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Exploding stars in near-solar space may have triggered at least two mass extinction events in Earth's history.
An analysis of the frequency of supernova explosions in the Milky Way, led by astrophysicist Alexis Quintana of the University of Alicante in Spain, reveals timing consistent with the Late Ordovician and Late Devonian extinctions; devastating events that saw vast swathes of life wiped out.
The conditions that led to these extinction events – numbered among the 'Big Five' that have ravaged the planet – are not well understood.
"Supernova explosions are some of the most energetic explosions in the Universe. 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 astrophysicist Nick Wright of Keele University in the UK.
"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."
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Supernovae are a part of the normal life cycle of stars more than eight times the mass of our own Sun. Larger stars live relatively short lifespans, millions of years rather than the billions that stars like the Sun hang around for.
Once they run out of fuel to fuse in their cores, these stars become unstable, and finally explode, spraying space with the metallic products of their fusion in a massive burst of light and energy.
If such an explosion were to take place close enough to Earth, the results would be pretty devastating, lashing our planet with powerful enough radiation to deplete the ozone layer. The resulting increase in ultraviolet radiation that could then reach the surface would cause irreparable harm to the planet's ecology.
The Ordovician and Devonian extinctions occurred around 445 million and 372 million years ago, respectively, each wiping out the majority of species living on Earth at those times. Both were also correlated with a significant depletion in the ozone layer, which has led to speculation that supernovae may be implicated.
The Crab Nebula supernova remnant, as imaged by JWST. (NASA, ESA, CSA, STScI, T. Temim/Princeton University)Quintana and colleagues discovered a plausible link between the extinctions and supernovae when they undertook a census of massive OB-type stars within a kiloparsec (around 3,260 light-year) radius of the Sun.
Because these stars live such short lives, a census of their current numbers allows astronomers to calculate the rates at which they are born, and the rates at which they die in a blaze of fury.
In their census, the researchers counted 24,706 OB stars, and calculated a supernova rate of 15 to 30 per million years across the entire Milky Way galaxy.
In order for a supernova to devastate Earth, it needs to be relatively close to the Solar System, so the team used that figure as a basis to work out the rate of OB supernovae within a 20-parsec radius, or around 65 light-years.
This gave a rate of 2.5 near-Earth OB supernovae per billion years – a figure that can explain both the late Ordovician and late Devonian extinction events.
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Luckily, there are currently no stars nearby that are likely to blow up anytime soon. Red giant stars Antares and Betelgeuse are getting close; but 'close' could be tens of thousands to over a million years in cosmic time, and both are hundreds of light-years away – too far to affect Earth.
Luckily, there are plenty of other possible triggers for a catastrophic extinction event, such as a rogue asteroid, or massive volcanic upheaval. Since neither of these can really be prevented, either, we've still got more than enough existential anxiety fuel to keep us trucking.
The research is due to be published in the Monthly Notices of the Royal Astronomical Society, and is available on preprint server arXiv.
