The names of the inner, or terrestrial, planets are Mercury, Venus, Earth and Mars. The outer planets are Jupiter, Saturn, Uranus and Neptune. Pluto was once designated a planet but has now been demoted to a dwarf planet.
The outer planets are also known as the Jovian planets because of the dominance of Jupiter, the largest planet in the solar system. Jupiter is so large that scientists believe that if it had a bit more mass, it would have ignited and become a star. It and the sun make up well over 90 percent of the mass of the solar system.
The Jovian planets are much larger than the terrestrial planets and are made largely out of gas. In the case of Neptune and Uranus, they are made out of ice. The gases are mostly hydrogen and helium, with traces of other chemicals, such as ammonia and methane. The Jovian planets are also believed to have rocky cores that might be the size of the Earth or somewhat larger.
These giant planets also have many moons, probably the result of debris left over from their formation. This is in contrast with Mercury and Venus, which have no moons, the Earth, which has one moon, and Mars, which has two.