There’s a new hope for Star Wars fans dreaming of a rocky planet that orbits a binary star like Luke Skywalker’s homeworld of Tatooine: astrophysicists from the University of Utah and the Smithsonian Astrophysical Observatory say that such a planet could theoretically exist.
To date, only uninhabitable gas-giants have been discovered circling twin-star systems, but the new study (which has been submitted to the Astrophysical Journal for review) uses mathematical simulations to show that habitable Tatooine-like planets not only exist, but may be widespread.
In their paper, Ben Bromley of the University of Utah and Scott Kenyon from the Smithsonian Astrophysical Observatory write that “Tatooine sunsets may be common after all… Our main result is that outside a small region near a binary star, [either rocky or gas-giant] planet formation can proceed in much the same way as around a single star.”
Dust bunnies under the the bed
Using a series of mathematical formulas to describe how binary stars can be orbited by asteroid-sized rocks that clump together to form planets (also known as planetesimals), the study authors create a scenario in which “planets are as prevalent around binaries as around single stars.”
Bromley explained that their work reveals that orbiting around a binary star system can be every bit as ordinary and uneventful as travelling around a single sun, and that the same recipe that let planets form in our solar system could work in a system like Tatooine’s. Their work was funded by NASA’s Outer Planets Program and was a spinoff of the duo’s previous research into how the dwarf planet Pluto and its primary moon Charon function much like a binary system.
Kenyon, whose observatory is part of the Harvard-Smithsonian Center for Astrophysics, said that planets form “like dust bunnies under the bed” from a disk of gas and dust surrounding a young star. They come together to form increasingly larger objects. In a binary system, however, these so-called dust bunnies have to be travelling along just the right orbit to form – an orbit known as a “most circular orbit” that is actually oval-shaped with several tiny wave-like ripples.
Mixed up paths
Scientists have long believed that planets similar to Earth would be unable to form around most binary stars, or at the very least not in the habitable zone (the area close enough to the suns to support life), the study authors explained. Planetesimals need to merge gently together to grow, and when they orbit around a single star, they tend to follow circular paths that do not cross, and if they approach each other, they can gently merge together, they added.
When planetesimals orbit two stars, however, “their paths get mixed up by the to-and-fro pull of the binary stars,” explained Bromley. “Their orbits can get so tangled that they cross each other’s paths at high speeds, dooming them to destructive collisions, not growth.”
While previous work looked at circular orbits when attempting to figure out if rocky planets are able to form around binary stars, the new study indicates that planets will naturally seek out these oval orbits, not circular ones, when they are small. “If the planetesimals are in an oval-shaped orbit instead of a circle,” Bromley added, “their orbits can be nested and they won’t bash into each other. They can find orbits where planets can form.”
He and Kenyon demonstrated using mathematical formulas and computer models to show that Earth-sized, non-gas planets could form around binary stars if they use the oval “most circular” orbit. While they did not run their simulations all the way up to the actual planet formation point, they did demonstrate that planetesimals could survive in these orbits for tens of thousands of years around a binary star without experiencing any collisions.
“We are saying you can set the stage to make these things,” said Bromley. “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.”