The Science of Discworld Revised Edition

solar system has to explain. It was all a lot easier when we thought there were six planets, plus the Sun and the Moon, and that was
it
. As for the solar system being an act of special creation by a supernatural being – why would any self-respecting supernatural being make the thing so
complicated
?
    Because it makes
itself
complicated – that’s why. We now think that the solar system was formed as a complete package, starting from quite complicated ingredients. But it us took a while to realize this.
    The first theory of planetary formation that makes any kind of sense by modern standards was thought up by the great German philosopher Immanuel Kant about 250 years ago. Kant envisaged it all starting as a vast cloud of matter – big lumps, small lumps, dust, gas – which attracted each other gravitationally and clumped together.
    About 40 years later the French mathematician Pierre-Simon de Laplace came up with an alternative theory of enormous intrinsic beauty, whose sole flaw is that it doesn’t actually work. Laplace thought that the Sun formed before the planets did, perhaps by some cosmic aggregation process like Kant’s. However, that ancient Sun was much bigger than today’s, because it hadn’t fully collected together, and the outer fringes of its atmosphere extended well beyond what is now the orbit of Pluto. Like the wizards of Unseen University, Laplace thought of the Sun as a gigantic fire whose fuel must be slowly burning away. As the Sun aged, it would cool down. Cool gas contracts, so the Sun would shrink.
    Now comes a neat peculiarity of moving bodies, a consequence of another of Newton’s laws, the Law(s) of Motion. Associated with any spinning body is a quantity called ‘angular momentum’ – a combination of how much mass it contains, how fast it is spinning, and how far out from the centre the spinning takes place. According to Newton, angular momentum is conserved – it can be redistributed, but it neither goes away nor appears of its own accord. If a spinning body contracts, but the rate of spin doesn’t change, angular momentum will be lost: therefore the rate of spin must increase to compensate. This is how ice skaters do rapid spins: they start with a slow spin, arms extended, and then bring their arms in close to their body. Moreover, spinning matter experiences a force, centrifugal force , which seems to pull it outwards, away from its centre.
    Laplace wondered whether centrifugal force acting on a spinning gascloud might throw off a belt of gas round the equator. He calculated that this ought to happen whenever the gravitational force attracting that belt towards the centre was equal to the centrifugal force trying to fling it away. This process would happen not once, but several times, as the gas continued to contract – so the shrinking Sun would surround itself with a series of rings of material, all lying in the same plane as the Sun’s equator. Now suppose that each belt coalesced into a single body … Planets!
    What Laplace’s theory got right, but Kant’s did not, was that the planets lie roughly in a plane and they all rotate round the Sun in the same direction that the Sun spins. As a bonus, something rather similar might have occurred while those belts were coalescing into planets, in which case the motion of satellites is explained as well. It’s not hard to combine the best features of Kant’s and Laplace’s theories, and this combination satisfied scientists for about a century. However, it slowly became clear that our solar system is far more unruly than either Kant or Laplace had recognized. Asteroids have wild orbits, and some satellites revolve the wrong way. The Sun contains 99% of the solar system’s mass, but the planets possess 99% of its angular momentum: either the Sun is rotating too slowly or the planets are revolving too quickly.
    As the twentieth century opened, these deficiencies of the Laplacian theory became too great for astronomers to bear, and several people independently came up with the idea that a star developed a solar system when it made a close encounter with another star. As the two stars whizzed past each other, the gravitational attraction from one of them was supposed to draw out a long cigar-shaped blob of matter from the other, which then condensed into planets. The advantage of the cigar shape was that it was thin at the ends and thick at the middle, just as the planets are small close to the Sun or out by Pluto, but