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.

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.

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

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).

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.

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 dai-ly 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.”

Courtesy World Science staff