Seismic studies have shown that the interior of the Earth has a very organized structure. The iron is not mixed throughout the Earth, but has collected in an iron core down at the center. Shortly after they formed, most planets were molten due to radioactive heating and the heat from impacts. This process of gravitational separation of light and dense material in a liquid planet is called differentiation . Iron and rock are two of the most abundant substances in the inner solar system.
Terrestrial planets have iron cores and rocky surfaces.