Jupiter, Saturn, Uranus, Neptune and Pluto are called the outer planets. The first four planets are also often referred to as "gas giant" planets. This refers primarily to their similar physical characteristics. The "gas giant" planets feature large masses and diameters. In contrast to the Earth-like planets, their density is rather low. The first four of the outer planets are of gigantic dimensions.
They mainly consist of light elements such as hydrogen. The outermost planet of the solar system is Pluto. It is the smallest planet of the solar system and is also farthest away from the Sun. Pluto does not fit into the above categories. Similar to the outer planets, its surface temperature is very low. Due to the enormous distances between these planets and the Sun, the energy emitted by the Sun is not sufficient to heat their surfaces.
