Originally Posted by TriangleMan

WAG, but uranium, and all elements after it, are radioactive and decay over time. Most of these isotopes have half-lives of less than a second although some, such as uranium, can have isotopes with half-lives of billions of years. Elements after uranium are formed in supernova explosions but over millions of year they decay into other elements, which is why we don't find natural sources of Plutonium. This is also the reason why there is no Technetium (sp?) naturally found on Earth. Despite being right in the middle of the periodic table (element #43?) it has no stable isotopes so all of the Technetium formed in the stellar fusion that formed the solar system has decayed into other elements.