Planets have different layers because different materials have different densities. Density is how heavy matter is at a certain size. The planets formed as dust, gas, asteroids and comets collided together. We call this “accretion.” Accretion makes matter very hot, hot enough to melt most rocks and metal! While the planets were still hot, the densest matter (pure metal) sank to the cores and the lightest matter (gas) floated to the surface. There are four main materials in the solar system: metal, rock, water, and gas. Metal is the densest and so it is at the core of every planet. Around the metal cores, the planets have a mantle and crust made of rock. Outside of the rock is where you find the lightest materials (least dense), like water and gas. Each planet is a little different, but remember, it is all about density! (Ask Scienceline about how the solar system formed if you want to know more!)

Please look at this interesting picture the Earth's layers click here

Denser substances fall faster than lighter substances, and thus settle to the inside of planets, where the gravity draws it. Lighter things float on top of the denser material. Air sits on top of water, water on top of rock, and rock on top of metal - that's basically the layering of the Earth. Gas planets have layers, too, but I don't think we've understood those as well.

Different materials will settle at different levels according to their density. For instance, the interior of the Earth is iron and nickel, which are solid and dense materials compared to water. Water, in turn, is dense compared to air, which is further "out" from the center of the Earth. I hope this helps!