One of the most influential astronomical theories in science fiction has been the Nebula Hypothesis. Before exploring how this shaped stories in science fiction, I'd first like to explore the background of the hypothesis, and how it has been influential.
The Nebula Hypothesis was a theory
proposed for the formation of the solar system. It was suggested in a very
simple form by the philosopher Immanuel Kant in 1755, but the more mature
development came with Laplace in 1796.
What this did was to provide a
model for the formation of the solar system that gave reasons for the
following problems. How it was that:
(1) the planets contain most of the
angular momentum in the Solar System
(2) all the planets are in roughly the
same plane, which is the same plane as the Sun's equator
(3) most planets
rotate in the same direction
With this model, the solar system forms from
a hot rotating nebular of dust and gas. As the rotating gas cools, the
nebular decreases in size, but as it contracts, there is a corresponding
increase in rotational speed. The centrifugal force eventually overtakes the
gravitational force, and there is expelled a rotating ring of gas and dust,
which would be rather like a vast ring around Saturn, on one plane, but
instead composed of large quantities of dust and gases. Gravitational forces
would cause those to coalesce and contract along its orbit into a planet.
The process would repeat a number of times, as the central nebula's speed
led to further expelling of matter, causing the formation eventually of all
of the planets. So the gravitational collapse of the nebula gives rise to
the sun and planets.
Now there are a number of problems with Laplace's
original hypothesis. One problem in particular is how a separated ring would
aggregate into one planet, and not several planets along the same orbit.
There are also major problems in the angular momentum of the complete
system, which is insufficient to explain why 98% of the angular momentum of
the solar system is concentrated in the planets.
As a result, this
theory of planet formation was largely abandoned at the beginning of the
20th century and alternatives were sought, building on the original "big
picture" of a nebula at the core of the solar system's formation, but with
quite a different basis for planetary formation. The consensus today largely
comes from the Soviet astronomer Victor Safronov, and builds on what is
called the Solar Nebular Disk Model. That still has some problems, but
lesser ones. But we need not concern ourselves with that today.
Rather - for the purposes of science fiction - we need to
remember that Laplace's hypothesis dominated the 19th century and left a
lasting legacy in the 20th century.
With Laplace's original
hypothesis, the age of the planets is fixed by their orbit. Because the
nebular intermittently throws off a rotating ring, which forms a planet, the
outermost planets are the oldest, and those closest to the sun are the
youngest. This is a fundamental premise which trickles down and influences
how science fiction writers approach the planets.
Because of Laplace's
Nebula Hypothesis, science fiction writers consider the planets closer to
the sun than earth as younger planets, and correspondingly in an earlier
state of evolution. Venus is a younger earth. Mars on the other hand, is an
older, dying planet, the Earth of the future.
In my next posting on this
subject, I'll look at how Venus is portrayed, and how that reflects the
Nebula Hypothesis, and what sort of Venus was portrayed in science fiction before the Mariner probe falsified some common misconceptions.
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