Pluto is definitely not made out of Plutonium, nor is it abundant there. Element #94, Plutonium was named according to a trend set by the two elements before it on the periodic table, #92, Uranium, and #93, Neptunium. Also, it is highly unlikely to be found on any planet that has been around for the 4-6 billion years our solar system has. It is formed in minute amounts by the natural radioactive decay of Uranium, and in larger amounts from nuclear reactors. Plutonium itself is radioactive and therefore will not accumulate as a decay product.