A protoplanetary disk is formed when an interstellar gas cloud begins to
contract or collapse under the force of gravity. The cloud then begins to
rotate rapidly and flatten into a disk due to the conservation of angular
momentum.
This rotating disk may break up into many smaller condensations which form
asteroids or protoplanets. The protoplanets then grow into full-size
planets
by attracting more material from the surrounding interstellar medium.
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