The above image from 2011 is an artist's concept of how a Jupiter-like rogue planet might appear as it drifted through space. Image Credit: NASA/JPL-Caltech
There is new research suggesting that billions of stars in our galaxy have captured rogue planets that once roamed between stars.
These nomad worlds were kicked out of the star systems in which they formed. Astronomers propose that, occasionally, they could find a new home with a different sun. This theory could explain the existence of some planets that orbit surprisingly far from their stars. It could even explain the existence of a known double-planet system.
The new study, appearing in the April 20 issue of The Astrophysical Journal, is co-authored by Haggai Perets of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and Thijs Kouwenhoven of Peking University, China. Perets and Kouwenhoven created computer simulations of 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 capture a planet over time. The more massive a star, the more likely it is to snag a planet drifting by.
They studied simulations of young star clusters because capture is presumed more likely when stars and free-floating planets are crowded together in a small space. Over time, the clusters disperse due to close interactions between the stars, so any planet-star encounters have to happen early in the cluster’s history.
Astronomers think that rogue planets are a natural consequence of star formation. The concept goes like this: Newborn star systems often contain multiple planets. If two planets interact, one can be ejected and become an interstellar traveler. But if the drifting body encounters a different star moving in the same direction at the same speed, it might hitch a ride and be gravitationally “adopted” by the new parent star.
Rather than nuzzling close to its new parent, a captured world tends to end up in an orbit hundreds or thousands of times farther from its star than Earth is from the sun. It is also likely that the new addition will have an orbit that is tilted relative to any native planets, and might even revolve around its star backward compared to the rest of the family.
Astronomers 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 mimics the signature of a captured world, Perets and Kouwenhoven noted. Finding a planet in a distant orbit around a low-mass star would be a good sign of capture, because the planet-forming disk originally surrounding the star wouldn’t have had enough material to form the planet so far out, they argue.
Researcher say the European Southern Observatory (ESO) provided the best evidence to date in support of planetary capture. In 2006 the ESO announced the discovery of two planets (weighing 14 and 7 times that of Jupiter) orbiting each other without a star. The reasoning goes that they could have captured each other. This rogue double-planet system is the closest thing to a “smoking gun” on the theory of planetary capture. More study and statistics are needed.
Could our own Sol system harbor an alien world far beyond Pluto? Astronomers have not found anything yet. Large planets may be ruled out at this point, but some suggest it is possible that a small world might lurk on the fringes of our system. Sound like just an old sci-fi "B" movie plot? Maybe. And maybe not...
The Astrophysical Journal is a peer-reviewed scientific journal covery astronomy and astrophysics. To read more, visit the publication URL here: http://iopscience.iop.org/0004-637X
To learn more about exoplanets and NASA's planet-finding program, PlanetQuest, visit this URL: http://planetquest.jpl.nasa.gov/
To learn more about the work of the European Southern Observatory (ESO), visit their site here: http://www.eso.org/