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Is our galaxy’s black hole shredding asteroids, planets?

Nov. 9, 2011
World Science staff

The black hole at the cen­ter of our gal­axy may be shred­ding as­ter­oids on a daily ba­sis, and may­be a plan­et eve­ry few hun­dred years—each time with a bang, some as­tro­no­mers pro­pose.

Kastytis Zubo­vas of the Uni­vers­ity of Leices­ter, U.K. and col­leagues de­vel­oped the the­o­ry to ex­plain small, near-daily flares de­tected around an ob­ject be­lieved to be the black hole. Their pa­pe­r on the study has been sub­mit­ted to the re­search jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty and is al­so posted on­line.

A black hole is an ob­ject so com­pact that its gra­vity over­whelms and draws in an­y­thing that strays too close, even light rays. Par­tic­u­larly heavy, or “su­pe­r­mas­sive,” black holes are be­lieved to lurk at the cen­ters of most ga­lax­ies, an­chor­ing their stars to­geth­er.

The black hole at the cen­ter of our Milky Way gal­axy is dubbed Sgr A*. While black holes aren’t di­rectly vis­i­ble, the ac­ti­vity around them gives them away, as­tro­no­mers say. This ac­ti­vity in­cludes long-dis­tance gravita­t­ional ef­fects, and shorter-dis­tance ef­fects in which the mighty gravita­t­ional field lit­er­ally shreds ob­jects be­ing dragged in­to the black hole. “Ti­dal” ef­fects like that oc­cur be­cause the gra­vity is stronger at one end of the dis­turbed ob­ject than the oth­er. The broken-up ob­jects shine as they dis­in­te­grate.

The new study is­n’t the first pro­pos­al re­gard­ing plan­ets po­ten­tially de­stroyed near black holes.

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 near the black holes, some say. A mem­ber of Zubo­vas’ team, Sergei Nayashkin, put forth that pro­pos­al in the Oct. 31 ad­vance on­line is­sue of the same jour­nal, along with two oth­er re­search­ers.

The new study looks more spe­cif­ic­ally at “our” su­pe­r­mas­sive black hole, and the mys­te­ri­ous mini-flares sur­round­ing it.

A typ­i­cal su­pe­r­mas­sive black hole in a gal­axy is ex­pected to tear apart a star eve­ry 10,000 years or so, but as­ter­oids are much smaller and more com­mon, Zubo­vas and col­leagues wrote. Plan­ets may lie be­tween as­ter­oids and stars in terms of size and abun­dance.

The re­search­ers wrote that in their new work, they es­ti­mat­ed “the as­ter­oid dis­rup­tion rates, and the dis­tri­bu­tion of the ex­pected lu­mi­nos­i­ties” from the re­sulting flares, in the mid­dle of our gal­axy. The re­sult was a rea­son­a­ble fit with ob­serva­t­ions, they added. The team wrote that they as­sumed that the as­ter­oid popula­t­ion per star near the ga­lac­tic cen­ter is not too dif­fer­ent from that near the Sun.

“The fact that there are many more as­ter­oids than stars and that the as­ter­oids are much smaller than stars would nat­u­rally ex­plain why Sgr A* flares are much more fre­quent but much less lu­mi­nous and shorter than the stel­lar tid­al dis­rup­tion events,” they added. The flares oc­cur daily and last for a few hours, as op­posed to months in the case of stars, the sci­en­tists went on.

Tid­al dis­rup­tions are ex­pected to pro­duce light be­cause of the tre­men­dous amount of en­er­gy re­leased as a gi­ant ob­ject is torn apart, like a rub­ber band snap­ping in slow mo­tion. “The com­bina­t­ion of tid­al ‘grind­ing’ of large as­ter­oids in­to smaller frag­ments and evapora­t­ion of the lat­ter may de­stroy the as­ter­oids ef­fi­ciently and turn their bulk en­er­gy in­to heat,” the group wrote.

Dis­rup­tions of full plan­ets are probably much rar­er, but there one might have oc­curred 300 years ago, Zubo­vas and col­leagues wrote. The di­rect ev­i­dence would be gone, they added, but the ef­fects pos­sibly are vis­i­ble as a “light echo” on a near­by cloud of gas—whose sur­face ap­pears brighter to this day as a pos­sible re­sult of light from the orig­i­nal event bounc­ing off it. The cloud is is known as mo­lec­u­lar cloud Sgr B2.

A plan­etary dis­rup­tion near a black hole might not it­self be a trag­ic event for liv­ing be­ings on that plan­et, Nayashkin and col­leagues wrote in their ear­li­er stu­dy. That’s be­cause it’s un­likely that an­y­thing could be alive at that dis­tance from a black hole anyway.

* * *

The black hole at the center of our galaxy may be shredding asteroids on a daily basis, and maybe a planet every few hundred years—each time with a bang, some astronomers propose. Kastytis Zubovas of the University of Leicester, U.K. and colleagues developed the theory to explain small, near-daily flares detected around an object believed to be the black hole. Their paper on the study has been submitted to the research journal Monthly Notices of the Royal Astronomical Society and is also posted online. A black hole is an object so compact that its gravity overwhelms and draws in anything that strays too close, even light rays. Particularly heavy, or “supermassive,” black holes are believed to lurk at the center of most galaxies, anchoring their stars together. The black hole at the center of our Milky Way galaxy is dubbed Sgr A*. While black holes aren’t directly visible, the activity around them gives them away, astronomers say. This activity includes long-distance gravitational effects, and shorter-distance effects in which the mighty gravitational field literally shreds objects being dragged into the black hole. “Tidal” effects like that occur because the gravity is stronger at one end of the object than the other. The broken-up objects shine as they disintegrate. The new study isn’t the first proposal regarding planets potentially destroyed near black holes. 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 near the black holes, some say. A member of Zubovas’ team, Sergei Nayashkin, put forth that proposal in the Oct. 31 advance online issue of the same journal, along with two other researchers. The new study looks more specifically at “our” supermassive black hole, and the mysterious mini-flares surrounding it. A typical supermassive black hole in a galaxy is expected to tear apart a star every 10,000 years or so, but asteroids are much smaller and more common, Zubovas and colleagues wrote. Planets may lie between asteroids and stars in terms of size and abundance. The researchers wrote that in their new work, they estimated “the asteroid disruption rates, and the distribution of the expected luminosities” from the resulting flares, in the middle of our galaxy. The result was a reasonable fit with observations, they added. The team wrote that they assumed that the asteroid population per star near the galactic center is not too different from that near the Sun. “The fact that there are many more asteroids than stars and that the asteroids are much smaller than stars would naturally explain why Sgr A* flares are much more frequent but much less luminous and shorter than the stellar tidal disruption events,” they added. The flares occur daily and last for a few hours, as opposed to months in the case of stars, the scientists went on. Tidal disruptions are expected to produce light because of the tremendous amount of energy released as a giant object is torn apart, like a rubber band snapping in slow motion. “The combination of tidal ‘grinding’ of large asteroids into smaller fragments and evaporation of the latter may destroy the asteroids efficiently and turn their bulk energy into heat,” the group wrote. Disruptions of full planets are probably much rarer, but there one might have occurred 300 years ago, Zubovas and colleagues wrote. The direct evidence would be gone, they added, but the effects possibly are visible as a “light echo” on a nearby cloud of gas—whose surface appears brighter to this day as a possible result of light from the original event bouncing off it. The cloud is is known as molecular cloud Sgr B2. A planetary disruption near a black hole might not itself be a tragic event for living beings on that planet, Nayashkin and colleagues wrote in their earlier study. That’s because it’s unlikely that anything could be alive at that distance from a black hole.