• The accumulation of a number of objects to form a single larger one. Accretion is among the commonest of ways for astronomical objects to form. In particular it seems that the planets, including the Earth, were built up by the accretion of planetesimals which formed a cloud around the Sun almost five billion years ago. Most of the planetesimals were swept up in the accretion process, leaving a small number to appear as comets and other small solar system objects.

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