The four largest planets in the Solar System are Jupiter, Saturn, Uranus, and Neptune. These planets are very different from the terrestrial, or Earth-like, planets. The most obvious difference is that they are all enormous compared to the terrestrial planets. Even the two smallest, Uranus and Neptune, are nearly 4 times the diameter of Earth. Another distinguishing feature is their very low density. All four are composed almost entirely of light materials such as hydrogen, helium, and water, rather than the rock and metal that makes up the terrestrial planets. Collectively, Jupiter, Saturn, Uranus, and Neptune are known as the giant planets (Table 10.1), although they are sometimes referred to as the Jovian planets, after Jupiter, the largest of the giant planets. (Jove is another name for Jupiter, the highest-ranking Roman deity.)