Cassiopeia A
© NASA
Supernova remnant Cassiopeia A. A supernova might have occurred around the right time and at the right distance from Earth to contribute to a minor mass extinction several million years ago.
An exploding star hundreds of light-years from Earth may have played a role in a minor mass extinction that happened 2.59 million years ago, new research indicates. Scientists modeled the light and radiation that would have reached Earth from relatively close exploding stars, or supernovae.

The impacts on Earth and its lifeforms could help explain the die-off that happened as the Pliocene Epoch wrapped up and the Pleistocene began, they say.

It's generally accepted that several stars have gone supernova about 300 light years from Earth within the past few million years. Recent evidence for these supernovae comes from two studies published in April. In one, researchers traced the amount of iron-60, a radioactive form of iron, in deep-sea crusts.

Iron-60 is catapulted into space by supernovae or in winds from massive stars; its presence can reveal when a star exploded nearby. Scientists found two influxes of iron-60, one about 1.5 to 3.2 million years ago, another at 6.5 to 8.7 million years ago.

Another group calculated the likely trajectories of recent supernovae, and found that the stars were probably nine times the size of our own sun, and exploded about 300 light years from Earth.

In the new study, scientists were curious about how these recent supernovae might have affected life on Earth, as well as our planet's atmosphere. To cause a truly catastrophic extinction, you'd need a supernova within about 26 light-years from Earth.

"This event is not close enough to have precipitated a major mass extinction, but may have had noticeable effects," wrote the researchers, who recently published the findings in Astrophysical Journal Letters.

Their work indicates that blue light from a supernova at that distance might have flared in the night sky for several weeks, disturbing animals' slumber. The supernovae would have also belched out cosmic rays, which travel at the speed of light. Once they reached Earth, these rays would have tripled the amount of radiation organisms on land and in the ocean's upper waters were exposed to. The extra radiation would likely have boosted cancer risk and mutations. "This is not disastrous, but might be noticeable in the fossil record," the team wrote.

Cosmic rays might also have influenced Earth's climate by ionizing, or changing the charge of, particles in the atmosphere. "The high-energy cosmic rays...tear up molecules, they can rip electrons off atoms, and that goes on right down to the ground level," coauthor Adrian Melott, of the University of Kansas, said in a statement. "Normally that happens only at high altitude."

This would have caused a tenfold increase in the ionization in the lowest layer of Earth's atmosphere for at least 1,000 years. "It is possible that this could trigger climate change, especially if instability was already present," wrote Melott and his colleagues.

Radiation from a recent supernova could be related to the extinction that happened at the end of the Pliocene Epoch, the team concluded. Further work will be needed to show how it would have influenced cloud formation, lightning strikes, or other events in the atmosphere.