"Long before it's in the papers"
April 17, 2012

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Homeless planets may get adopted

April 17, 2012
Courtesy of the Harvard-Smithsonian Center for Astrophysics
and World Science staff

New re­search sug­gests that bil­lions of stars in our gal­axy have cap­tured rogue plan­ets that once roamed the voids be­tween stars.

The no­mad worlds, which were kicked out of the star sys­tems in which they formed, could oc­ca­sion­ally find a new home with a dif­fer­ent sun, as­tro­no­mers pro­pose. This could ex­plain the ex­ist­ence of some plan­ets that or­bit sur­pris­ingly far from their stars, and even the ex­ist­ence of a double-plan­et sys­tem.

In this artist's con­cep­tion, a cap­tured world drifts at the out­er edge of a dis­tant star sys­tem. (Cred­it: Chris­tine Pul­liam (CfA))

“S­tars trade plan­ets just like base­ball teams trade play­ers,” said Ha­gai Perets of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass. The stu­dy, co-authored by Perets and Thijs Kou­wen­hoven of Pe­king Uni­vers­ity, Chi­na, is to ap­pear in the April 20 is­sue of The As­t­ro­phys­i­cal Jour­nal.

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 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 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 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, ac­cord­ing to as­tro­no­mers. New­born star sys­tems of­ten con­tain mul­ti­ple plan­ets. If two plan­ets in­ter­act, one can be ejected and be­come an in­ter­stel­lar trav­el­er. If the drift­ing chunk of real es­tate lat­er en­coun­ters a dif­fer­ent star mov­ing in the same di­rec­tion at the same speed, it might hitch a ride.

A cap­tured world 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.

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.

The best ev­i­dence to date in sup­port of plan­etary cap­ture, the re­search­ers say, comes from the Eu­ro­pe­an South­ern Ob­serv­a­to­ry, which an­nounced in 2006 the disco­very of two plan­ets (weigh­ing 14 and 7 times Ju­pi­ter) or­biting each oth­er with­out a star. They could have cap­tured each oth­er, the rea­son­ing goes. “The rogue double-plan­et sys­tem is the clos­est thing we have to a ‘smok­ing gun’ right now,” said Perets. “To get more proof, we’ll have to build up sta­tis­tics by stu­dying a lot of plan­etary sys­tems.”

Could our so­lar sys­tem har­bor an al­ien world far be­yond Plu­to? As­tro­no­mers have looked, and haven’t found an­ything yet. “There’s no ev­i­dence that the Sun cap­tured a plan­et,” said Perets. “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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New research suggests that billions of stars in our galaxy have captured rogue planets that once roamed interstellar space. The nomad worlds, which were kicked out of the star systems in which they formed, could occasionally find a new home with a different sun, astronomers propose. This 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 in Cambridge, Mass. The study, co-authored by Perets and Thijs Kouwenhoven of Peking University, China, is to appear in the April 20 issue of The Astrophysical Journal. 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 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 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 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, according to astronomers. Newborn star systems often contain multiple planets. If two planets interact, one can be ejected and become an interstellar traveler. If the drifting chunk of real estate later encounters a different star moving in the same direction at the same speed, it might hitch a ride. A captured world 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. 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 star’s disk wouldn’t have had enough material to form the planet so far out. The best evidence to date in support of planetary capture, the researchers say comes from the European Southern Observatory, which announced in 2006 the discovery of two planets (weighing 14 and 7 times Jupiter) orbiting each other without a star. They could have captured each other, the reasoning goes. “The rogue double-planet system is the closest thing we have to a ‘smoking gun’ right now,” said Perets. “To get more proof, we’ll have to build up statistics by studying a lot of planetary systems.” Could our solar system harbor an alien world far beyond Pluto? Astronomers have looked, and haven’t found anything yet. “There’s no evidence that the Sun captured a planet,” said Perets. “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.”