The inner planets of the solar system are small, rocky worlds that have very few moons. The outer planets are composed primarily of gases and are much larger than the inner planets. All four of the outer planets have a system of rings and are orbited by multiple moons.
The four planets of the inner solar system, often called terrestrial planets, orbit much closer to the sun. Mercury, Venus, Earth and Mars are composed of heavy metals such as nickel and iron, and all have a solid surface. The inner planets are much smaller than the gas giants found in the outer solar system, and each has either no moons or few moons.
The outer planets are gas giants, sometimes referred to as jovian planets. Unlike the inner planets, the gas giants are composed mostly of hydrogen and helium. Jovian planets lack a clearly defined boundary between the atmosphere and the planetary interior, although planets in the outer solar system are thought to possess solid or liquid cores. The outer planets comprise the bulk of planetary mass within the solar system. Jupiter, Saturn, Uranus and Neptune are all orbited by numerous moons, and each planet has a complex system of rings comprised of dust and ice.