Considering the dangers lurking out there, it’s a wonder that our little planet is not in the firing line more often. We are just 93 million miles from a star that, while mostly well-behaved, occasionally has temper tantrums that could bring our civilisation to its knees. Our solar system is home to a swarm of comets, rocks, boulders and flying mountains, tens of thousands of which are big enough to wipe out anything from a small city to the entire biosphere. And further out lurk delinquent stars whose death explosions are the largest since the Big Bang. If one of these went off nearby, it would be curtains for all of us.
In fact, Earth can be considered rather lucky to have not suffered a total cataclysm in at least 3.5 billion years – the period during which we have an unbroken record of life existing on the Earth’s surface. Before then, global sterilisation events, caused by collisions with huge space rocks, almost certainly took place many times – perhaps once every few hundred years. Each may well have wiped out early versions of life. Then, after one final cataclysmic impact, it was plain sailing all the way.
That was what we used to think, anyway. But in recent decades, it has become clear that our cosmic neighbourhood, in more recent times, has not been as benign as was thought. In the 1980s, for example, it was confirmed that Earth has been hit several times in its history by objects from space – none big enough to sterilise the planet completely, but a handful packing enough of a punch to change the course of life forever. The most famous of these was our collision with a six-mile-wide asteroid 65 million years ago, whose fiery passage into the Mexican coast has been blamed for killing off, or at least delivering the coup de grâce to, the dinosaurs.
It is not just rocks we have to worry about. Japanese scientists have uncovered evidence from the study of tree rings that in the year 775 the Earth was hit by a colossal solar flare. The scientists found a spike in radioactive carbon-14, taken up by the ancient cedar trees they were studying. In Finland, Ilya Usoskin and his colleagues found an identical spike on the other side of the world. One theory is that this was caused by a nearby exploding star – a supernova – showering the Earth with radiation.
The trouble, says Usoskin, was that they could see no sign in the skies of a supernova remnant within the required distance. So the scientists turned to the historical record to see if there were any clues. The handful of supernovae that have burst into existence in historical times have often been well-recorded. But, 1,238 years ago, there were reports not of a brilliant “new star” but – as one English chronicler, Roger of Wendover, put it – of the skies themselves catching fire: “Fiery and fearful signs were seen in the heavens after sunset; and serpents appeared in Sussex, as if they were sprung out of the ground, to the astonishment of all.”
This was, says Usoskin, probably an account of an aurora borealis – the Northern Lights. “Anyone who has seen aurorae knows they look like serpents,” the Finnish scientist told New Scientist this week. The conclusion is that the Earth was hit by a huge mass of charged particles ejected from the Sun.
This was far from a one-off: such flares probably happen every few centuries or so. In late August and early September 1859, the Earth was hit by a smaller flare that had equally dramatic effects. Named the Carrington Event, after the astronomer who documented it fully, the solar storm caused Californian Gold Rush miners to be woken in their tents by the bright northern lights. Aurorae were seen as far north as Queensland in the southern hemisphere and as far south as Washington DC in the northern.