The formation of our solar system is one of the big questions that astronomy has tried to answer, up there with the origin of the universe and the beginning of life (ok never mind, that’s for biology). Although by the early 20th century much was known about the planetary motions and a few hints of the chemistry could be found from meteorites, much of the initial theories required advances in astronomy, physics and computational power that had yet to come. Today, when we see computer-rendered simulations of planet formation it's easy to forget that the theory took decades of refinement and the ingenious contributions of many astronomers. In this article, we’ll look at the rise and fall of what is widely considered the origin of these origin theories - Laplace's nebular theory of solar system formation.
Descartes' world of vortices
Our story begins just a little bit after Galileo. René Descartes was influenced by him and thought about the origin of the heliocentric solar system. He postulated that the Universe was filled with a fluid that had vortices in it. These swirling vortices contracted and smaller eddies formed around them. A giant vortex formed the Sun, and the smaller vortices around it formed the planets and the moons. The hypothesis was an attempt to explain the circular motion of the planets and the planarity of their paths, but sadly Descartes had this notion before Newton and thus didn’t realize that it wouldn't work. His vortex theory explanation for the forces holding the universe was superseded by Newton's gravity
Newton himself didn’t delve into this question, believing that it was simply a consequence of the divine. He states in his Principia Mathematica "This most beautiful system of the sun, planets and comets, could only proceed from the counsel and dominion of an intelligent and powerful Being".
After Newton, in 1734 philosopher Emmanuel Swedenborg proposed that the planets were formed from a shell of matter that broke off from the Sun. Immanuel Kant took this a step further in 1755, supposing that the collapse of matter in a nebula could be responsible for the formation of the planets. Although Kant was essentially on the right track, he did not understand Newton’s gravity well enough to give a proper description of the process.
The convincing conjecture
The first proper theory finally came up in 1796 by Pierre-Simon Laplace. Alongside Lagrange, he was the first to show the stability of planetary orbits by developing techniques that could consider the perturbations that the other planets had on any particular planet’s orbit. Thus he knew the implications of Newton’s theory of gravity better than the others before him. Laplace debunked a previous theory by Comte de Buffon (1749) who suggested that the planets were formed from the remains of a collision between a comet and the Sun, by pointing out that such a planet would necessarily fall back into the Sun.
Laplace took Kant’s nebular hypothesis and modified it as follows: He started off with a hot spinning cloud of gas and dust. As time progressed it cooled, collapsed due to gravity and spun faster as its angular momentum increased. The cloud starts to flatten along the spin axes and its shape goes from ellipsoidal to lenticular. Eventually, matter at the edges starts to break off in rings and the cloud contracts inwards. The central core of the cloud forms the Sun and the matter in each ring coalesces to form planets. The same process on the planetary scale gives us the moons. It should be noted that Laplace did not actually perform any calculations for his theory, it was essentially a clever reasoning based on his qualitative understanding.
Interestingly, Laplace himself was not very optimistic about his theory. He only mentioned the theory in his ‘Systeme du Monde’ (1796) - a popular science book (think of it like 'The Brief History of Time' but nearly 200 years older!) and is cautious to say “These are conjectures on the formation of the stars and the solar system, I present them with the distrust which everything that is not a result of observation or calculation must inspire.” And yet, this very conjecture remained the best explanation for the formation of our solar system for over a century.
The theory had reasonable observational evidence in the then recently observed nebulae (including ‘planetary’ nebula) and was supported by most astronomers throughout the 19th century. In a very plausible and mechanistic way, it explained the formation of not just the planets but also the Sun and it gave an explanation for why the orbits were planar and the planets all revolved in the same sense. But gradually people began to realize that the same principle of conservation of angular momentum that led to the success of the theory, also led to its downfall.
Holes in the hypothesis
J.C. Maxwell won the 1856 Adam’s Prize when he mathematically showed that the stability of Saturn’s rings could only be explained if it was made of solid particles. His work also implied that if the matter in the planets had previously been spread out in a disk, the shearing forces due to rotation would prevent the rings from condensing into planets unless the rings were hundreds of times more massive than the planets themselves.
Asteroids and comets were discovered with highly eccentric orbits, and much later moons were found that did not revolve in the same direction as their planet’s spin e.g. Neptune’s moon Triton. This was a problem because the rings formed by the theory would have given rise to nearly circular orbits with the planets rotating in the same sense. Although it could be explained through collisions that disrupted these specific bodies.
But the biggest problem by far was this: 99.86% of the solar system’s mass is in the Sun, but only 0.5% of the angular momentum of the solar system is due to the Sun. This means that either the Sun had to slow down a lot or the revolution of the planets had to speed up, neither of which can be justified by this theory.
The entire mass of the Sun was initially distributed in a nebular disk that extended till at least the orbit of Neptune. Even if we did the calculation for Mercury, if the disk took 88 days (the orbital period of Mercury) to complete one rotation, after it shrunk to the Sun’s radius, conservation of angular momentum means that it would spin once every 17 min! This is nowhere close to the observed period of 27 days and moreover, the Sun would be ripped apart long before it would reach such a speed.
Eduard Roche tried to modify the theory by considering a nebula in which most of the mass was at the centre. But this did not improve the angular momentum problem by much and also led to there being insufficient mass to actually form planets. Thus some astronomers in the early 20th century began to look for other formation theories.
Glossary
Heliocentric: helios - sun, centric - centre, the theory that the sun was at the centre of the universe
vortex theory: Much later in 1944, Carl von Weizsäcker (known also for the liquid drop model of the nucleus) came up with a vortex theory of solar system formation which in some respects resembled Descartes's ideas. The theory explained the Titius-Bode law but ultimately was rejected as it involved turbulence.
Planetary nebula: An emission nebula formed by the gas and dust remaining from the outer layers of a dying star.
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