There are really two kinds of planets in our solar system. One kind is called a terrestrial planet because it is very much like Earth. The other is called a jovian planet because it is more like Jupiter.
When the solar system formed, the planets took shape out of clouds of dust and gas. Those planets that formed near the sun, like Earth, formed mainly out of heavy dust containing elements such as iron and carbon, while the lighter gases were burned away by the heat of the sun. So the terrestrial planets, Earth, Mars, Mercury, and Venus, are small and solid.
But the jovian planets formed far from the sun, where the sun’s heat was not strong enough to burn away the gases. So the jovian planets, Jupiter, Saturn, Neptune, and Uranus, are much larger and lighter, made up of liquids or gases, with a small center of solid matter. Perhaps these planets have no real surface, and their atmosphere gradually thickens to form what appears to us to be a solid ball.
Pluto, the farthest planet from the sun, is so far away that we don’t know what it’s made of!
Because Saturn is less dense than water, if you could place it in a huge body of water, it would float!