Why did the four inner planets of the solar system form as rocky planets and the four outer planets form as gas planets?

There have two types of planets: Inner planets and outer planets.

Explanation:

Mercury, Venus, Earth and Mars are called as Inner planets where Jupiter, Saturn, Uranus, Neptune are called as Outer Planets. The inner planets are known as rocky planets and the outer are known as gaseous planets. The Inner planets or the rocky planets are closer to the Sun and after them there is an Asteroid belt and then the Outer or gaseous planets. The inner planets are mostly made up of heavy metals like iron and nickel, and for this reason they are known as rocky planets. The outer planets are some time known as gaseous planets or the gas giants because these are made up mostly of gasses like hydrogen and helium.