What evidence do we judge these theories against? We have to decide first what sort of information we can judge the theories against. As we go through we shall see there is a plethora of facts about the physics and chemistry of the system against which we can measure these ideas. There are ese ideas. There are distributions of elements, the details of the dynamics, overall compositions etc which all need to be fitted in. But we can start by just looking at some of the basic outline information that we have. We know for instance that the planets can be divided into two main groups - the "terrestrial" planets, small rocky bodies with densities 3-6 times that of water in the inner part of the system, and gaseous, low density bodies in the outer part, divided from the terrestrial planets by the asteroid belt. We also know that the gaseous planets can be further subdivided: there is Jupiter and Saturn, the "Gas Giants" which are roughly the same size (though different in density), and Uranus and
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