The outer planets (Jupiter, Saturn, Uranus and Neptune) are also known as gas giants. These planets are larger than the inner planets, but are made primarily of gases and ices. As the name "gas giant" might suggest, the outer planets are largely made up of different kinds of gasses, such as methane or ammonia. The temperature on the outer planets is far cooler than the inner planets as well; therefore, ices of various compositions are also a major component of the outer planets.

Scientists believe that the outer planets formed in the less-dense outer reaches of the nebula disk, where temperatures were cooler. The gas was too far from the Sun to be boiled away, and the cooler temperatures allowed ice to form. Additionally, some matter escaped the inner solar system to make it farther out. This matter is lighter elements such as oxygen. As a result, small cores of ice and rock formed. Gravity caused the ice, gas and heavier elements that make up the planets'cores to collide and begin to compress together. The result was the formation of the outer planets.