The Solar System
The Solar System consists of (from the centre outwards) The Sun, an average sized star of average brightness, Mercury, Venus, the Earth and Mars, four small rocky dense planets and then, seperated from these by the Asteroid belt, Jupiter, Saturn, Neptune and Uranus, four giant and not very dense 'gas' planets. Then there are the dwarf planets - a category of object invented in 2006 to describe other large objects orbiting the Sun which are however too small to be termed 'planets'. Currently there are three dwarf planets - Ceres, which is situated in the asteroid belt and is the largest known asteroid in the Solar System, Pluto, which lies beyond Neptune, and Eris, thought to be the largest of the dwarfs, which lies beyond Pluto.
In figures, the Solar System is made up of:
As these statistics show, planets make up the bulk of the non-solar mass in the Solar System, followed by comets and then satellites (i.e. planets' moons). Most of the comets in the Solar System can be found in the 'Oort cloud' - a large grouping of comets which surrounds the solar system far beyond the orbit of Pluto at about 500,000AU, or 2 light years, from the Sun. These comets are not easy to detect as being this far out from the Sun, and given their relatively small sizes, they do not reflect back enough sunlight for us to see them directly. It is only when a comet comes closer to the Sun that Solar radiation causes it to shine and to grow its characteristic 'tail'.
Between the Oort cloud and the planets there lies the Kuiper belt, a belt of icy, frozen, lifeless lumps of ice which sometimes enter the inner Solar System. It encircles the Sun in a highly elliptical shape, at its closest being just beyond Neptune and at its furthest linking up with the Oort cloud.
The Solar System was born when a cloud of gas (made up of about 75% Hydrogen, 25% Helium and nearly 1% heavier elements) condensed enough to form solid objects, which slowly joined to make the Solar System as we know it today.