Radioactive elements in rocks decay in a predictable way, like the ticking of a well-made clock that can run for millions of years. The decay marks a change in character of the elements; one type of uranium, for example, decays into lead. To measure the age of meteorites, and thereby deteremine the age of Earth itself, Harvard researcher Stein Jacobsen and his colleagues used a radioactive type of hafnium, a rare heavy metal, which decays into tungsten, a more familiar gray-white metal. The ratio of this type of tungsten to a stable variety of the same metal reveals how much hafnium decayed away, or how long the clock has ticked. According to new evidence found in meteorites, our planet is 50 to 90 million years older than previously thought.