Scraps flung from nearby exploding stars rained down on Earth twice in the past 10 million years, a pair of studies in Nature show, and may have had a hand in life-changing climate change.
An analysis of iron in the Earth's crust uncovered two events, around three and eight million years ago, which coincide with periods of global cooling.
The most recent event, the second study shows, was produced by two stars around nine times the size of the Sun that exploded only a few hundred light-years away – a cosmological stone's throw.
Aaron Wallner, a nuclear physicist at the Australian National University and lead author of the first study, says "it's an interesting coincidence" that event occurred during a period of glaciation on Earth and the rise of modern humans.
When a massive star explodes at the end of its life, cataclysmic forces crush atoms into heavy elements, such as iron, which are sprayed into space.
The common, stable form of iron is iron-56, with 26 protons and 30 neutrons in its nucleus. But with four extra neutrons, it becomes the rare and unstable iron-60, which decays with a half-life of 2.6 million years into lighter elements.
As Earth formed more than four billion years ago, any iron-60 that formed with it has long disappeared. And “iron-60 from space is a million billion times less abundant than the iron that exists naturally on Earth”, Wallner says. So only recently has technology been developed to measure these minuscule amounts.
In the late 1990s, a German team detected iron-60 in core samples drilled from the Earth's crust. So Wallner and his team took 120 samples drilled from the ocean floor at different sites around the world spanning the past 11 million years to see if they contained iron-60 too.
After extracting all the iron from the cores, they separated the tiny traces of iron-60. When they dated those layers using radioactive dating, they saw interstellar fallout across the planet occurred in two periods: eight million years ago and 1.7-3.2 million years ago.
Rather than being a nuclear bomb-like radiation blast, supernova-induced climate change would likely be relatively gentle.
“We were very surprised that there was debris clearly spread across 1.5 million years,” Wallner says. “It suggests there were a series of supernovae, one after another."
To trace the origins of that shower, Dieter Breitschwerdt from the Berlin Institute of Technology and colleagues modelled how iron-60 transports across space following a stellar explosion.
The amounts of iron-60 found in the core layers corresponded to supernovae around 400 light-years from Earth, within what is called the "Local Bubble" – a patch of mostly cleared space in our galaxy, the Milky Way.
Their calculations showed the first explosion occurred 2.3 million years ago, thanks to a star 9.2 times as massive as the Sun, while another was slightly smaller – 8.8 times the Sun's mass – 1.5 million years ago.
While estimates suggest "16 supernovae have exploded during the past 13 million years", they write, they only analysed two. Any other blew up too far away or too long ago to detect in the core samples.
Wallner and colleagues suggest that the Solar System was either sprayed by supernovae or passed through a "polluted" region as it orbited the Milky Way.
So how might exploding stars affect life on Earth?
This is not clear. But rather than being a nuclear bomb-like radiation blast, supernova-induced climate change would likely be relatively gentle. One theory is cosmic rays bombarded our atmosphere, exciting atoms and jostling electrons around, which increased cloud cover and lightning.
But scientists have bandied around the idea since the 1950s when German palaeontologist Otto Schindewolf suggested a nearby supernova caused the end-Permian mass extinction 250 million years ago, which wiped out more than 90% of species.
Any iron-60 evidence of such a blast has decayed to as-yet undetectable levels. But it's been calculated that a supernova within what's known as the "kill zone" (around 26 light-years' radius from Earth) should happen once every 800 million years or so.
So – hopefully – we should be right for a while yet.