2.3 Million Years Ago, These Stars Exploded — And Left a Radioactive Mark on Earth

The bottom of the ocean may not be the first place you'd look for remnants of ancient star explosions. 

But that's exactly what scientists have found in patches of ocean sediment in the Atlantic, Pacific and Indian oceans: deposits of an iron isotope called iron-60, produced when a star explodes in a supernova.

Combined with supernova modeling, suggests that million of years ago, at least two stars exploded and some of the elemental debris rained down on Earth. Astronomers estimate the first explosion happened about 2.3 million years ago, and the second about 1.5 million years ago. The stars must have been no more than 300 light-years away from Earth. (Two new papers describing the discovery are published in the journal Nature here and here.)

But other than a shower of iron-60, it's unclear what kind of impact the explosions had on Earth. 

They didn't make us burn up: If you're within about 75 light-years of a supernova, you get completely roasted. These stars probably weren't close enough to do too much damage, and the timing of the explosions don't coincide with any mass extinctions in Earth's history. 

Still, it's possible the explosions were close to enough to influence the planet's climate. The explosions happened when Earth was cycling through periods of regular ice ages.

If you're within about 75 light-years of a supernova, you get completely roasted.

"It's an interesting coincidence that they correspond with when the Earth cooled and moved from the Pliocene into the Pleistocene period," Anton Wallner, one of the scientists who worked on the research, told BBC. 

The Pleistocene period is when modern humans began to emerge, so it's possible the supernovae played a very important role in our history. This is only a loose correlation and not direct evidence, though.

"Our local research group is working on figuring out what the effects were likely to have been," Adrian Melott, an astronomer not involved with the research, said in a statement. "We really don't know. 

"The events weren't close enough to cause a big mass extinction or severe effects, but not so far away that we can ignore them either," Melott said. "We're trying to decide if we should expect to have seen any effects on the ground on the Earth."