We usually divide the planets into two groups, but we have two ways of doing this. One way is by location in the Solar System, and the other is by what they are like. The planets are located in the inner and outer parts of the Solar System, so two groups are then formed. The planets are also further classified in several ways. Those ways are by composition, size, position relative to the sun, position relative to Earth and history. .

             What determines what group each planet will be placed in, when discussing its composition, is whether it is terrestrial and rocky or gas. Terrestrial planets are composed primarily of rock and metal and relatively high densities, slow rotation, solid surfaces, no rings and few satellites. Gas planets are composed primarily of hydrogen and helium, and generally have low densities, rapid rotation, deep atmospheres, rings and a lot of satellites. The terrestrial planets are Mercury, Venus, Earth and Mars. The gas planets are Jupiter, Saturn, Uranus and Neptune. Then there is Pluto which is in a group all its own. .

             The next method of classification is by size. The small planets are Mercury, Venus, Earth, Mars and Pluto. In order to be classified in the small planets group, the planet must have a diameter less than 13,000 km. The giant planets are Jupiter, Saturn, Uranus, and Neptune. The giant planets must have the diameter greater than 48,000 km. The .

             giant planets are sometimes referred to as gas giants, while Mercury and Pluto are sometimes called lesser planets. .

             When discussing the position relative to the sun, the planets are categorized in groups of inner and outer planets. The planets in the inner Solar System are the Sun, Mercury, Venus, Earth and Mars, and in the outer Solar System are Jupiter, Saturn, Uranus, Neptune and Pluto. There is a belt called the asteroid belt, which is between Mars and Jupiter. This belts purpose is to form a boundary line between the inner and outer Solar System.