How Dust on the Ocean Floor Hints at a Recent Near-Earth Supernova
On the evidence for recent supernovae close to Earth
However big you imagine a supernova to be, the reality is certainly bigger. To put it one way, an exploding star can briefly outshine the combined light of every other star in a galaxy; to put it another, a supernova at the distance of Pluto would hit you with more energy than a hydrogen bomb exploding just outside your front door.
Not that you’d want either to happen. But whereas the bomb might simply leave your neighbourhood a smoking crater, the supernova would strip the atmosphere, boil the oceans dry, and then vaporise whatever was left of the Earth. It would not be a good day.
Fortunately such a thing is unlikely to happen in the foreseeable future. But even a relatively nearby supernova – one, say, within fifty light-years of our solar system – would be a catastrophe. The blast would send a pulse of radiation smashing into the Earth, and though the atmosphere would offer some early protection, things would quickly get bad.
For thousands of years afterwards, the remains of the supernova would bombard the Earth with intense cosmic rays. These would erode the ozone layer, and then leave the Earth exposed to the Sun’s dangerous ultraviolet rays. Alongside that, a layer of deadly nitrogen dioxide gas would form in the atmosphere, cooling the planet and possibly triggering an ice age.
Three hundred and sixty million years ago, something very much like this seems to have happened. Back then, as life was making a tentative move from the oceans to solid land, and the first vast forests began to spread across the continents, a mass extinction took place. Unlike the impact that wiped out the dinosaurs, this extinction was slower, spread over hundreds of thousands of years.
In that time, around forty percent of species vanished from existence. And, intriguingly, it was not a constant wave of death, but rather a long decline punctuated by sudden pulses of extinction. It could be, one theory runs, that this was caused by a series of nearby supernovae.
The idea posits that the solar system passed within a hundred light-years of a cluster of dying stars. As they died, one by one, they exploded into supernova. Each would have sent a burst of radiation towards our world, causing a brief spike in extinctions. Afterwards, as their cosmic rays kept on bombarding the solar system, the ozone layer would have faded and life on Earth would have experienced a long and relentless decline.
I. A Recent Near-Earth Supernova?
Fortunately, there are few nearby stars that could cause anything like this in the next million years. The closest known candidate for a future supernova is a star called IK Pegasi B, a white dwarf lying a hundred and fifty light-years away. It has a companion, a star slightly larger and brighter than the Sun. One day – but probably not for a billion years or so – that star will exhaust its fuel, cool, and begin to swell up.