Who can forget the dramatic moment, early in the original Star Wars film, when Luke Skywalker gazed wistfully at the twin setting suns on his home planet of Tatooine? The image was so arresting that it didn't seem to matter how fast and loose the story played with science. After all, two stars so close together couldn't really support a stable planetary system, right? Well, not so fast.
Astronomers using the Spitzer Space Telescope to survey the local galactic neighborhood have found that binary star systems produce protoplanetary disks--the necessary precursors for planet formation--at least as often as single stars such as our sun. But that's only half the story. It turns out that binary systems whose stars orbit very close together are even more likely to harbor disks--and, potentially, planets--than single-star systems. "There appears to be no bias against having planetary system formation in binary systems," says team leader David Trilling of the University of Arizona in Tucson. "There could be countless planets out there with two or more suns."
Astronomers have long known that certain binary star systems could host planetary disks. It's just that all of the systems they observed have comprised two stars orbiting far apart--on the order of 1000 times the distance between Earth and the sun. Many astronomers believed that closely orbiting stars would create too much gravitational chaos to allow stable protoplanetary disks--and eventually planets--to form. But as Trilling and colleagues report in the 1 April issue of the Astrophysical Journal, when they aimed Spitzer's infrared telescope at 69 binary systems, they found evidence of debris disks in 40% of them. That's actually slightly more frequent than disks have been discovered around single stars.
Even more notable, Trilling and colleagues found disks around 60% of systems with the most tightly orbiting stars, the kind capable of producing twin sunsets like those seen on the fictional Tatooine. "We were very surprised to find that the tight group had more disks," Trilling says, adding that it could mean that tight binaries actually favor planet formation.
The discovery should serve as another cautionary tale for anyone who relies too much on our own solar system as a model, says astrophysicist Mario Livio of the Space Telescope Science Institute in Baltimore, Maryland. Astronomers used to think that all gas giant planets such as Jupiter would be far from their suns, for example, he says. But they've now found several "hot Jupiters" close to their stars. Likewise, Livio says, we should no longer assume that one-star systems are the ideal planet breeding grounds. The Spitzer findings just "confirm the general notion that planets are everywhere," he says.