Rogue Planets Could Find Homes Orbiting Alien Stars

A new study suggests that billions of rogue, free-floating worlds could become attached to the planetary systems of foreign stars.

Planets that somehow were ejected or broke free from the gravitational hold of their parent stars may have eventually found themselves captured by other stars, a new study has found. In fact, even though there is thought to be billions of rogue planets wandering the Milky Way, there very well could have been billions more that have settled into orbits around billions of non-parent stars.

"Stars trade planets just like baseball teams trade players," Hagai Perets of the Harvard-Smithsonian Center for Astrophysics, the study's lead author, said in a statement, according to Space.com.

Perets and his co-author, Thijs Kouwenhoven of China's Peking University, constructed a simulation model of young star clusters wherein the number of rogue planets were comparable to the number of stars. Their findings indicated that 3 to 6 percent of the free-floating worlds would have found new stellar hosts. Most reattachments would have occurred during the younger stages of a star cluster's evolution, the relative proximity of the member stars working to corral rogue planets via gravitational attraction. The study noted that the more massive the star, the more likely it could attract a wandering planet. As the star cluster dispersed over billions of years, free-ranging worlds would become more difficult to capture (although it could be done with rogue planets matching a star's relative speed in its galactic revolution around the Milky Way, eventually being captured by the star's gravitational field).

Perets and Kouwenhoven also posited that such snaring of alien rogue planets could account for some of the strange orbits being found among the extra-solar worlds being discovered. It might also help explain how some planets circle their host star in a distant fashion, even though the star's mass would indicate that the planet should not actually be in its planetary train.

The study appears in the Astrophysical Journal released on April 20.

Even though vagabond worlds would undoubtedly be attracted to other stars given the right circumstances, the number that would not be attracted exceeds 90 percent. Those particular findings appear to support the revelations of a study released in May 2011 that suggested that rogue planets could actually outnumber those circling stars.

Various studies in the past year seem to indicate that our universe isn't as stable as we once thought it was.

A study released in March found that the Milky Way galaxy was also home to rogue "hyper-velocity" planets. These worlds have been shot out from the galactic center due to a close call with the Milky Way's supermassive black hole. The runaway planets travel between 7 and 30 million miles per hour and are flung at "warp-speed" toward the galactic frontier, eventually escaping -- barring collisions with other interstellar traffic -- the home galaxy and reaching -- and continuing onward through -- intergalactic space.

And just as a wandering planet can be captured by a star's gravitational force, it's distance from it's parent or host star might also make it easier to eject from a planetary system. A computer simulation study released in September 2011 indicated that the current mass and layout of the Solar System is an improbable configuration and that there would have had to be at some point in its evolutionary history a fifth gas giant planet. The simulation posited that a planet the size of Uranus or Neptune once circled the Sun in a tight cluster with its sister gas giants but over time, the larger gas giants drifted inward toward the Sun as the lighter worlds drifted further away. A close encounter with massive Jupiter caused the "slingshot effect," hurling the fifth gas giant out of the Solar System.

(photo credit: NASA and G. Bacon (STScI), Wikimedia Commons)