The two elements named for planets are Neptunium and Plutonium. Neptunium is a radioactive element with the atomic number 93, while Plutonium is a radioactive element with the atomic number 94. Both elements were discovered in 1940 by scientists at the University of California, Berkeley. Neptunium was named after the planet Neptune, while Plutonium was named after the planet Pluto. Both elements are used in nuclear reactors and weapons.