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Comets may have come from other solar systems

June 10, 2010
Courtesy of Southwest Research Institute
and World Science staff

Many of the best known comets, in­clud­ing Hal­ley, Hale-Bopp and Mc­Naught, may have been born or­bit­ing oth­er stars, ac­cord­ing to a new the­o­ry.

The pro­pos­al comes from a team of as­tro­no­mers led by Hal Lev­i­son of the South­west Re­search In­sti­tute in Boul­der, Co­lo., who used com­put­er sim­ula­t­ions to show that the Sun may have cap­tured small icy bod­ies from “si­b­ling” stars when it was young.

Comet McNaught over the Pa­ci­fic Ocean in Jan­u­ary 2007. (Cred­it: Se­bas­tian Deir­ies, Eu­ro­pe­an South­ern Ob­serv­a­tory)


Sci­en­tists be­lieve the Sun formed in a clus­ter of hun­dreds of stars closely packed with­in a dense gas cloud. Each star would have formed many small icy bod­ies, Lev­i­son and col­leagues say—comets. These would have aris­en from the same disk-shaped zone of gas and dust, sur­round­ing each star, from which plan­ets formed. 

Most of these comets were slung out of these fledg­ling plan­e­tary sys­tems due to gravita­t­ional in­ter­ac­tions with newly form­ing gi­ant plan­ets, the the­o­ry goes. The comets would then have be­come ti­ny, free-float­ing mem­bers of the clus­ter. 

The Sun’s clus­ter came to a vi­o­lent end, how­ev­er, when its gas was blown out by the hot­test young stars, ac­cord­ing to Lev­i­son and col­leagues. The new mod­els show that the Sun then gravita­t­ionally cap­tured a large cloud of comets as the clus­ter dis­persed.

“When it was young, the Sun shared a lot of spit with its sib­lings, and we can see that stuff to­day,” said Lev­i­son, whose re­search is pub­lished in the June 10 ad­vance on­line is­sue of the re­search jour­nal Pro­ceed­ings of the Na­tio­n­al Aca­de­my of Sci­en­ces.

“The pro­cess of cap­ture is sur­pris­ingly ef­fi­cient and leads to the ex­cit­ing pos­si­bil­ity that the cloud con­tains a pot­pour­ri that sam­ples ma­te­ri­al from a large num­ber of stel­lar sib­lings of the Sun,” added Mar­tin Dun­can of Queen’s Uni­vers­ity, Can­a­da, a co-author of the stu­dy.

The team cites as ev­i­dence a bubble-shaped re­gion of comets, known as the Oort cloud, that sur­rounds the Sun, ex­tend­ing half­way to the near­est star. It has been com­monly as­sumed this cloud formed from the Sun’s proto-plan­e­tary disk, the struc­ture from which plan­ets formed. But be­cause de­tailed mod­els show that comets from the so­lar sys­tem pro­duce a much more ane­mic cloud than ob­served, anoth­er source is needed, Lev­i­son’s group con­tends.

“More than 90 per­cent of the ob­served Oort cloud comets [must] have an extra-so­lar orig­in,” as­sum­ing the Sun’s proto-plan­e­tary disk can be used to es­ti­mate the Oort Cloud’s in­dig­e­nous popula­t­ion, Lev­i­son said.


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Many of the best known comets, including Halley, Hale-Bopp and, most recently, McNaught, may have been born orbiting other stars, according to a new theory. The proposal comes from a team of astronomers led by Hal Levison of the Southwest Research Institute in Boulder, Colo., who used computer simulations to show that the Sun may have captured small icy bodies from “sibling” stars when it was young. While the Sun has no companion stars now, scientists think it formed in a cluster containing hundreds of stars closely packed within a dense gas cloud. Each star would have formed many small icy bodies, Levison and colleagues say—comets. These would have arisen from the same disk-shaped cloud of gas and dust, surrounding each star, from which planets formed. Most of these comets were slung out of these fledgling planetary systems due to gravitational interactions with newly forming giant planets, becoming tiny, free-floating members of the cluster, the theory goes. The Sun’s cluster came to a violent end, however, when its gas was blown out by the hottest young stars, according to Levison and colleagues. The new models show that the Sun then gravitationally captured a large cloud of comets as the cluster dispersed. “When it was young, the Sun shared a lot of spit with its siblings, and we can see that stuff today,” said Levison, whose research is published in the June 10 advance online issue of the research journal pnas. “The process of capture is surprisingly efficient and leads to the exciting possibility that the cloud contains a potpourri that samples material from a large number of stellar siblings of the Sun,” added Martin Duncan of Queen’s University, Canada, a co-author of the study. The team cites as evidence a bubble-shaped region of comets, known as the Oort cloud, that surrounds the Sun, extending halfway to the nearest star. It has been commonly assumed this cloud formed from the Sun’s proto-planetary disk, the structure from which planets formed. But because detailed models show that comets from the solar system produce a much more anemic cloud than observed, another source is needed, Levison’s group contends. “More than 90 percent of the observed Oort cloud comets [must] have an extra-solar origin,” assuming the Sun’s proto-planetary disk can be used to estimate the Oort Cloud’s indigenous population, Levison said.