The Science of Discworld Revised Edition

distances, it’s solid ice. You can pick up a lot of solid mass if you’re condensing in a region where ice can form. So the planets there are bigger, and (at least to begin with) they are icy. Nearer the star, the planets are smaller, and rocky. But now the big guys can parlay their initial weight advantage into an even bigger one. Anything that is ten times the mass of the Earth, or greater, can attract
and retain
the two most abundant elements of the disc, hydrogen and helium. So the big balls soak up large amount of extra mass in the form of these two gases. They can also retain compounds like methane and ammonia, which are volatile gases closer to the star.
    This theory explains rather a lot. It gets all the main features of the solar system pretty much right. It allows for the odd exceptional motion, but not too many. It agrees with observations of condensing gas clouds in distant regions of space. It may not be perfect, and some special pleading might be necessary to explain odd things like Pluto, but most of the important features click neatly into place.
    It also seems likely that huge numbers of planets exist without a central star. In 2000 a team led by Rafael Rebolo observed isolated large planets. A survey of such bodies in the Sigma Orionis cluster shows that the smaller these bodies are, the more numerous they become. If this relationship continues down to Earth-sized bodies (which are too small to observe with current methods) then ‘isolated planets’ will litter the galaxy. There are probably hundreds of them within 30 light years of Earth, for example. But without a nearby star, there is no way we can observe them directly. There is no star to wobble, no light output to dim as a planet gets in the way, and the planets themselves emit only reflections of distant starlight, far too faint to be seen from here. The conventional theory of planetary formation, in which a star and its accompanying solar system come into being together, cannot apply to such worlds. Small gas clouds are not massive enough to collapse under gravity in the right way, but magnetic effects might cause a collapsing gas cloud round a star to break up and be ejected before its planets are fully formed. Or perhaps these worlds came into being in the usual way, but were then ejected from their solar systems.
    The future of the solar system is at least as interesting as its past. The picture of the solar system that emerged from the ideas of Newton and his contemporaries was very much that of a clockwork universe – a celestial machine that, once set ticking, would continue to follow some simple mathematical rules and continue ticking merrily away forever. They even
built
celestial machines, called orreries, with lots and lots of cogwheels, in which little brass planets with ivory moons went round and round when you turned a handle.
    We now know that the cosmic clockwork can go haywire. It won’t happen quickly, but there may be some big changes to the solar system on the way. The underlying reason is chaos – chaos in the sense of ‘chaos theory’, with all those fancy multicoloured ‘fractal’ things, a rapidly expanding area of mathematics which is invading all of the other sciences. Chaos teaches us that simple rules need not lead to simple behaviour – something that Ponder Stibbons and the other wizards are in the process of discovering. In fact, simple rules can lead to behaviour that in certain respects has distinct elements of randomness. Chaotic systems start out behaving predictably, but after you cross some ‘prediction horizon’ all predictions fail. Weather is chaotic, with a prediction horizon of about four days. The solar system, we now know, is chaotic, with a prediction horizon of tens of millions of years. For example, we can’t be sure which side of the Sun Pluto will be in a hundred million years’ time. It will be in the same
orbit
, but its position in that orbit is completely uncertain.
    We know this because of some mathematical work that was done, in part, with an orrery – but this was a ‘digital orrery’, a custom-built computer that could do celestial mechanics very fast. The digital orrery was developed by Jack Wisdom’s research group, which – in competition with its rival headed by Jaques Laskar – has been extending our knowledge of the solar system’s future. Even though a chaotic system is unpredictable in the long run, you can make a whole series of independent attempts at