In February 2020, Betelgeuse, a prominent star 642 light-years away in the Orion constellation, began to dim, suggesting it was in the throes of death. Astronomers’ telescope observations and computer simulations revealed the real culprit: a roving dust cloud that temporarily crossed in front of the star. When Betelgeuse eventually does run out of fuel and enters the supernova stage of a star’s life, it will generate a brilliant display of stellar fireworks in the night sky.
Astronomers estimate that a handful of stars in our galaxy go supernova every century. Throughout Earth’s history, it’s likely some of these stellar explosions have been close enough to cause catastrophic damage to our planet and, as some researchers believe, potentially alter the evolutionary trajectory of life. The charge has drawn skepticism—but has revived a debate about how susceptible life is to cosmic influence.
The explosion of a near-Earth supernova has the potential to unleash a cascade of events that would dramatically impact our planet. Visible light from the supernova would reach Earth first, and the burst star would appear to shine as bright as the moon for months. While not ultimately harmful to humans, it could possibly be bright enough to alter the biological systems of nocturnal animals, says Adrian Melott, Ph.D., an astronomer at the University of Kansas, Lawrence.
Soon after the first light of a supernova appears, a blast wave packed with cosmic rays, an amalgam of high-energy particles, would begin its race toward Earth. "The guts of the star are launched into space at speeds that are a few percent of the speed of light," says Brian Fields, Ph.D., an astronomer at the University of Illinois Urbana-Champaign. This wave radiates out into space, sweeping gases and other interstellar matter up like a cosmic snowplow. It could take thousands of years for these rays to reach Earth because their trajectory is influenced by the magnetic fields they encounter. If their path is free of magnetic field lines, they’ll travel in a straight line, says Melott.
I Spy: Iron-60
One telltale sign of a near-Earth supernova is the presence of the radioactive isotope Iron-60. The isotope, which is carried to Earth by the gaseous remnants of these burst stars, has a half-life of millions of years, meaning it must have arrived on Earth long after our planet formed. Traces of Iron-60 have been found in rocky crusts plucked from the seafloor, in Antarctic snow, and even in lunar soil collected during the Apollo missions.
Earth’s atmosphere would bear the brunt of these charged particles, according to a 2016 study in Astrophysical Journal Letters, led by Melott and his colleagues. They suggest that the cosmic rays generated by a supernova 300 light-years away would slice through nitrogen molecules in the air, generating nitrogen oxide compounds that can rain down and fertilize vegetation. This would spur Earth’s plant life to gulp up carbon dioxide from the atmosphere and cool the climate.
The excess nitrogen oxide in the atmosphere could wash away as much as 7 percent of Earth’s ozone layer, too, according to the study. Erasing this protective shield would subject animals and plants to sun damage, potentially altering the food web for thousands of years. "You and I would put on a hat and some sunblock—but if you’re a phytoplankton, you don’t have that option, and just get cooked," says Fields.
Furthermore, when cosmic rays pierce through Earth’s atmosphere, they generate secondary particles called muons, which are similar to electrons but heavier. "[Muons] can come all the way down to the ground and even under the ground," Fields says. "You can’t hide from them." These muons would subject animals on Earth’s surface to three times the normal amount of radiation.
Scouring the planet for geologic evidence of a near-Earth supernova is more difficult than, say, searching for a five-mile-wide asteroid crater. Still, researchers like Melott and Fields are combing through the geologic record for instances where supernovae could have played a role in shaping Earth’s environment and the evolution of life on Earth.
Last year, for instance, a team of researchers studying fossilized leaves from a notable extinction event in the late Devonian period, roughly 359 million years ago, found evidence of warped plant spores—suggesting these plants may have absorbed excessive amounts of ultraviolet radiation. Fields and his colleagues argued in a subsequent paper published in the Proceedings of the National Academy of Sciences last September that the jump in radiation could be the result of an ozone-free Earth.
And in a study published in The Journal of Geology last year, Melott and his colleagues suggest that widespread wildfires—possibly spurred by cosmic ray–induced lightning—helped push our early human ancestors to move from forests to the savanna and embrace bipedalism at the beginning of the Pleistocene, 2.5 million years ago. Deposits of the radioactive isotope Iron-60 (see sidebar) found on Earth and on the moon seem to correspond with this timing.
Fields admits more evidence is needed to understand exactly what role these stellar explosions may have played in charting our evolutionary path. Pinpointing the exact causes of global-scale changes across the geologic record is a difficult task.
One thing is certain, however: Earth is safe from future supernovae. Of the stars in our galaxy that are approaching the end of their life cycle and might go supernova in the near future, none of them are likely to cause catastrophic damage. At most, Fields says, they’ll just provide a captivating show.
Jennifer Leman is a science journalist and senior features editor at Popular Mechanics, Runner's World, and Bicycling. A graduate of the Science Communication Program at UC Santa Cruz, her work has appeared in The Atlantic, Scientific American, Science News and Nature. Her favorite stories illuminate Earth's many wonders and hazards.
