Promethium, pruh MEE thee uhm (chemical symbol Pm), is a chemical element and one of the rare-earth metals. It does not occur naturally on Earth, but astronomers have detected it in a star. It exists as radioactive isotopes among the fission products of uranium, thorium, and plutonium. An element's isotopes have the same number of protons but different numbers of neutrons. Promethium was first isolated in 1945, by the American chemists J. A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell.