Paleontologists recognize five big mass extinctions in the fossil record, marked by the loss of a significant number of species in a (geologically speaking) short span of time.
Theories to explain mass extinctions range from terrestrial causes, like volcanic eruptions and catastrophic environmental changes, to extraterrestrial causes, like the impact of an asteroid or similar high-energy events. One theory even blames supernovae — super-powerful blasts triggered by the gravitational collapse of a massive star.
Based on their research, a team of scientists believe they can link both the late Devonian and Ordovician extinction events, which occurred 372 and 445 million years ago respectively, to supernova explosions.
The Ordovician extinction killed 60 percent of marine invertebrates at a time when life was largely confined to the sea, while the late Devonian extinction wiped out around 70 percent of all species, including some of the earliest groups of fish found in the fossil record.
Past research has failed to identify a clear cause for either extinction, although they are thought to have been linked to sea level changes or the onset of an ice age, but an extraterrestrial cause can’t be ruled out.
Supernovae occur when massive stars reach the end of their lives, run out of fuel, cool off, and then collapse under the pressure of gravity. The collapse may cause violent expulsion of the outer layers of the star, resulting in a shock wave and radiation burst. The shock wave in combination with high-energy burst could alter the planet’s atmosphere or strip away its outermost protective ozone layer, exposing life to harmful cosmic radiation.
“If a massive star were to explode as a supernova close to the Earth, the results would be devastating for life on Earth. This research suggests that this may have already happened,” explains study coauthor Dr. Nick Wright from Keele University.
The researchers compiled a census of massive stars within a kiloparsec (around 3,260 light-years) of the sun, comparing their numbers to known stellar collapses, including black holes and neutron stars formed when the gravitational force was strong enough to prevent an explosion, and clouds of gas and material formed by an expulsion.
This census allowed the researchers to calculate the rate at which supernovae occur less than 1,000 light-years away from Earth, close enough to affect our planet.
“We calculated the supernova rate close to Earth and found it to be consistent with the rate of mass extinction events on our planet that have been linked to external forces such as supernovae,” concludes Dr. Wright.
So far, this remains a possible yet speculative link. There is no unambiguous evidence, like specific elements released by an exploding star and preserved in oceanic sediments, that such nearby supernovae ever happened.
We may also have overestimated the possible effects of a supernova. Some experts note that Earth’s magnetic field likely would act as a shield, preventing the blast from stripping away the planet’s atmosphere. And even if Earth loses parts of its atmosphere, the remaining layers, in combination with the magnetic field, could mitigate the impact of harmful radiation on Earth’s surface.
The preprint, “A census of OB stars within 1 kpc and the star formation and core collapse supernova rates of the Milky Way,” was published in the Monthly Notices of the Royal Astronomical Society.
Additional material and interviews provided by the Royal Astronomical Society.
