Exploding stars may have assaulted ancient Earth

The Crab nebula is the remains of a supernova more than 6000 light-years away—too far to harm Earth.

NASA; ESA; J. Hester and A. Loll/Arizona State University

For our Australopithecus ancestors who roamed Africa 2.5 million years ago, the bright new star in the sky surely would have aroused curiosity. As luminous as the full Moon, it would have cast shadows at night and been visible during the day. As the supernova faded over the following months, it probably also faded from memory. But it left other traces, now coming to light.

Over the past 2 decades, researchers have found hundreds of radioactive atoms, trapped in seafloor minerals, that came from an ancient explosion marking the death of a nearby star. Its fusion fuel exhausted, the star had collapsed, generating a shock wave that blasted away its outer layers in an expanding ball of gas and dust so hot that it briefly glowed as bright as a galaxy—and ultimately showered Earth with those telltale atoms.

Erupting from hundreds of light-years away, the flash of x-rays and gamma rays probably did no harm on Earth. But the expanding fireball also accelerated cosmic rays—mostly nuclei of hydrogen and helium—to close to the speed of light. These projectiles arrived stealthily, decades later, ramping up into an invisible fusillade that could have lasted for thousands of years and might have affected the atmosphere—and life.

In a flurry of studies and speculation, astronomers have sketched out their potential effects. A cosmic ray barrage might have boosted mutation rates by eroding Earth’s protective ozone layer and generating showers of secondary, tissue-penetrating particles. Tearing through the atmosphere, the particles would have also created pathways for lightning, perhaps kindling a spate of wildfires. At the same time, atmospheric reactions triggered by the radiation could have led to a rain of nitrogen compounds, which would have fertilized plants, drawing down carbon dioxide. In that way, the celestial event could have cooled the climate and helped initiate the ice ages 2.5 million years ago, at the start of the Pleistocene epoch. Even taken together, the effects are “not like the dinosaur extinction event—it’s more subtle and local,” says Brian Thomas, an astronomer at Washburn University who has studied the earthly effects of cosmic catastrophes for nearly 2 decades.

Few astronomers are suggesting that the supernovae caused any great extinction at the time, and even fewer paleontologists are ready to believe them. “Death from space is always really cool,” says Pincelli Hull, a paleontologist at Yale University. “The evidence is interesting but has not quite really reached the threshold to incorporate into my mental register.”

Yet the supernova hunters believe other blasts, more distant in time, went off closer to Earth. And they think these supernovae could explain some extinction events that lack customary triggers such as volcanic outbursts or asteroid impacts. Adrian Melott, an astronomer at the University of Kansas, Lawrence, who explores how nearby cosmic cataclysms might affect Earth, says it’s time to more carefully probe Earth’s history for ancient supernova strikes. Not only will that help astrophysicists understand how the blasts shaped the neighborhood of the Solar System and seeded it with heavy elements, but it could also give paleontologists a new way to think about bouts of global change. “This is new and unfamiliar,” Melott says. “It will take time to be accepted.”

Astronomers believe a few supernovae go off in the Milky Way every century. By the law of averages, a handful must have exploded very close to Earth—within 30 light-years—during its 4.5-billion-year lifetime, with potentially catastrophic effects. Even blasts as far as 300 light-years away should leave traces in the form of specks of dust blown out in the shell of debris known as a supernova remnant. When physicist Luis Alvarez set out in the 1970s with his geologist son Walter Alvarez to study the sediment layers associated with the dinosaurs’ extinction 65 million years ago, they were expecting to find supernova dust. Instead, they found iridium, an element that is rare on Earth’s surface but abundant in asteroids.

The Alvarezes didn’t have the tools to look for supernova dust, in any case. Because Earth is already largely made of elements forged in supernovae billions of years ago, before the Sun’s birth, most traces of more recent explosions are undetectable. Not all of them, however. In the 1990s, astrophysicists realized supernova dust might also deposit radioactive isotopes with half-lives of millions of years, far too short to have been around since Earth’s birth. Any that are found must come from geologically recent sprinklings. One key tracer is iron-60, forged in the cores of large stars, which has a half-life of 2.6 million years and is not made naturally on Earth.