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Planets may be smashed to dust near black holes

Nov. 1, 2011
Courtesy of the University of Leicester
and World Science staff

Fat, doughnut-shaped shrouds of dust sur­round­ing about half of the big­gest black holes could be the re­sult of high speed crashes be­tween plan­ets and as­ter­oids, some as­tro­no­mers say.

A group of sci­en­tists led by Sergei Nayak­shin of the Uni­vers­ity of Leices­ter, U.K., has pub­lished the the­o­ry in the re­search jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty.

Black holes are ob­jects so com­pact that their gra­vity be­comes over­pow­ering, draw­ing in an­y­thing that strays too close, even light. The larg­est, most pow­er­ful black holes are de­scribed as “super­massive” and sit at the cen­ters of most ga­lax­ies. About half of these seem to be hid­den from view by mys­te­ri­ous dust clouds of un­clear or­i­gin. 

The new the­o­ry is in­spired by our own so­lar sys­tem, where the so-called zo­di­a­cal dust is known to or­i­ginate from col­li­sions be­tween sol­id bod­ies such as as­ter­oids and comets, re­search­ers say. The sci­en­tists pro­pose that the cen­tral re­gions of ga­lax­ies con­tain not only black holes and stars but al­so plan­ets and as­ter­oids.

Col­li­sions be­tween these rocky ob­jects would oc­cur at co­los­sal speeds as large as 1,000 km (600 miles) per sec­ond, con­tin­u­ously shat­ter­ing the ob­jects un­til eventually they end up as mi­cro­scop­ic dust. 

The harsh en­vi­ron­ment—radia­t­ion and fre­quent col­li­sions—would make the plan­ets ster­ile long be­fore they break up, Nayak­shin said. “Too bad for life on these plan­ets” if it ev­er did man­age to evolve, he added. “On the oth­er hand the dust cre­at­ed in this way blocks much of the harm­ful radia­t­ion from reach­ing the rest of the host gal­axy. This in turn may make it eas­i­er for life to prospe­r else­where in the rest of the cen­tral re­gion of the gal­axy.” 

He al­so be­lieves that un­der­stand­ing the or­i­gin of the dust near black holes is im­por­tant in our mod­els of how these mon­sters grow and how ex­actly they af­fect their host ga­lax­ies. “We sus­pect that the super­massive black hole in our own gal­axy, the Milky Way, ex­pelled most of the gas that would oth­erwise turn in­to more stars and plan­ets,” he con­tin­ued. “Under­stand­ing the or­i­gin of the dust in the in­ner re­gions of ga­lax­ies would take us one step clos­er to solv­ing the mys­tery of the super­massive black holes.”


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Fat, doughnut-shaped shrouds of dust surrounding about half of the biggest black holes could be the result of high speed crashes between planets and asteroids, some astronomers say. A group of scientists led by Sergei Nayakshin of the University of Leicester, U.K., has published the theory in the research journal Monthly Notices of the Royal Astronomical Society. Black holes are objects so compact that their gravity becomes overpowering, drawing in anything that strays too close, even light. The largest, most powerful black holes are described as “supermassive” and sit at the centers of most galaxies. About half of these seem to be hidden from view by mysterious dust clouds of unclear origin. The new theory is inspired by our own solar system, where the so-called zodiacal dust is known to originate from collisions between solid bodies such as asteroids and comets, researchers say. The scientists propose that the central regions of galaxies contain not only black holes and stars but also planets and asteroids. Collisions between these rocky objects would occur at colossal speeds as large as 1000 km (600 miles) per second, continuously shattering the objects until eventually they end up as microscopic dust. The harsh environment—radiation and frequent collisions—would make the planets orbiting supermassive black holes sterile, even before they break up, Nayakshin said. “Too bad for life on these planets” if it ever did manage to evolve, he added. “On the other hand the dust created in this way blocks much of the harmful radiation from reaching the rest of the host galaxy. This in turn may make it easier for life to prosper elsewhere in the rest of the central region of the galaxy.” He also believes that understanding the origin of the dust near black holes is important in our models of how these monsters grow and how exactly they affect their host galaxies. “We suspect that the supermassive black hole in our own galaxy, the Milky Way, expelled most of the gas that would otherwise turn into more stars and planets,” he continued. “Understanding the origin of the dust in the inner regions of galaxies would take us one step closer to solving the mystery of the supermassive black holes.”