The Earth has been through five major mass extinction events since its formation roughly 4.6 billion years ago (Neil deGrasse Tyson talks about some of these on Cosmos). One of those events, the Cretaceous-Tertiary, wiped out the dinosaurs, but the biggest extinction event happened at the end of the Permian period roughly 248 million years ago, causing the extinction of roughly 96% of all species on the planet—it’s known as the “Great Dying.” There are numerous hypotheses for what caused these events, ranging from asteroids to volcanic eruptions to a drastic and cataclysmic release of methane. Now, in a study published in Physical Review Letters, scientists point to another possible cause: gamma ray bursts.
Gamma ray bursts are short, powerful electromagnetic radiation explosions that generate as much energy as the sun has since it formed. It’s not totally clear what causes gamma ray bursts, but scientists think hypernovas, massive exploding stars, account for longer bursts, and collisions between neutron stars account for shorter ones. Scientists believe the long bursts tend to occur in dwarf galaxies with low amounts of elements heavier than hydrogen and helium—if they occur in the Milky Way, they’d be in regions with similar properties.
Gamma rays are so powerful that even if they emanate from the far end of the solar system, they could seriously damage Earth’s atmosphere, particularly the ozone layer. It’s also possible that gamma ray bursts could emit cosmic rays tantamount to a nuclear blast. Scientists from Harvard, The Hebrew University of Jerusalem, and the University of Barcelona wanted to research the likelihood that one impacted Earth at some point in the past.
They concluded that there is a 50% chance a long gamma ray burst caused mass extinctions on Earth in the past 500 million years. The odds increase to 60% over the past billion years, and jump to 90% when considering the past 5 billion years, or to the beginning of the planet’s formation. While short gamma ray bursts occur much more frequently than long ones, the researchers concluded that because those bursts aren’t as powerful, they would likely not cause mass extinctions on Earth. They also don’t think long gamma ray bursts from outside the Solar System could cause extinctions on Earth.
Thus, the researchers believe that a relatively close long gamma ray burst accounts for one of the five mass extinctions. They also suspect that these bursts could also threaten other life that might exist in the Solar System. The center of the galaxy is the most vulnerable spot, given the density of stars there, so the scientists estimate that there’s a 95% chance that planets within 6,500 light years of the Solar System’s core experienced a catastrophic such as this over the past billion years.
Gamma ray bursts could threaten life across the universe, too. The scientists concluded that only 10% of galaxies in the universe could support life safe from such bursts. Any life in those areas likely developed over the past 5 billion years—before that, the galaxies were smaller, and gamma ray bursts would have done even more widespread damage.
The scientists suggest that damage caused by gamma ray bursts might explain the Fermi paradox—why alien life hasn’t made itself known to us when the size of the universe suggests that such life is out there somewhere. Of course, it’s possible that other life forms wouldn’t be cataclysmically affected by gamma ray bursts, but those forms that are could have been wiped out.