According to Universe Today, the four outer planets of the solar system are all gas giants. This means they lack a conventional surface, and are made up of dense gases around a possible rocky or metallic core. These planets are called Jovian planets to distinguish them from the four rocky terrestrial planets closer to the sun.
When the solar system formed, leftover matter from the creation of the sun orbited in a large, flat disc around the star. Heavier and more solid elements orbited closer to the new star, while lighter frozen gases escaped further out into the fledgling solar system. Eventually, some of these molecules began to accumulate together, forming small discs and then eventually spheres.
The outer planets all formed from frozen gases, and as a result they are made up primarily of elements such as helium and hydrogen. They are also considerably larger than the terrestrial planets, and their enormous size results in massive gravitational forces. At the center of planets such as Saturn and Jupiter, these gases may even be forced into a solid phase, creating cores of metallic hydrogen or other elements. The Jovian planets also tend to have more moons than the terrestrial planets.