Which planet has the highest density?

Earth has the highest average density of all the planets. Its mean density is about 5.51 grams per cubic centimetre, slightly higher than Mercury's, even though Mercury is made of an unusually large proportion of metal.

Earth's high density comes from its composition and size. The planet contains a substantial iron-and-nickel core surrounded by a rocky mantle and crust. Earth's gravity compressed these materials strongly during formation, increasing the planet's average density. Density also rises with depth inside Earth, reaching its greatest values in the inner core.

A common mix-up is to confuse density with mass or size. Jupiter is by far the most massive planet, but its hydrogen-and-helium composition gives it a much lower average density. Mercury is the densest planet after Earth and has the highest metal-to-rock ratio, while Venus and Mars are less dense. The comparison refers to average planetary density, not the density of a particular surface rock or internal layer.