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 in the constellation Cygnus the Swan. The findings are described in a new study published Sept. 16 in the journal Science.
Unlike Star Wars’ Tatooine, this planet 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 because of a planet crossing directly in front of it.
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.
“This system is so fascinating since it is viewed edge-on, and all three bodies—the two stars and the planet—are all eclipsing each other,” said Doyle, who is lead author of the Science paper.
When the smaller star partially blocks the larger star, a primary eclipse occurs, and a secondary eclipse occurs when the smaller star is occulted (completely blocked) 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 brightness-dimming events, called tertiary and quaternary eclipses, reappeared at irregular intervals of time, indicating that the stars were in different positions in their orbit each time the third body passed. This showed that the third body was circling not just one, but both stars, in a wide circumbinary orbit.
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.
“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.”
Kepler-16b is an inhospitable, cold world about the size of Saturn and thought to be made up of about half rock and half gas. The parent stars are smaller than our Sun. One is 69 percent the mass of the Sun and the other only 20 percent. Kepler-16b orbits around both stars every 229 days.
“Working in film, we often are tasked with creating something never before seen,” said visual effects supervisor John Knoll of Industrial Light & Magic, a division of Lucasfilm Ltd., which created the Star Wars films. “However, more often than not, scientific discoveries prove to be more spectacular than anything we dare imagine. There is no doubt these discoveries influence and inspire storytellers. Their very existence serves as cause to dream bigger and open our minds to new possibilities beyond what we think we ‘know.’ ”
You can contact Bob Eklund at [email protected], or visit his websites at www.bobeklund.com and http://firststarbook.com.