Comets

Comets may have come from other solar systems.

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.


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.

World Science