Jupiter, Saturn, Uranus and Neptune are all similar in size and material composition. These planets are known as gas giants, and they are made almost entirely of hydrogen and helium. They also lack a solid surface.
The outer planets are all very large, and together they make up 99 percent of the planetary mass of the solar system. The four gas giants of the outer solar system do not have clearly defined boundaries between atmosphere and surface. Although they are composed mostly of gas, the outer planets are thought to have a core made of rock, heavy metals and hydrogen compounds. Jupiter and Saturn have similar interiors with layers of gaseous hydrogen topped by a layer of visible clouds, while Uranus and Neptune have higher levels of water and methane.
Although moons are scarce in the inner solar system, the outer planets have dozens of moons, some of which rival the inner planets in terms of size and mass. All of the outer planets have a system of rings, although only the rings of Saturn can be seen from Earth. Planetary rings are made of small pieces of dust, rock and ice that are held in orbit by planetary gravity just like a moon.