The Science of Discworld Revised Edition

big in the middle where Jupiter and Saturn live. Mind you, it was never entirely clear
why
the blob had to be cigar-shaped …
    One important feature of this theory was the implication that solar systems are rather uncommon, because stars are quite thinly scattered and seldom get close enough together to share a mutual cigar. If you were the sort of person who’d be comforted by the idea that human beings are unique in the universe, then this was a rather appealing suggestion: if planets were rare, then
inhabited
planets would be rarer still. If you were the sort of person who preferred to think that the Earth isn’t especially unusual, and neither are its life-forms, then the cigar theory definitely put a crimp on the imagination.
    By the middle of the twentieth century, the shared-cigar theory had turned out to be even less likely than the Kant-Laplace theory. If you rip a lot of hot gas from the atmosphere of a star, it doesn’t condense into planets – it disperses into the unfathomable depths of interstellar space like a drop of ink in a raging ocean. But by then, astronomers were getting a much clearer idea of how
stars
originated, and it was becoming clear that planets must be created by the same processes that produce the stars. A solar system is not a Sun that later acquires some tiny companions: it all comes as one package, right from the start. That package is a disc – the nearest thing in our universe (so far as we know) to Discworld. But the disc begins as a cloud and eventually turns into a lot of balls (Stibbons’s Third Rule).
    Before the disc formed, the solar system and the Sun started out as a random portion of a cloud of interstellar gas and dust. Random jigglings triggered a collapse of the dustcloud, with everything heading for roughly – but not exactly – the same central point. All it takes to start such a collapse is a concentration of matter somewhere, whose gravity then pulls more matter towards it: random jigglings will produce such a concentration if you wait long enough. Once the process has started, it is surprisingly rapid, taking about ten million years from start to finish. At first the collapsing cloud is roughly spherical. However, it is being carried along by the rotation of the entire galaxy, so its outer edge (relative to the centre of the galaxy) moves more slowly than its inner edge. Conservation of angular momentum tells us that as the cloud collapses it must start spinning, and the more it collapses, the faster it spins. As its rate of spin increases, the cloud flattens out into a rough disc.
    More careful calculations show that near the middle this disc thickens out into a dense blob, and most of the matter ends up in the blob. The blob condenses further, its gravitational energy gets traded for heat energy, and its temperature goes up
fast
. When the temperature rises enough, nuclear reactions are ignited: the blob has become a star. While this is happening, the material in the disk undergoes random collisions, just as Kant imagined, and coalesces in a not terribly ordered way. Some clumps get shoved into wildly eccentric orbits, or swung out of the plane of the disc; most clumps, however, are better behaved and turn into decent, sensible planets. A miniature version of the self-same processes can equip most of those planets with satellites.
    The chemistry fits, too. Near the Sun, those incipient planets get very hot – too hot for solid water to form. Further out – around the orbit of Jupiter for a dustcloud suitable for making our Sun and solar system – water can freeze into solid ice. This distinction is important for the chemical composition of the planets, and we can see the main outlines if we focus on just three elements: hydrogen, oxygen, and silicon. Hydrogen and oxygen happen to be the two most abundant elements in the universe, apart from helium which doesn’t undergo chemical reactions. Silicon is less abundant but still common. When silicon and oxygen combine together, you get silicates – rocks. But even if the oxygen can mop up all the available silicon, some 96% of the oxygen is still unattached, and it combines with hydrogen to make water. There is so much hydrogen – a thousand times as much as oxygen – that virtually all of the oxygen that doesn’t go into rocks gets locked away in water. So by far the most common compound in the condensing disc is water.
    Close to the star, that water is liquid, even vapour, but out at Jovian