SOLAR SYSTEM

Detailed explanation-1: -At the center of the Earth lies the core, which is nearly twice as dense as the mantle because its composition is metallic (iron-nickel alloy) rather than stony.

Detailed explanation-2: -Now that the Earth has cooled somewhat, pressure deep in the Earth increases local density by forcing liquids (e.g. the liquid outer core) into a solid state (the solid inner core), and then as the pressure increases, solids shift into new, more tightly packed crystal structures.

Detailed explanation-3: -Gravity pulls more strongly on denser materials so denser materials are at the center of things. Earth’s core, at its center, is denser than its crust. The lowest layer of the atmosphere is denser than the upper layer.