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Nebular Hypothesis

It is thought that the Solar System formed according to what is known as the Nebular Hypothesis. About 4.5 billion years ago it is believed that the Solar System consisted of a large cloud of gas and dust, called a nebula. This cloud started rotating, and the dust particles combined to form planetesimals. As the cloud rotated faster, it flattened, and the planetesimals combined to form, first of all, the Sun at the center, and secondly, the planets in orbit around the Sun. This model explains qualitatively many features of the Solar System, including the fact that the planets essentially all revolve around the Sun in the same plane.