Inner planet

The inner planets. From left to right: Mercury, Venus, Earth, and Mars

The inner planets are the four planets closest to the Sun: Mercury, Venus, Earth and Mars. They are small and dense, made up of mostly rock and metal with a distinct internal structure and a similar size. The composition of these bodies and variable density (higher and lower on Mars and Mercury) provide important clues to solar system formation. The four have solid surfaces. Three also have an atmosphere. The study of the four planets gives information about geology outside the Earth.