Some Stars Capture Rogue Planets

Posted on April 17, 2012

New research suggests that billions of stars in our galaxy have captured rogue planets that once roamed interstellar space. These nomad planets - booted out of the star systems in which they formed - occasionally find a new home with a different sun.

Hagai Perets of the Harvard-Smithsonian Center for Astrophysics (CfA) says, "Stars trade planets just like baseball teams trade players."

The researchers say the finding could explain the existence of some planets that orbit surprisingly far from their stars. It can also explain the existence of a double-planet system. 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.

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 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.

Astronomers haven't detected any clear-cut cases of captured planets yet. The researchers say the best evidence to date in support of planetary capture comes from the European Southern Observatory, which announced the discovery of two planets (weighing 14 and 7 times Jupiter) orbiting each other without a star in 2006.

Perets says, "The rogue double-planet system is the closest thing we have to a 'smoking gun' right now. To get more proof, we'll have to build up statistics by studying a lot of planetary systems."

Perets also says there is no evidence our solar system contains a captured planet, but it's not impossible that there is one far from our Sun. Perets says, "There's no evidence that the Sun captured a planet. 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."


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