Billions of stars in our galaxy may have captured rogue planets, kicked out of the star systems in which they formed.
Newborn star systems often contain multiple planets - and, if two interact, one can be ejected and attach itself to a different star.
According to Hagai Perets of the Harvard-Smithsonian Center for Astrophysics, "Stars trade planets just like baseball teams trade players."
He and Thijs Kouwenhoven of Peking University simulated young star clusters containing free-floating planets, and found that if the number of rogue planets equaled the number of stars, then three to six percent of the stars would grab one eventually. The more massive a star, the more likely this is to happen.
A captured planet tends to end up hundreds or thousands of times farther from its star than Earth is from the sun. It's also likely to have a orbit that's tilted relative to any native planets, and may even revolve around its star backward.
However, no clear-cut cases have yet been detected. A planet in a distant orbit around a low-mass star would be a good candidate, as the star's disk wouldn't have had enough material to form the planet so far out.
The best evidence so far comes from the European Southern Observatory, which announced in 2006 the discovery of two planets orbiting each other without a star.
"The rogue double-planet system is the closest thing we have to a 'smoking gun' right now," says Perets. "To get more proof, we'll have to build up statistics by studying a lot of planetary systems."
It's even possible - though unlikely - that our own solar system contains such an interloper, far beyond Pluto.
"There's no evidence that the sun captured a planet," said Perets. "We can rule out large planets. But there's a non-zero chance that a small world might lurk on the fringes of our solar system."