Mercury is one of four inner planets in the Solar System, and has a rocky body like the Earth. It is the smallest planet in the Solar System, with a radius of . Mercury is even smaller than some of the largest moons in the solar system, such as Ganymede and Titan. However, it has a greater mass than the largest moons in the solar system. Mercury is made of about 70% metallic and 30% silicate material. Mercury's density is the second highest in the Solar System at 5.427 g/cm³, only a little bit less than Earth’s.