The Origin of the
Solar System

The Nebular Hypothesis in its original form was proposed by Kant and Laplace in the 18th century. The initial steps are indicated in the following figures.

Collapsing Clouds of Gas and Dust

A great cloud of gas and dust (called a nebula) begins to collapse because the gravitational forces that would like to collapse it overcome the forces associated with gas pressure that would like to expand it (the initial collapse might be triggered by a variety of perturbations---a supernova blast wave, density waves in spiral galaxies, etc.).

In the Nebular Hypothesis, a cloud of gas and dust collapsed by gravity begins to spin faster because of angular momentum conservation

It is unlikely that such a nebula would be created with no angular momentum, so it is probably initially spinning slowly. Because of conservation of angular momentum, the cloud spins faster as it contracts.

The Spinning Nebula Flattens

Because of the competing forces associated with gravity, gas pressure, and rotation, the contracting nebula begins to flatten into a spinning pancake shape with a bulge at the center, as illustrated in the following figure.

The collapsing, spinning nebula begins to flatten into a rotating pancake

Condensation of Protosun and Protoplanets

As the nebula collapses further, instabilities in the collapsing, rotating cloud cause local regions to begin to contract gravitationally. These local regions of condensation will become the Sun and the planets, as well as their moons and other debris in the Solar System.

As the nebula collapses further, local regions begin to contract gravitationally on their own because of instabilities in the collapsing, rotating cloud

While they are still condensing, the incipient Sun and planets are called the protosun and protoplanets, respectively.

Evidence for the Nebular Hypothesis

Because of the original angular momentum and subsequent evolution of the collapsing nebula, this hypothesis provides a natural explanation for some basic facts about the Solar System: the orbits of the planets lie nearly in a plane with the sun at the center (let's neglect the slight eccentricity of the planetary orbits to simplify the discussion), the planets all revolve in the same direction, and the planets mostly rotate in the same direction with rotation axes nearly perpindicular to the orbital plane.

The nebular hypothesis explains many of the basic features of the Solar System, but we still do not understand fully how all the details are accounted for by this hypothesis. As we discuss in the next section, we now have some direct observational evidence in support of the nebular hypothesis.

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