III. Formation of the planets
- The first solid particles were microscopic in size. They orbited the Sun in nearly circular orbits right next to each other, as the gas from which they condensed. Gently collisions allowed the flakes to stick together and make larger particles which, in turn, attracted more solid particles. This process is called accretion.
- The objects formed by accretion are called planetesimals (small planets): they act as seeds for planet formation. At first, planetesimals were closely packed. They coalesced into larger objects, forming clumps of up to a few kilometers across in a few million years, a small time compared to the age of the solar system (movie).
- Once planetesimals had grown to these sizes, collisions became destructive, making further growth more difficult (movie). Only the biggest planetesimals survived this fragmentation process and continued to slowly grow into protoplanets by accretion of planetesimals of similar composition.
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