The Science of Discworld Revised Edition

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.
    These bodies are known as ‘Kuiper Belt Objects’ or KBOs. A few years back there was a big fuss because some astronomers wanted to redefine Pluto as a KBO rather than a planet. Pluto probably wouldn ’t have minded either way, but an awful lot of textbook publishers would have. The scientific case was strong: Pluto is weird in almost every respect, as we’ve just seen, and it could easily be a KBO that accidentally strayed into the outer reaches of the solar system when disturbed by other bodies. If so, that would explain why it’s so weird. It doesn’t look like a planet because it isn’t one. Other astronomers disagreed strongly with this proposal – for sentimental reasons, for historical ones, or because we don’t know for sure that Pluto is a wandering KBO. In the end, Pluto remained on the list of planets. But whether it can hang on to that status for much longer is unclear.
    Then there are meteorites, lumps of rock of various sizes that wander erratically through the whole thing …
    Each of these celestial objects, moreover, is a one-off. Mercury is a blisteringly hot lump of cratered rock. Venus has a sulphuric acid atmosphere, rotates the wrong way compared to nearly everything else in the solar system, and is believed to resurface itself every hundred million years or so in a vast, planetwide surge of volcanic activity. Earth has oceans and supports life; since we live on it we find it the most congenial of the planets, but many aliens would probably be aghast at its deadly, poisonous, corrosive oxygen atmosphere. Mars has rock-strewn deserts and dry ice at its poles. Jupiter is a gas giant, with a core of hydrogen compressed so much that it has become metallic, and maybe a small rocky core inside that – ‘small’ compared to Jupiter, but about three times the diameter of the Earth. Saturn has its rings – but so do Jupiter, Uranus, and Neptune, though these are nowhere near as extensive or spectacular. Uranus has an icy mantle of methane and ammonia, and its axis of rotation is tilted so far that it is slightly upside down. Neptune is similar to Uranus but without that ridiculous axial tilt. Pluto, as we’ve said, is just crazy. We don’t even know accurately how big it is or how massive it is, but it’s a Lilliputian in the country of the Gas Giants.
    Right …
all that
is what a theory of the origins of the