The Giant Planets

The giant planets of the outer solar system are Jupiter, Saturn, Uranus and Neptune. They do not have any solid surfaces and are mostly atmosphere. The main properties of the giant planets are that they are large, massive and of low density. Jupiter is 318 times the mass of the Earth, Saturn about 95 times the mass of Earth, Uranus and Neptune about 20 times the mass of Earth.

The large mass of the giant planets means that gravitational forces hold each planet together, holding in even the lightest gas, hydrogen. Inside both Jupiter and Saturn the pressures are great enough to make hydrogen metallic. At the very core of each of the giant planets it is believed that there is a small core of rock and iron - about 10 times the mass of the Earth. That's a pretty respectable terrestrial planet!

Jupiter and Saturn have a composition very similar to the Sun, but their masses are far too low to provide sufficient pressure needed for nuclear fusion. Calculations show that nuclear fusion is possible only with a mass at least 80 times that of Jupiter. Some people call Jupiter a "failed star". Some prefer to think of it as a very successful planet.

The pressures inside Uranus and Neptune are not sufficient to make hydrogen metallic. Instead, the temperatures are cooler so that hydrogen can bind with other elements to make compounds.