Solar System

The Solar System is the system that revolves around the star "Sol", or our Sun.

Planets in the Solar System
The eight major planets in the Solar System are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Smaller planets, known as "dwarf planets", are Ceres, Pluto, Haumea, Makemake, and Eris.

How the Planets are Classified in the Solar System
The four "terrestrial", or inner, planets are Mercury, Venus, Earth, and Mars. They are generally "rocky" planets, and have matter that exists in all 3 (on Earth, 4) phases of matter: solid, liquid, and gas. On Earth scientists discover plasma on it. They all are relatively small, and have relatively high densities.

In between the inner and outer planets, there is the asteroid belt.

The four "gas giants", or outer planets, are Jupiter, Saturn, Uranus, and Neptune. They are generally "gassy" planets, and have matter that only exists in two phases: liquid and gas. They all are relatively large, and have relatively low densities. The density of Saturn (0.7 g/cm^3) is less that that of water (1.0 g/cm^3)! If there were a big enough tub to hold both the water and Saturn, and the water and the tub don't collapse or turn into superheated plasma, Saturn could float in water.