Answer:
They are all composed of gases.
Step-by-step explanation:
The inner planets are mainly composed of types of rock. The surface and interior is rocky but the atmosphere is composed of gases. The outer planets happen to be composed mainly of gas, or in other terms, elements. The two primary elements that make up these planets are hydrogen and helium. Because the inner planets are made up of primarily, they have given the nickname the "Terrestial Planets" and the outer planets are called "Gas Giants" because of their gas (elements) composed atmospheres and overall interiors.