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Death from across the galaxy

Feb. 27, 2007
Special to World Science  

A type of co­los­sal cos­mic ex­plo­sion could beam le­thal ra­di­a­tion across a ga­laxy, fry­ing any life forms in its path, a new anal­y­sis has found.

The blasts are thought to oc­cur rare­ly in our Milky Way gal­axy, but more of­ten in those where stars are born and die more fre­quent­ly. These in­clude ar­eas where as­tro­no­mers hope to find Earth-like plan­ets ripe for life.

Artist's con­cep­tion of a gam­ma-ray burst. (Cour­te­sy NA­SA)

In a 1995 stu­dy, Steve Thor­sett of Prince­ton Uni­ver­si­ty in Prince­ton, N.J. cal­cu­lat­ed that such events, called gam­ma-ray bursts, might wreak hav­oc on an Earth-like plan­et if they oc­curred near it. But sci­en­tists don’t ful­ly un­der­stand the ex­tent of the pos­si­ble dam­age. Es­pe­cial­ly un­clear is how far a burst would have to oc­cur to af­fect life, ac­cord­ing to the au­thors of the new stu­dy.

Gamma-ray bursts are flashes of high-en­er­gy ra­di­a­tion found to oc­cur ran­dom­ly in space. At least some are thought to be as­so­ci­at­ed with ex­treme­ly mas­sive stars that, hav­ing burnt out, col­lapse to form black holes.

In the new re­search, Doug­las Galante and Jor­ge Er­nes­to Hor­vath of the Uni­ver­si­ty of São Pa­o­lo, Bra­zil, ar­gued that gam­ma-ray bursts could shine their le­thal ef­fects across a whole gal­axy, and dam­age life over great­er dis­tances still. The study is to ap­pear in a forth­com­ing is­sue of the In­ter­na­tion­al Jour­nal of As­tro­bi­ol­o­gy.

The bursts could cause “global en­vi­ron­men­tal changes and bio­spheric dam­age” even at dis­tances five times the Milky Way’s width, they wrote. Our Milky Way is a rel­a­tively large, spir­al gal­axy, about 100,000 light-years wide (a light-year is the dis­tance light trav­els in a year).

Artist's concept of life in the Or­do­vi­cian era (490 mil­lion to 443 mil­lion years ago). (Cour­te­sy NA­SA)

Gamma-ray bursts are thought to emerge main­ly from the poles of a col­laps­ing star. This cre­ates two, op­po­site­ly-shin­ing beams of ra­di­a­tion shaped like nar­row cones. Plan­ets not ly­ing in these cones would be com­par­a­tive­ly safe; the chief wor­ry is for those that do.

Galante and Hor­vath iden­ti­fied three as­pects of gam­ma-ray bursts as par­t­i­cu­lar­ly deadly. 

The first is a flash of gam­ma rays, the high­est-en­er­gy form of light. The flash can im­pe­r­il even the most ra­di­a­tion-resistant or­gan­isms known, the bac­te­ri­um Deinococ­cus ra­dio­du­rans, the re­search­ers wrote. This mi­crobe can take 3,000 times the ra­di­a­tion that would kill a hu­man: the as­sault shreds its ge­nome to hun­dreds of bits, but the har­dy bug stitches them back to­geth­er.

Galante and Hor­vath cal­cu­lat­ed that for a plan­et with a thin at­mos­phere, the gam­ma flash could kill 90 per­cent of D. ra­dio­du­rans from dis­tances up to three times our gal­ax­y’s width. A thick at­mos­phere would pro­tect the mi­crobes from this, but not nec­es­sar­i­ly from a sec­ond com­po­nent of the beam, ul­t­ra­vio­let ra­di­a­tion. Ul­t­ra­vio­let is a type of light slight­ly low­er in en­er­gy than gam­ma rays, but le­thal, large­ly be­cause it pen­e­trates DNA very eas­i­ly.

D. ra­dio­du­rans, the most ra­di­a­tion-re­sis­tant bac­te­ria known. In this im­age, a clus­ter of four of the cells are repli­cat­ing syn­chro­nous­ly. (Cour­te­sy M.J. Da­ly, US­UHS)

For thick-at­mos­phere plan­ets, a gam­ma-ray burst’s ul­t­ra­vio­let rays would kill 90 per­cent of D. ra­dio­du­rans at dis­tances rang­ing from 13,000 to 62,000 light years, about two-thirds the ga­lac­tic width, the re­search­ers cal­cu­lated.

Life sur­viv­ing that on­slaught would have to con­tend with a third ef­fect, de­ple­tion of the at­mos­phere’s pro­tective ozone lay­er by the burst. This would kill 90 per­cent of D. ra­dio­du­rans at up to 40 per­cent of the dis­tance across the Milky Way, Ga­lante and Hor­vath es­ti­mat­ed.

