The Sun and its Planets Part II

Virtually all of the material of the solar system is contained within the Sun, with only a small fraction in the planets and other objects in orbit.

There are only two distinct kinds of planets. Near the Sun, in the "inner" solar system, are planets like the Earth --relatively small, rocky, high-density worlds. These are called the terrestrial planets, and include Mercury, Venus, Earth, Mars, and although it is not a real planet Earth's Moon. Farther out from the Sun, in the "outer" solar system, are huge worlds made primarily of liquids and gases. We call them "gas giants," orJovian planets, and they are Juipter, Saturn, Uranus,and Neptune. Plutothe outermost planet, is something of an anomaly, being small and rocky.

Interspersed with the planets are a large number of other kinds of objects. All the planets execpt the innermost Mercury and Venus are orbited by one or more moons. While some moons are little more than boulders a few kilometers across, others are much larger, and Saturn's largest moon Titan is about the smae size as Mercury. Saturn also has a dramaticrings composed of millions of tiny moons. Small, rocky asteroids that circle the Sun like minuature planets are found primary in orbits between Mars and Juipter, in which is called the asteriod belt, although some have orbits that cross the Earth's. Far beyoung Pluto, we find a swarm of icy coments with compositions something like a "dirty snowball." Occasionally, one is jostled loose from its orbit and become part of the realm of the planets, creating a spectacular display in the sky.