Lecture 2:  Solar System

 
The Big Ones

Terrestrial Planets

The Origin of the Solar System
(The original and other links may be found at the Royal Greenwich Observatory)

The earliest accounts of how the Sun, the Earth and the rest of the Solar System were formed are to be found in early
myths, legends and religious texts. None of these can be considered a serious scientific account.

The earliest scientific attempts to explain the origin of the solar system invoked collisions or condensations from a gas
cloud. The discovery of `island universes', which we now know to be galaxies, was thought to confirm this latter theory.

During this century Jeans proposed the idea that material had been dragged out of the Sun by a passing star and that
this material had then condensed to form the planets. There are serious flaws to this explanation but recent
developments have been made suggesting that a filament was drawn out of a passing protostar at a time when the
Sun was a member of a loose cluster of stars but the most favoured theories still involve the gravitational collapse of a
gas and dust cloud.
 

The problems to be faced by any theory for the formation of the Solar System

Any theory has to account for certain rather tricky facts about the Solar System. These are in addition to the obvious
facts that the Sun is at the centre with the planets in orbit around it. There are 5 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. The planets carry the rest of the angular momentum.

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.

(Bode's `law' takes the form of a series in which the first term is 0, the second is 3 and each term is then double the
previous one, to each term add 4 and divide the result by 10. This yields the series of numbers,

0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10.0, 19.6, 38.8;

which may be compared to the mean distances of the planets from the Sun in AU,

0.39, 0.72, 1.0, 1.52, 5.2, 9.52, 19.26, 30.1, 39.8.

The agreement for all but Neptune and Pluto is remarkable. The lack of a planet at 2.8 led to the discovery of the
asteroids.)

There are 5 theories which are still considered to be `reasonable' in that they explain many (but not all) of the
phenomena exhibited by the solar system.
 

The Accretion theory

This assumes that the Sun passed through a dense interstellar cloud and emerged surrounded by a dusty, gaseous
envelope. It thus separates the formation of the Sun from that of the planets thus losing problem 1.

The problem which remains 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

This assumes that initially there is a dense interstellar cloud which will eventually produce 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.

Thus many of the problem areas are covered 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

This theory is a version of Jeans's theory in which 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 Nebula theory

Observations of very young stars indicate that they are surrounded by dense dusty disks. While there are still
difficulties in explaining some of the problem areas outlined above, in particular ways to slow down the rotation of the
Sun, it is believed that the planets originated in a dense disk which formed from material in the gas and dust cloud
which collapsed to give 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.
 

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 and it is possible that there will further developments which may better explain the known facts.

We do believe, however, that we understand the overall mechanism which is that 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
was responsible for the formation of 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.

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PJA Thu Apr 18 10:43:06 GMT 1996

webman@mail.ast.cam.ac.uk

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