Once confined to science fiction, a planet circling two stars has left the cinema and landed in reality: Scientists have spied a Saturn-sized world, called Kepler-16b, orbiting a binary star system.
The sunset on Kepler-16b would look something like the iconic double sunset depicted in Star Wars, only better, says astrophysicist Laurance Doyle of the SETI Institute in Mountain View, Calif. “Nature is always more amazing,” says Doyle, coauthor of a Sept. 16 Science paper that describes Kepler-16b. “There, you’d get a different sunset every day!”
The two suns are different sizes, different colors, and always changing places. Every 41 days, these dancing stars complete a circle around one another, periodically eclipsing each other and reversing their position in the sky. The bigger star is orange and about 69 percent the mass of the sun, and the smaller star is quite red and only 20 percent the sun’s mass.
The finding also reinforces astronomers’ growing realization that planets are hardy galactic residents capable of surviving nearly any conditions. Planets have been found orbiting pulsars, surviving supernova explosions, wandering through interstellar space and maybe even forming near black holes.The newly discovered planet takes 229 Earth days to complete its orbit around its two stars. The planet is thought to be half gas, half rock and ice and similar to Saturn in both mass and size, the scientists report. No word on rings yet, but Doyle says he is looking for exo-moons orbiting the planet.
“This is a really important discovery,” says astronomer Andrew Howard of the University of California, Berkeley. “When the theorists and modelers sit down and predict the family of things that should happen, this is one of them,” he says, referring to a binary star system with planets. “But it’s still so new and different that it still shocks at least me.” Astronomers detected the oddity using NASA’s Kepler space telescope, which stares at a patch of sky studded with 155,000 stars and looks for diagnostic dips in luminosity that signal the passage of a planet.
There might be as many as 2 million detectable transiting planets in eclipsing binary star systems in the galaxy, Doyle says, a number he reached by considering the number of Kepler stars, the number of candidate binaries and the number of stars in the galaxy. “Now that we know how to find them, what they look like, and the things to look for, we’re probably going to find a few more,” Doyle says.
Kepler-16 is visible in the night sky in the constellation Cygnus. Near the swan’s western wingtip are three stars, and the Kepler-16 stars are nearest the middle of the three, visible with binoculars, Doyle says. Next year, Kepler-16b’s transit will cause a 1.7 percent drop in the brightness of the bigger star, which will be visible in parts of Asia to amateur astronomers with telescopes.