Nearly 5 billion years old, the Solar System is made up of celestial bodies orbiting around the central Sun and bounded to it by gravity. The celestial objects consist of eight planets, a handful of so-called dwarf planets and its more than 170 Moons, gas, dust, thousands of comets and asteroids. The eight planets are divided in four Inner Planets and four Outer Planets. The Inner or terrestrial planets are Mercury, Venus, Earth and Mars and are composed mainly of rock and metal. The Outer Planets or Gas Giants are Jupiter, Saturn, Uranus and Neptune. They are composed mostly of hydrogen and helium and are considerably bigger than the terrestrials.