The inner and outer planets all follow an elliptical orbit, share the same orbital plane, are spherical and contain some of the same elements. Besides those attributes, each planet is very different.
The inner planets are those that exist between the sun and the asteroid belt. They are:
The outer planets are those that are between the asteroid belt and the end of the solar system. These are:
The outer planets are known as Jovian or gas giants. They are much larger in size than the inner planets and are primarily composed of gases that surround a liquid metal core.
The inner planets are all smaller and denser than the outer planets. They are mostly solid and composed of rock rather than gas.
The reason for the difference in the size, density, and composition of the planets is that the gravity of the sun tends to attract heavier, solid elements, such as heavy metals, while lighter gases, such as hydrogen, helium, nitrogen, and oxygen, are not pulled as strongly inward.
The elements that exist in all of the planets are iron and nickel. Mercury, the planet closest to the sun, is almost entirely made of these two elements, while the outer planets only have trace amounts located in their core.