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We saw our first extraterrestrial visitor in 2017 when ‘Oumuamua rocketed across the solar system, but there are extraterrestrial elements hiding right here on Earth. An analysis of isotopes in the ocean crust reveals radioactive materials that could only have arrived here from outside our solar system, and their presence could help us better understand the physics of cataclysmic events like supernovae. 

Most heavy elements are unstable, meaning they decay into smaller, more stable atoms. The amount of time it takes for half of a radioactive material to decay is known as a half-life, and scientists from the Australian National University report finding two important isotopes with short half-lives in high concentrations in samples taken from the ocean floor. 

One of the isotopes, iron-60, has been found in traces on Earth and in higher concentrations elsewhere in the solar system. With a half-life of just 2.6 million years, all the iron-60 that was originally part of Earth has long since decayed into stable nickel atoms. So where’s it coming from? Scientists know that iron-60 is commonly produced in supernovae explosions along with many other heavy elements, and some of it ends up in our solar system. Finding it in higher concentrations on the ocean floor, isolated from artificial human processes, suggests an influx of the isotope in the geologically recent past. 

The team actually detected two spikes of iron-60 within the past 10 million years. Knowing this is most likely the result of nearby supernovae (within a few hundred light-years), they decided to see what other isotopes were present in the same areas of the crust. The team uncovered a small but notable quantity of plutonium-244, which has a half-life of 80 million years. That’s long for plutonium, but all of Earth’s original traces are gone after billions of years. 

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Unlike iron-60, the story of plutonium-244 is complicated. This unusual isotope has to come from somewhere, but there is disagreement as to whether supernovae are a major driver of plutonium-244 production. Some scientists believe it takes more calamitous events like neutron star collisions to pump out the isotope. Finding plutonium-244 associated with confirmed supernovae products like iron-60 is the first direct evidence that this material does indeed come from dying stars. 

However, we don’t know how much of it came from the same local supernovae as the iron. It’s possible some proportion of this newly identified plutonium came from other celestial events. Plutonium-244 might also be present in the interstellar medium, allowing it to be swept up by supernovae shockwaves. Scientists will need to gather more samples to unravel this mystery, but the end result could be a fuller understanding of the processes at work in supernovae and other space-rending explosions.

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