Why are there two main types of planets?

There are two main types of planets because the two categories separate the planets into more specific groups.

1. Terrestrial planets have a rocky surface and are inside of the asteroid belt that separates the Terrestrial planets from the Gas Giants

The Terrestrial planets are: (in order of distance to the sun (close to far)

Mercury, Venus, Earth, and Mars.

2. Gas Giants are mostly made up of gasses and are very large in size

Gas Giants are: (in order of distance to the sun (close to far)

Jupiter, Saturn, Uranus, and Neptune.