Space factPlanets
Earth’s average density is 5,513 kilograms per cubic meter, about 5.5 times that of water. That is the highest of any planet in the solar system. Mercury is second at 5,429.
Mercury has a metal core that is huge for its size, so it looks like it should win. Earth still comes out ahead because it is much bigger. Earth has about 18 times Mercury’s mass, and its gravity squeezes its rock and metal into a tighter space than Mercury’s gravity can. Planet size alone does not decide density. Composition and gravity both play a part.
5,513 kg/m³
Earth's average density