Meteorites originating from asteroids are the oldest-known rocks in the Solar System, and many predate formation of the planets. Refractory inclusions in primitive chondrites are the oldest-known materials, and chondrules are generally a few million years younger. Igneous achondrites and iron meteorites also formed in the first five million years of the protoplanetary disk and escaped accretion into planets. Isotopic dates from these meteorites serve as time markers for the Solar System's earliest history. Because of the unique environments in the protoplanetary disk, dating the earliest meteorites has its own opportunities and challenges, different from those of terrestrial geochronology.

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