25.5 The Origin of the Solar System

Figuring out how the solar system formed is a bit like detective work. You can't go back 4.6 billion years in time to see exactly what happened. But there are many clues that scientists have used to form a theory about what must have occurred. Any such theory must explain why all of the planets' orbits lie more or less in a single plane and why all of the planets orbit the sun in the same direction. It must also explain the difference in size and composition between the terrestrial planets and the gas giants.

The Nebular Theory

The generally accepted explanation for the formation of the solar system is the nebular theory, illustrated in Figure 28. The nebular theory states that the solar system formed from a rotating cloud of dust and gas. A large, thin cloud of dust and gas like the one that eventually formed our solar system is called a solar nebula. According to the nebular theory, the solar nebula formed from the material expelled by previous stars.

Figure 28 The contraction of the solar nebula, a large cloud of dust and gas, led to a disk-shaped protoplanetary disk, from which the sun and planets formed. Illustrations are not shown to scale with respect to one another.
Applying Concepts What force caused the solar nebula to contract?

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