Plutonium is a radioactive, man made substance. It's periodic table number is Pu and it's atomic number is 94. It was discovered in 1941 by Dr. Glenn T Seaborg. It was named after the planet Pluto because Uranium had been named after the planet Uranus. The discovery was kept secret because of the Second World War, which started in 1939 and went on until it was ended by the nuclear bombing of Hiroshima in Japan in 1945.