Answer:
The temperature of the early solar system explains why the inner planets are rocky and the outer ones are gaseous.
Step-by-step explanation:
The outer planets are much larger than the inner planets. Because of their large mass and cooler temperature, they have a very different composition; they are mostly made of light elements with extended atmospheres in a gaseous form. Outer planets exist in much bigger systems than inner planets.
The reason for different composition of planets has to do with how the solar system formed. In the early solar system, there was a disk of material rotating around the Sun, from which the planets eventually formed. However, to form planets required some kind of initial clump in the protoplanetary disk. These clumps were probably formed from collisions of solid particles. Different elements become solid at different temperatures: the most common elements, hydrogen and helium, never became solid, but hydrogen compounds (like water) can solidify at colder temperatures, like those in the outer solar system. In the inner solar system, it was too hot for these compounds to solidify; only rocks and metals can solidify at these temperatures. Hence, only small planetesimals formed in the inner solar system.