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Scientists trying to work out whether a star exploded and the resulting debris crashed into Earth 10 million years ago may be one step closer to solving the mystery. They discovered an unusual amount of Beryllium-10 (¹⁰Be) at the bottom of the Pacific Ocean, which may be explained by a supernova, an exploding star.

Beryllium-10 is a long-lived radioisotope of beryllium that is formed when cosmic rays from outside our solar system strike atoms in Earth's atmosphere. Earlier this year, Dominik Koll at the Helmholtz-Zentrum Dresden-Rossendorf in Germany and his colleagues were looking for concentrations of the isotope in deep-ocean crusts to help date these layers of rock. They found a concentration that was much higher than they expected about 10 million years ago.

While several theories can explain the deposits, such as a shift in currents distributing ¹⁰Be away from the north and south poles, another group of scientists had a hunch that the cause was a massive star exploding near the sun.

To test this theory, Efrem Maconi at the University of Vienna in Austria and his colleagues traced the past orbits of the sun and 2,725 star clusters over 20 million years. They wanted to see if a supernova from one of these clusters could have occurred close enough to our planet (within 100 parsecs, or roughly 326 light-years) and at around the same time as the spike in ¹⁰Be in the ocean crusts.

The team's analysis found that the probability of a massive star exploding within 100 parsecs during the time of the ¹⁰Be spike is 68%. The chance of a blast happening much closer at around 35 parsecs (114 light-years) is 1%. They also identified 19 star clusters that each had more than 1% chance of producing at least one supernova within the 100 parsec range.

"Our results support the possibility of an SN (supernova) origin for the ¹⁰Be anomaly and highlight the importance of additional ¹⁰Be records from independent terrestrial archives to determine whether the anomaly is of astrophysical or terrestrial origin," wrote the scientists in their paper in Astronomy & Astrophysics.

The researchers also pinpointed two possible sources of the massive star explosion—ASCC 20 and OCSN 61, two star clusters from the Orion star-forming region.

While the research provides strong support for the cosmic theory, the mystery remains unsolved. Scientists need to confirm the Beryllium-10 anomaly in other deep crusts around the world, which would suggest a single, universal cause such as a supernova. However, if the Pacific is the only place with the excess ¹⁰Be, it suggests a terrestrial cause, such as shifting currents.

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