How did the planets, moons and asteroids in the Solar System form? There is a surprising amount of debate and several strong and competing theories.

What makes a good theory?

Any theory has to account for certain rather tricky facts about the Solar System. These are in addition to the obvious fact that the Sun is at the centre with the planets in orbit around it. There are five of these problem areas:

  1. The Sun spins slowly and only has 1 percent of the angular momentum of the Solar System but 99.9 percent of its mass.
  2. The formation of the terrestrial planets with solid cores.
  3. The formation of the gaseous giant planets.
  4. The formation of planetary satellites.
  5. An explanation of Bode's law (which states that the distances of the planets from the Sun follow a simple arithmetic progression).

There are 5 theories considered to be 'reasonable' in that they explain many (but not all) of the phenomena exhibited by the Solar System.

The Accretion theory

The Sun passes through a dense interstellar cloud and emerges surrounded by a dusty, gaseous envelope.

The problem is that of getting the cloud to form the planets. The terrestrial planets can form in a reasonable time but the gaseous planets take far too long to form. The theory does not explain satellites or Bode's law and must be considered the weakest of those described here.

The Protoplanet theory

A dense interstellar cloud produces a cluster of stars. Dense regions in the cloud form and coalesce; as the small blobs have random spins the resulting stars will have a low rotation rates. The planets are smaller blobs captured by the star.

The small blobs would have higher rotation than is seen in the planets but the theory accounts for this by having the 'planetary blobs' split to give a planet and satellites but it is not clear how the planets came to be confined to a plane or why their rotations are in the same sense.

The Capture theory

The Sun interacts with a nearby protostar dragging a filament of material from the protostar. The low rotation speed of the Sun is explained as being due to its formation before the planets, the terrestrial planets are explained by collisions between the protoplanets close to the Sun and the giant planets and their satellites are explained as condensations in the drawn out filament.

The Modern Laplacian theory

Laplace in 1796 first suggested that the Sun and the planets formed in a rotating nebula, which cooled and collapsed. It condensed into rings, which eventually formed the planets and a central mass, which became the Sun. The slow spin of the Sun could not be explained.

The modern version assumes that the central condensation contains solid dust grains which create drag in the gas as the centre condenses. Eventually, after the core has been slowed its temperature rises and the dust is evaporated. The slowly rotating core becomes the Sun. The planets form from the faster rotating cloud.

The Modern Nebular theory

The planets originate in a dense disk formed from material in the gas and dust cloud that collapses to give us the Sun. The density of this disk has to be sufficient to allow the formation of the planets and yet be thin enough for the residual matter to be blown away by the Sun as its energy output increased.

In 1992 the Hubble Space Telescope obtained the first images of proto-planetary disks in the Orion nebula. They are roughly on the same scale as the Solar System and lend strong support to this theory.

Conclusion

There have been many attempts to develop theories for the origin of the Solar System. None of them can be described as totally satisfactory. We do believe, however, that we understand the overall mechanism.

The Sun and the planets formed from the contraction of part of a gas/dust cloud under its own gravitational pull and that the small net rotation of the cloud created a disk around the central condensation. The central condensation eventually formed the Sun while small condensations in the disk formed the planets and their satellites. The energy from the young Sun blew away the remaining gas and dust leaving the Solar System as we see it today.