While the sun was forming in the center of the solar nebula, planets were forming in the outer regions.  Small bodies from which a planet originated in the early stages of formation of the solar system are called planetesimals.  This process, known as accretion, happens as larger particles attract smaller particles and they form even larger particles like planetesimals.   Some planetesimals joined together through collisions and through the force of gravity to form larger bodies called protoplanets.   The protoplanets’ gravity attracted other planetesimals in the solar nebula. These planetesimals collided with the protoplanets and added their masses to the protoplanets.  Eventually, the protoplanets became very large and condensed to form planets and moons.  Moons are the smaller bodies that orbit the planets. Planets and moons are smaller and denser than the protoplanets.


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