The Origin of the
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
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
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
As we discuss in the next section, we now have some direct observational
evidence in support of the nebular hypothesis.