The Nebular Hypothesis
How does a solar system like our own
originate? There is very strong evidence that the origin of our solar system and
presumably of others is
accounted for by a set of ideas known loosely as the nebular hypothesis.
This hypothesis in its original form was proposed by Immanuel Kant and
Pierre-Simon de Laplace
in the eighteenth 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 such as
a supernova blast wave or waves of density variation in spiral galaxies.)
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, just as a spinning ice skater spins
faster as extended arms are pulled in.
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.
