The inner planets are DIFFERENT from the outer planets mainly because they are:
Larger
Colder
Comprised of rock
Comprised of gas
The answer is:
Comprised of rock
The inner planets, also known as terrestrial planets, are made up primarily of solid rock and metal. They have a relatively small size compared to the outer planets. In contrast, the outer planets, also known as gas giants, are mainly composed of gas, such as hydrogen and helium, with a smaller rocky core.