数亿恒星曾“捕获”过行星

New research suggests that billions of stars in our galaxy¹ have captured rogue² planets that once roamed interstellar space（星际空间） . The nomad³（流浪的） worlds, which were kicked out of the star systems in which they formed, occasionally find a new home with a different sun. This finding could explain the existence of some planets that orbit surprisingly far from their stars, and even the existence of a double-planet system. "Stars trade planets just like baseball teams trade players," said Hagai Perets of the Harvard-Smithsonian Center for Astrophysics.

The study, co-authored by Perets and Thijs Kouwenhoven of Peking University, China, will appear in the April 20th issue of The Astrophysical Journal.

To reach their conclusion, Perets and Kouwenhoven simulated young star clusters containing free-floating planets. They found that if the number of rogue planets equaled the number of stars, then 3 to 6 percent of the stars would grab a planet over time. The more massive a star, the more likely it is to snag（抓住机会） a planet drifting by.

They studied young star clusters because capture is more likely when stars and free-floating planets are crowded together in a small space. Over time, the clusters disperse⁴ due to close interactions between their stars, so any planet-star encounters have to happen early in the cluster's history.

Rogue planets are a natural consequence of star formation. Newborn star systems often contain multiple planets. If two planets interact, one can be ejected and become an interstellar traveler. If it later encounters a different star moving in the same direction at the same speed, it can hitch⁵ a ride.

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 tilted⁶ relative to any native planets, and may even revolve⁷ around its star backward.

Astronomers⁸ haven't detected any clear-cut cases of captured planets yet. Imposters can be difficult to rule out. Gravitational interactions within a planetary system can throw a planet into a wide, tilted orbit that mimics⁹ the signature of a captured world.