'I'm afraid that when I was dropping rocks on the ball of iron there were a few that went out of control.'
'Could happen to anyone, Stibbons,' said the Archchancellor generously. 'Did you add gold?'
'Oh yes, sir. And other metals,'
'Gold does give a crust some style, I think. Are these volcanoes?'
'Sort of, sir. They are the, er, acne of young worlds. Only unlike ours, where the rock is melted in the internal magical fields generated in the sub-strata, the magma is kept molten by the heat trapped inside the sphere.'
'Very smoky atmosphere. I can hardly see anything.'
'Yes, sir.'
'Well, I don't call it much of a world,' said the Dean, sniffing. 'Practically red hot, smoke belching out everywhere ...'
'The Dean does have a point, young man,' said Ridcully. He was extra kind, just to annoy the Dean. 'It's a brave attempt, but you just seem to have made another ball.'
Ponder coughed. 'I just put this one together for demonstration purposes, sir.' He fiddled with the controls of the omniscope. The scene flickered, and changed. 'Now this,' he said, and there was a twinge of pride in his voice, 'is one I made earlier.'
They stared into the lens.
'Well? Just more smoke,' said the Dean.
'Cloud, sir, in fact,' said Ponder.
'Well, we can all make clouds of gas...'
'Er ... it's water vapour, sir,' said Ponder.
He reached over and adjusted the omniscope.
The room was filled with the roar of the biggest rainstorm of all time.
By lunchtime it was a world of ice.
'And we were doing so well,' said Ridcully.
'I can't think what went wrong,' said Ponder, wringing his hands. 'We were getting seas!'
'Can't we just warm it up?' said the Senior Wrangler.
Ponder sat down on his chair and put his head in his hands.
'Bound to cool a world down, all that rain,' said the Lecturer in Recent Runes, slowly.
'Very good ... er, rocks,' said the Dean. He patted Ponder on the back.
'Poor chap looks a bit down,' hissed the Senior Wrangler to Ridcully. 'I don't think he's been eating properly.'
'You mean ... not chewing right?'
'No eating enough, Archchancellor.'
The Dean picked up a piece of paper from Ponder's crowded desk.
'I say, look at these,' he said.
On the paper was written, in Ponder's very neat handwriting:
THE RULES
1 Things fall apart, but centres hold.
2 Everything moves in curves.
3 You get balls.
4 Big balls tell space to bend.
5 There are no turtles anywhere.
6 ... It's so depressing.
'Always been a bit of a one for rules, our Ponder,' said the Senior Wrangler.
'Number Six doesn't sound incredibly well formulated,' said Ridcully.
'You don't think he's going a bit bursar, do you?' said the Lecturer in Recent Runes.
'He always thinks everything has to mean something,' said Ridcully, who generally took the view that trying to find any deep meaning to events was like trying to find reflections in a mirror: you always succeeded, but you didn't learn anything new.
'I suppose we could simply heat the thing up,' said the Senior Wrangler.
'A sun should be easy,' said Ridcully 'A big ball of fire should be no problem to a thinking wizard.' He cracked his knuckles. 'Get some of the students to put Mister Stibbons to bed. We'll soon have his little world all warm or my name's not Mustrum Ridcully.'
14. DISC WORLDS
TO THE WIZARDS OF UNSEEN UNIVERSITY, the heavens include two obviously different types of body: stars, which are tiny pinpricks of light, and the sun, which is a hot ball, not too far away, and passes over the Disc during the day and under it at night. It's taken humanity a while to realize that in our universe it's not like that. Our Sun is a star, and like all stars it's huge, so those tiny pinpricks must be a very long way off. Moreover, some of the pinpricks that seem to be stars aren't: they betray themselves by moving differently from the rest. These are the planets, which are a lot closer and a lot smaller, and together with the Earth, Moon, and Sun they form the solar system. Our solar system may look like a lot of balls whizzing around in some kind of cosmic game of pool, but that doesn't mean that it started out as balls or rock and ice. It is the outcome of a physical process, and the ingredients that went into that process are not obliged to resemble the result that comes out. The more we learn about the solar system, the more difficult it is to give a plausible answer to the question: how did it start? It is not the 'answer' part that gets harder, it's the plausibility. As we learn more and more about the solar system, the reality-check that our theories have to pass becomes more and more stringent. This is one reason why scientists have a habit of opening up old questions that everybody assumed were settled long ago, and deciding that they weren't. It doesn't mean that scientists are incompetent: it demonstrates their willingness to contemplate new evidence and re-examine old conclusions in its light. Science certainly does not claim to get things right, but it has a good record of ruling out ways to get things wrong.
What must a theory of the formation of the solar system explain? Principally, of course, the planets, nine of them, dotted rather randomly in space; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. It must explain their differences in size. Mercury is a mere 3,032 miles (4,878 km) in diameter, whereas Jupiter is 88,750 miles (142,800 km) in diameter, 29 times as big, 24,000 times the volume, an enormous discrepancy. It must explain their differences in chemical composition: Mercury is made of iron, nickel, and silicate rock; Jupiter is made from hydrogen and helium. It must explain why the planets near the Sun are generally smaller than those further out, with the exception of tiny Pluto, out in the cold and the dark. We don't know a great deal about Pluto, but most of what we do know is strange. For instance, all the other planets lie pretty close to a single plane through the centre of the Sun, but Pluto's orbit is inclined at a noticeable angle. All the other planets have orbits that are pretty close to circles, but Pluto's orbit is much more elongated, to the extent that some of the time it is closer to the Sun than Neptune is.
But that's not all that a theory of the origin of the solar system has to get right. Most planets have smaller bodies in orbit around them, our own familiar Moon; Phobos and Deimos, the diminutive twin satellites of Mars; Jupiter's 16 satellites; Saturn's 17 ... Even Pluto has a satellite, called Charon, and that's weird too. Saturn goes one better and also has entire rings of smaller bodies surrounding it, a broad, thin band of encircling rocks that breaks up into a myriad distinct ringlets, with satellites mixed up among them as well as more conventional satellites elsewhere. Then there are the asteroids, thousands of small bodies, some spherical like planets, others irregular lumps of rock, most of which orbit between Mars and Jupiter, except for quite a few that don't. There are comets, which fall in towards the Sun from the huge 'Oort cloud' way out beyond the orbit of Pluto, a cloud that contains trillions of comets. There is the Kuiper belt, a bit like the asteroid belt but outside Pluto's orbit: we know over 30 bodies out there now, but we suspect there are hundreds of thousands. There are meteorites, lumps of rock of various sizes that wander erratically through the whole thing ...