A rare version of the radioactive element plutonium embedded in Earth’s crust below the deep sea is providing new clues as to how heavy metals form in the stars.

The new research finds that the isotope, called plutonium-244, may arrive on Earth in tandem with iron-60, a lighter metal known to form in supernovas, explosions that occur during the death throes of many types of stars. This finding suggests that supernovas may create both heavy metals — although it’s possible that other events, such as the mergers of neutron stars, are responsible for at least some of the plutonium-244.