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Part of Hall of Meteorites.
How do we know that calcium-aluminum inclusions, or CAIs, are the oldest rocks that formed in our solar system? In a technique somewhat like the radiocarbon dating method used on fossils, researchers measure the amounts of certain forms, or isotopes, of the elements uranium and lead present in CAIs. Isotopes of uranium undergo radioactive decay to form lead at known rates, providing a clock that can be used to date material from the early solar system. These studies indicate that the oldest CAIs formed 4.568 billion years ago—less than one million years after our Sun first ignited.