The Solar System is a gravitationally bound system consisting of the Sun and the celestial bodies that orbit it. After the Sun, the largest objects in the Solar system are the eight planets, consisting of the four gas and ice giants as well as the four terrestrial planets. Many of the planets and larger dwarf planets in the Solar System also have moons of their own. The sun is orbited by several belts of small Solar System bodies: the asteroid belt between the orbits of Mars and Jupiter, the Kuiper belt just beyond the orbit of Neptune, and possibly the Oort cloud in the outer reaches of the Solar System. The entire system was formed roughly 4.6 billion years ago from the remnants of the Sun's molecular cloud, and the hydrogen and helium that was present in this cloud constitutes much of the Solar System's mass.