Space is full of threats to life, especially asteroids that smack into Earth. An even more explosive hazard looms in deep space: supernovas, which can unleash enough radiation to zap our life-shielding ozone layer. However, a new study, accepted for publication in the Astrophysical Journal, concludes that a supernova must blow up within 25 light-years of Earth to wreak major havoc--so close that it might happen just once or twice in a billion years.
In 1974, the risk seemed higher. Physicist Malvin Ruderman of Columbia University in New York City calculated that gamma rays and cosmic rays from a supernova about 50 light-years away would erase most of our ozone for decades, exposing Earth's surface to harmful ultraviolet (UV) light from the sun. Since then, researchers have debated how much radiation supernovas produce, how those rays damage the atmosphere, and how often stars explode near our sun. The latter estimates are "all over the map," says astrophysicist Neil Gehrels of NASA's Goddard Space Flight Center in Greenbelt, Maryland. Some teams have claimed that recent supernovas--perhaps within the last few million years--devastated life.
That's unlikely, according to work by Gehrels and his colleagues. The team used a detailed model of the atmosphere to gauge how nitrogen oxides--a chemical species catalyzed by a supernova's radiation--would destroy ozone. The researchers also used the energy from Supernova 1987A, which exploded in another galaxy in 1987, as a guide for how much radiation would reach Earth. The results are good news for Earthlings: In order to thin the ozone layer so that twice as much UV light reaches the surface, a star must explode within 25 light-years. There are no massive stars--the ones that die as supernovas--that close to the Earth today. Moreover, these stars approach our solar system so seldom that a nearby supernova should happen only every 700 million years or so, on average, according to the team's analysis of stellar motions in the galaxy, making them minor contributors to the history of mass extinctions on earth.
The study surpasses other attempts to quantify the effects of supernovas on Earth's atmosphere, says astronomer John Scalo of the University of Texas, Austin. "Their result depends sensitively on many things, but it's the best we have right now," he says. Lower-level radiation from more distant supernovas still might have triggered episodes of genetic mutation hundreds or thousands of times during Earth's history, Scalo notes.