revelations bring the official tally of extrasolar planets to 645.

Other findings from the Extreme Solar Systems II conference:
• Over at the "Dynamics of Cats" blog, Steinn Sigurdsson quotes Kepler team members as saying they have identified 1,781 candidate planets, with up to 27 of those confirmed. Among the reported candidates are 123 potential worlds that are less than 1.25 times as wide as Earth, and 121 that are in the nominal habitable zones of their parent stars.

Jon Lomberg

An artist's conception shows storms on a brown dwarf.

• Astronomers say they have observed brightness changes on a failed star, also known as a brown dwarf, that may indicate a storm grander than any seen yet on a planet. The stormy brown dwarf is known as 2MASS 2139.

"We found that our target's brightness changed by a whopping 30 per cent in just under eight hours," the University of Toronto's Jacqueline Radigan said in a news release. "The best explanation is that brighter and darker patches of its atmosphere are coming into our view as the brown dwarf spins on its axis."

Radigan is the lead author of a paper being presented this week at the Extreme Solar Systems II conference.

More about alien planets:

• Previously: Super-Earth on the 'edge of habitability'
• Interactive: How scientists search for other worlds
• 'Invisible' planet discovered using new technique
• Background: Looking for alien Earths? Here they come

Authors of  “The HARPS search for Earth-like planets in the habitable zone, I — Very low-mass planets around HD20794, HD85512, HD192310" include F. Pepe, C. Lovis, D.D. Ségransan, W. Benz, J. L. Bertaux , F. Bouchy, X. Dumusque, M. Mayor, D. Queloz, N.C. Santos and S. Udry.

Authors of "The HARPS search for southern extra-solar planets XXXIV. Occurrence, mass distribution and orbital properties of super-Earths and Neptune-mass planets" include M. Mayor, M. Marmier, C. Lovis, S. Udry, D.D. Ségransan, F. Pepe, W. Benz, J. L. Bertaux , F. Bouchy, X. Dumusque, G. Lo Curto, C. Mordasini, D. Queloz and N.C. Santos.

Authors of "High amplitude, periodic variability of a cool brown dwarf: Evidence for patchy, high-contrast cloud features" include Jacqueline Radigan, Ray Jayawardhana, David Lafreniere, Etienne Artigau, Mark Marley and Didier Saumon.

Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding me to your Google+ circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.