A scene from Star Wars shows Luke Skywalker and the double sunset on his home planet, Tatooine
The Star Wars movies weren't especially big on subtlety. Their heroes and villains were cartoonishly one-dimensional, the aliens were grotesquely alien, and the action was over the top. One scene in the first film was a notable exception, though. It showed a sunset on Tatooine, Luke Skywalker's home world — with not one, but two suns sinking in tandem toward the horizon. The essential strangeness of that image made it quietly but profoundly clear that you were visiting an utterly foreign world.
When astronomers actually began finding such worlds almost two decades after that first Star Wars movie, though, they didn't waste much time looking for places like Tatooine. Double-star systems are very common in the Milky Way — in fact, solitary stars like the sun are in the minority. But it wasn't clear, said theorists, that planets could form and survive in their vicinity: when two elephants are waltzing, it could be very difficult for mice to tiptoe safely under their feet.
That uncertainty is now gone. As described in the latest issue of Science, the Kepler space telescope has found just what George Lucas imagined back in the 1970s — sort of. The world known as Kepler-16b is a huge, gaseous planet, about the size of Saturn, and its temperature is at least 100°F below zero; Tatooine may have been a harsh desert world, but Kepler-16b is positively uninhabitable. Still, like Tatooine, it orbits a dual-star system. The planet's very existence suggests that the number of places to look for other worlds is much vaster than anyone had counted on.
Looking for such improbable planets wasn't high on the list of goals Kepler was meant to accomplish. Its primary mission is to figure out how common Earth-size planets in life-friendly orbits are in the Milky Way. Since early 2009, the satellite has been staring at a single patch of the northern sky, straddling the constellations Lyra and Cygnus, and monitoring about 150,000 stars around the clock to see if they dim — a sign that a planet may have passed across a star's face. But dimming can also be caused by an eclipsing binary, a pair of stars in an orbit that appears edge on as seen from Earth so that each star passes in front of the other once per cycle.
Even though these were considered poor neighborhoods for planet hunting, Kepler can't help seeing them since they're in the satellite's field of view anyway, and in fact, the space probe has discovered 2,000 previously unknown eclipsing binaries. So a team led by SETI Institute astronomer Laurance Doyle decided to take a closer look. One binary in particular, located about 200 light-years from Earth, caught their eye: it consists of two stars, both cooler and dimmer than the our own home star, orbiting each other once every 41 days. The bigger star is orange, and about two-thirds as big as the sun; the second is red, about a fifth the sun's size. The double dip in brightness as they play hide-and-seek with each other is unmistakable.
But when Doyle and his colleagues looked more closely, they saw another series of dips much fainter, suggesting there was a much smaller object passing in front of at least one of stars. These dips were irregularly spaced, though — an average of 229 days apart, but varying by about nine days from that average. The best explanation: the small object was passing in front of both, and since the stars themselves were constantly changing position, the timing was different on each pass.
"When we first saw it," says co-author Joshua Carter, of the Harvard-Smithsonian Center for Astrophysics, "I thought, 'Wow, this is just amazing.' It's hard not to get excited. This is too much fun."
Aside from the coolness factor, Kepler's precise measurements of both brightness dips and timing changes let the scientists calculate the sizes and masses of the stars and planet with unprecedented accuracy. "All you need, says Carter, "is Newton's laws of gravity and Euclid's geometry." For Star Wars fans, that's probably a minor detail, but for understanding the formation, structure and composition of the heavenly objects, it's crucial. "We'll be doing lots of follow-up," says Carter, "to figure out how this system came to be."
They may even find other planets in the system, and if they don't, it's likely they'll find other examples of planets orbiting binary stars in Kepler's treasure trove of data. It's even conceivable they can find a moon orbiting a planet that orbits a double star. "It's the same principle," says Carter. "It's just a lot harder."