The mesmerizing double-sunset of Luke Skywalker's home planet Tatooine might not be too far from reality after all, astrophysicists now say.
While it was once thought that only gas giant planets could orbit twin stars, scientists now believe that rocky Earth-like planets with two suns probably exist in the universe.
"For over a decade, astrophysicists believed that planets like Earth could not form around most binary stars, at least not close enough to support life... Tatooine sunsets may be common after all," said Professor Ben Bromley of the University of Utah in a written statement.
It was once thought that the uneven gravitational pull of twin stars would prevent orbiting rocks and dust from coalescing to form Earth-like planets, sending these asteroid-size rocks (planetesimals) on dangerous collision courses.
"Their paths get mixed up by the to-and-fro pull of the binary stars," Bromley writes. "Their orbits can get so tangled that they cross each other's paths at high speeds, dooming them to destructive collisions, not growth."
He compared planet formation to the creation of dust bunnies: they glom together to form increasingly larger objects. When planets form around a twin star, the uneven gravitational pull scrambles up the dust bunnies unless they are on the right orbit.
Using mathematical and simple computer simulations, Bromley showed that Earth-sized planets can form around binary stars if they have the oval and "most circular" orbit. Although they didn't follow the simulation through until the point of planet formation, they found that the planet-forming material could survive without collisions for tens of thousands of years if kept on a concentric, oval-shaped orbit around binary stars.
"We are saying you can set the stage to make these things," Bromley says. "It is just as easy to make an Earthlike planet around a binary star as it is around a single star like our sun. So we think that Tatooines may be common in the universe."
NASA's Kepler space telescope has found seven planets orbiting binary stars, but all of them are giant gaseous planets.
"The planets that Kepler has discovered so far around binary stars are larger, Neptune- or Jupiter-size gas giants," he writes. "None of those found so far are small and rocky like our Earth -- or like Tatooine in 'Star Wars.'"
Bromley believes Kepler hasn't yet spotted Earth-like planets around binary stars because they are small compared with gas giants, "so it's a hard measurement."
The study, "Planet Formation Around Binary Stars: Tatooine Made Easy," was submitted to the Astrophysical Journal for review and can be found here.
