Transcript: One of the most exciting things about exploration of the solar system with spacecraft in the last few decades is the discovery that many of the objects in the solar system have their own peculiar characteristics that make them very interesting to study. It's as if some of the moons and planets have their own personalities that we can learn through their physical properties. Regardless of these distinctive features, one of the most important things to remember when studying the solar system is called comparative planetology, the idea that there are general features of the planets that can be explained by simple physical principles. For instance, it's easy to see that the inner set of planets, Mercury, Venus, Earth, and Mars, are quite different in nature from the outer planets, Jupiter, Saturn, Uranus, and Neptune. The inner planets are small, dense, and rocky. The outer planets are large, gaseous, and have low density. Comparative planetology uses physical principles to relate planets to each other and to give us ideas for expecting what we might find when we start to investigate other planetary systems around other stars.