A gamma-ray burst, the universe's most powerful known type of cosmic explosion, could have generated mass extinctions on Earth within the past billion years, researchers say.
Gamma-ray bursts, thought to be caused by giant exploding stars known as hypernovas or possibly be collisions between dead stars called neutron stars, are brief but extremely intense eruptions of electromagnetic radiation.
Such bursts can, in just milliseconds to minutes, emit as much total energy as our sun will in its entire lifetime of 10 billion years, researchers say.
Scientists believe gamma-ray bursts may be the result of giant exploding stars known as hypernovas, or be created by collisions of pairs of dead stars known as neutron stars.
A gamma-ray burst within our Milky Way galaxy could wreak havoc on the Earth if its tightly focused beam was pointed our way, scientists say.
Such an event could damage our atmosphere, reducing or destroying the ozone layer that protects life on Earth from damaging ultraviolet rays, possible leading to mass extinctions, they suggest.
Tsvi Piran of the Hebrew University in Israel and Raul Jimenez from the University of Barcelona in Spain, investigating the likelihood that such a blast may have hit the Earth in the past, say there's a 50 percent chance a burst capable of triggering extinctions could have struck in the past 500 million years.
If a time frame of 5 billion years is considered, those odds could go as high as 90 percent, they said in a report of their research in the journal Physical Review Letters.
Our solar system is about 4.6 billion years old.
The Earth has experienced a number of mass extinctions in its history, and Piran and Jimenez say they believe its possible a gamma-ray burst could have caused the so-called Ordovician extinction of around 440 million years ago.
Looking outward into the universe, the scientists also suggest gamma-ray bursts may be part of the reason why, despite long and intense searching, we have never found evidence alien life anywhere.
"This may be an explanation, or at least a partial one, to what is called the Fermi paradox or the 'Big Silence,'" says Piran. "Why we haven't encountered advanced civilizations so far? The Milky Way galaxy is much older than the solar system and there was ample time and ample space -- the number of planetary systems with conditions similar to Earth is huge -- for life to develop elsewhere in the galaxy. So why haven't we encountered advanced civilizations so far?"
Part of the answer might be that because of gamma-ray bursts, life as it is known on Earth might safely develop in only 10 percent of galaxies, the researchers suggest.
