Astronomy and Science Fiction: Part 1

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.