Gamma-ray bursts are de­tected rough­ly once dai­ly some­where in the sky. The like­li­hood of one strik­ing Earth has been de­bat­ed. Re­search­ers at Ohio State Uni­ver­si­ty cal­cu­lat­ed, in a pape­r in the re­search jour­nal Ac­ta As­tro­no­mi­ca late last year, that the prob­a­bil­i­ty is vir­tu­al­ly nil. Our gal­ax­y’s chem­i­cal com­po­si­tion is in­com­pat­i­ble with strong gam­ma-ray bursts, they wrote. 

On the oth­er hand, as­tro­no­mers at the Uni­ver­si­ty of Kan­sas in Law­rence, Kan. and at NASA hy­poth­e­sized in 2004 that at least one has al­ready struck Earth, caus­ing the so-called Or­do­vi­ci­an Mass Ex­tinc­tion 450 mil­lion years ago. The Earth’s sec­ond most dev­as­tat­ing ex­tinc­tion, it de­stroyed an ar­ray of the life forms that had flour­ished un­til then, re­strict­ed in that time to the seas.

“Re­cently, gamma-ray bursts were rec­og­nized as some of the most en­er­get­ic as­t­ro­phys­i­cal events since the Big Bang” that gave birth to our uni­verse, Ga­lan­te and Hor­vath wrote. “It is un­de­ni­a­ble that dam­age to the bi­o­ta could be se­vere if a burst strikes (or has struck) the plan­et.”

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A type of colossal cosmic explosion could beam lethal radiation across a galaxy, sterilizing planets along its path, a new analysis has found. The blasts are thought to occur rarely in our Milky Way galaxy, but more often in those where stars are born and die more frequently. These include areas where astronomers hope to find Earth-like planets ripe for life. In a 1995 study, Princeton University’s Steve Thorsett calculated that such events, called gamma-ray bursts, might wreak havoc on an Earth-like planet if they occurred nearby. But scientists still don’t fully understand the extent of the possible damage. Especially unclear is how far a burst would have to be to affect life, according to the authors of the new study. Gamma-ray bursts are flashes of high-energy radiation found to occur randomly in space. At least some are thought to be associated with extremely massive stars that, having burnt out, collapse to form black holes. In the new research, Douglas Galante and Jorge Ernesto Horvath of the University of São Paolo, Brazil, argued that gamma-ray bursts could shine their lethal effects effects across a whole galaxy, and damage life over greater distances still. The study is to appear in a forthcoming issue of the Inter national Journal of Astro biology. The bursts could cause “global environ mental changes and biospheric damage” even at distances five times the Milky Way’s width, they wrote. Our Milky Way is a relatively large, spiral galaxy, about 100,000 light-years wide (a light-year is the distance light travels in a year). Gamma-ray bursts are thought to emerge mainly from the poles of a collapsing star. This creates two, oppositely-shining beams of radiation shaped like narrow cones. Planets not lying in these cones would be comparatively safe; the chief worry is for those that do. Galante and Horvath identified four key harmful effects of gamma-ray bursts, of which three would be particularly dangerous. The first of these is a flash of gamma rays, the highest-energy type of light known. The flash can imperil even the most radiation-resistant organisms known, the bacterium Deinococcus radiodurans, the researchers wrote. The microbe can take 3,000 times the radiation that would kill a human: the assault shreds its genome to hundreds of bits, but the hardy bug stitches them back together. Galante and Horvath calculated that for a planet with a thin atmosphere, the gamma flash could kill 90% of the D. radiodurans from distances up to three times our galaxy’s width. A thick atmosphere would protect the microbes from this, but not necessarily from a second component of the beam, ultraviolet radiation. Ultraviolet is a type of light slightly lower in energy than gamma rays, but lethal, largely because it penetrates DNA very easily. For thick-atmosphere planets, a gamma-ray burst’s ultraviolet rays would kill 90% of D. radiodurans at distances ranging from 13,000 to 62,000 light years, about two-thirds the galactic width, the researchers calculated. Those surviving that, would still have to contend with a third effect, depletion of the atmosphere’s protective ozone layer by the burst. This would kill 90% of D. radiodurans at up to 40% of the distance across the Milky Way, Galante and Horvath estimated. Gamma-ray bursts are detected roughly once per day somewhere in the sky. The likelihood of one striking Earth has been debated. Researchers at Ohio State University calculated, in a paper in the research journal Acta Astronomica late last year, that the probability is virtually nil. Our galaxy’s chemical composition is incompatible with strong gamma-ray bursts, they wrote. On the other hand, some astronomers at the University of Kansas and NASA hypothesized in 2004 that at least one has already struck Earth, causing the so-called Ordovician Mass Extinction 450 million years ago. The Earth’s second most devastating extinction, it destroyed an array of the life forms that had flourished until then, restricted in that time to the seas. “Recently, Gamma-Ray Bursts were recognized as some of the most energetic astro physical events since the Big Bang” that gave birth to our universe, Galante and Horvath wrote. “It is undeniable that damage to the biota could be severe if a burst strikes (or has struck) the planet.”