NASA's Kepler Mission Discovers a World Orbiting Two Stars
The existence of a world with a double sunset, as portrayed in the film Star Wars more than 30 years ago, is now scientific fact. NASA's Kepler mission has made the first unambiguous detection of a circumbinary planet -- a planet orbiting two stars -- 200 light-years from Earth.
Unlike Star Wars' Tatooine, Kepler-16b is cold, gaseous and not thought to harbor life, but its discovery demonstrates the diversity of planets in our galaxy. Previous research has hinted at the existence of circumbinary planets, but clear confirmation proved elusive. Kepler detected such a planet, known as Kepler-16b, by observing transits, where the brightness of a parent star dims from the planet crossing in front of it.
"This discovery confirms a new class of planetary systems that could harbor life," Kepler principal investigator William Borucki said. "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. This milestone discovery confirms a theory that scientists have had for decades but could not prove until now."
A research team led by Laurance Doyle of the SETI Institute in Mountain View, Calif., used data from the Kepler space telescope, which measures dips in the brightness of more than 150,000 stars, to search for transiting planets. Kepler is the first NASA mission capable of finding Earth-size planets in or near the "habitable zone," the region in a planetary system where liquid water can exist on the surface of the orbiting planet.
Read more: http://kepler.nasa.gov/news/index.cfm?FuseAction=ShowNews&NewsID=152
First Planet Orbiting Two Stars Discovered by the NASA Kepler Spacecraft
By Laurance Doyle, an astrophysicist at the SETI Institute, and lead author of a paper that will appear in the journal Science on September 15, 2011
For the first time, astronomers with the NASA Kepler spacecraft mission have discovered a planet orbiting two stars. This is a fundamentally different kind of planetary system than has ever been discovered before. The new system is known as "Kepler-16" and consists of two stars — one about 69% the mass of the Sun, and the other only 20% the mass of the Sun, which circle each other every 41 days. Around both of these circles the Saturn-mass planet, half rock and half gas, known as Kepler-16b, with a period of 229 days. Even though the planet has an orbital period of less than a year, it is still outside the habitable zone of the stars because the stars are much dimmer than our Sun.
The discovery that such "circumbinary" planets can exist increases the likelihood of success of the Kepler Mission, which is to detect the first habitable; i.e. Earthlike, planets around other stars. Perhaps half the stars in the galaxy are in double star systems. Understanding that planets can form in close binary systems means that these, too, can be targets in the search for habitable worlds.
The research team discovered the circumbinary planet in an eclipsing binary system. This is a double star system in which both stars orbit each other across our line of sight, so that eclipses of the stars occur with regularity. Historically, much of what we know about stars sizes comes from such eclipsing binary systems. In addition, the planet's orbit was found to lie very close to the same orbital plane -- the difference in the tilt of their orbits is less than 1/3 of a degree -- so that the planet also moves across the disc of each star, momentarily blocking some of the light. Such events are called "planetary transits" and most of what we know about the sizes of planets outside the Solar System comes from such transit events. Thus we have the best of both worlds, and Kepler-16 is probably the most accurately measured planetary system outside our own.
Some scientists have nicknamed the planet "Tatooine" after the name of the home planet of Luke Skywalker, the hero in the 1970s science fiction movie Star Wars. In the story -- in a hypothetical galaxy far, far away, -- a circumbinary planet's double sunset was first brought to the screen. The public's vision of circumbinary planets thus goes back decades. But today science fiction has become science fact, and that galaxy far, far away has become our own galaxy. A whole new kind of planetary system has been shown to exist and -- like Luke in the story -- the adventure is just getting started.
Source: SETI Institute http://www.seti.org/page.aspx?pid=1664
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