CAMBRIDGE, Mass., April 17 (UPI) -- Billions of stars in our galaxy may have captured rogue planets that once roamed interstellar space, adding them to their stellar families, U.S. scientists say.
The finding could explain the existence of planets that orbit surprisingly far from their stars, and even account for strange double-planet systems, the Harvard-Smithsonian Center for Astrophysics reported Tuesday.
"Stars trade planets just like baseball teams trade players," astrophysicist Hagai Perets said.
Rogue planets are a natural consequence of star formation, the researchers said, as newborn star systems often contain multiple planets.
If two planets interact, one can be ejected and become an interstellar traveler, and if it encounters a different star moving in the same direction at the same speed, it can hitch a ride.
However, the concept still remains a theory, and astronomers haven't detected any clear-cut cases of captured planets yet although they are looking for evidence.
One finding supporting planetary capture 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," Perets said. "To get more proof, we'll have to build up statistics by studying a lot of planetary systems."
Finding a planet in a distant orbit around a low-mass star would be a good sign of capture, he said, because the star's disk wouldn't have had enough material to form the planet so far out.
So could our solar system harbor an alien world far beyond Pluto?
"There's no evidence that the sun captured a planet," Perets said. "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."