Worlds with Double Sunsets Common

Astronomers might not have to search a galaxy far, far away after all to find a world with double sunsets like Luke Skywalker's home planet Tatooine. A new study suggests the universe is filled with them.

Astronomers using NASA's Spitzer Space Telescope have found that twin-star systems are just as likely to be surrounded by dusty debris disks as ones with only a single star. Debris disks are made up of asteroid-sized rock chunks and other material that could be leftovers of planets that have formed in the system.

The majority of stars like our Sun have at least one stellar companion. Astronomers have theorized that planets could form with little trouble in two-star systems, called binaries, despite the more complex gravitational tugging. The new study provides strong observational evidence to support that idea.

"There appears to be no bias against having planetary system formation in binary systems," said study leader David Trilling of the University of Arizona. "There could be countless planets out there with two or more suns."

A planetary nursery

Trilling and his team looked for disks in 69 binary systems between 50 and 200 light-years away from Earth. All the stars are more massive and younger than our middle-aged Sun. The researchers found that about 40 percent of the binary systems they looked at had disks. This frequency is a bit higher than that for a comparable sample of single stars and suggests planets are at least as common around binary stars as they are around single stars.

Deepak Rhagavan, an astronomer at Georgia State University who was not involved in the study, says the new findings are exciting because they are the first evidence of a planetary nursery in a multiple star system. "Until now, we knew planets existed [in multiple star systems], but I think this is the first time that we've gotten a comprehensive study that looks at the debris disk where planets are born," Rhagavan said.

Last year, Rhagavan's team reported that many star systems known to harbor planets actually contained two, and in some cases, even three, stars.

Alan Boss, a planet formation theorist at Carnegie Institution of Washington, says the finding is encouraging news for planet hunters. "It's pretty reassuring," said Boss, who also was not involved in the study. "This really goes in the direction of making planets more frequent than they would be otherwise."

Tight binaries

Surprisingly, most of the debris disks found in the new survey were around so-called tight binary systems, where the stars are separated by 500 AU or less. One AU is equal to the distance between the Earth and the Sun.

Scientists know of about 50 planets that have two Suns, but all of them belong to "wide" binary systems, where the stars are separated by about 1,000 AU.

"The fact that they've found some positive evidence of planet-forming disks being around close binaries is really a new step," Boss said.

Some scientists had previously argued that planet formation would be stifled in tight binary systems because of the large gravitational interactions between the stars.

"The idea was that the extra star would stir up the stuff in the planet forming disk so much that you would never form a planet," Trilling told SPACE.com.

Trilling said his team's results might mean that planet formation favors tight binaries over single stars. However, it could also be that tight binaries are just dustier, and thus easier to spot. Further observations will be required to determine which of these explanations is correct.

A human gazing at a double sunset on a world with two Suns like Skywalker's Tatooine might not find the scene so alien after all, Trilling said. "It would be kind of like what you see on Earth, but with an extra Sun following in the sky," he said. "Maybe it's a little hotter during the day."

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