Density

Density is the mass of an object divided by its volume. Thus, by definition, a cubic centimeter (cm³) of water has a mass of 1 gram (g), or a density of 1 gram/cm³. A planet with a density of 5 gram/cm³ and the same size as another with a density of 2.5 gram/cm³ will have twice the mass. A planet with one quarter the volume of another but twice its density will have half its mass.

Planet densities are useful because they provide a hint of what the inside of the planet is made of. For example, metals tend to have higher density than many rocks, so a very dense planet probably has a metallic core.