Solar system

The Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The Sun's objects circle it in a nearly flat disc called the ecliptic plane. The four smaller inner planets; Mercury, Venus, Earth and Mars are called the terrestrial planets and are primarily composed of rock and metal. The four outer planets, Jupiter, Saturn, Uranus and Neptune, are called gas giants and  are composed largely of hydrogen and helium.  They are far more massive than the terrestrials.  The Solar System is also home to two main belts of small bodies. The asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. The Kuiper belt lies beyond Neptune's orbit and is composed mostly of ices such as water, ammonia and methane. Within these belts, five individual objects, Ceres,  Pluto, Haumea, Makemake and Eris, are recognised to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets.