Plutonium-244

Plutonium-244 (²⁴⁴Pu) is an isotope of plutonium that has a half-life of 80 million years. This is longer than any of the other isotopes of plutonium and longer than any other actinide isotope except for the three naturally abundant ones: uranium-235, uranium-238, and thorium-232. Although studies are in conflict, given the mathematics of the decay of plutonium-244, an exceedingly small amount should still be present in the Earth's composition, making it a likely although unproven candidate as the shortest lived of the primordial elements.