Here's why:
* Rocky planets like Earth and Mars are primarily composed of rock and metal.
* Gas giants like Jupiter and Saturn are mostly made of gas (hydrogen and helium) with a smaller, solid core.
Pluto, on the other hand, is composed mostly of rock and ice. It's smaller than Earth's Moon and has a very thin atmosphere. It doesn't have enough gravity to clear its orbital path of other objects, which is one of the requirements for being classified as a planet.
