The Solar System is made up of one star (the Sun), the eight planets with their satellites (Table 1.1) and various minor members such as asteroids, comets and meteoroids, plus a vast amount of thinly spread interplanetary matter. The Sun contains 99.86% of the total mass of the System, while Jupiter and Saturn account for 90% of what is left. Jupiter is the largest member of the planetary family, and is in fact more massive than all the other planets combined. Mainly because of Jupiter, the centre of gravity of the Solar System lies just outside the surface of the Sun.

The Solar System is divided into two parts. There are four comparatively small, rocky planets (Mercury, Venus, the Earth and Mars), beyond which comes the zone of the Main-Belt asteroids, of which only one (Ceres) is over 900 km in diameter. Next come the four giants (Jupiter, Saturn, Uranus and Neptune), plus a swarm of trans-Neptunian objects, of which the largest known are Eris and Pluto. For many years after its discovery, in 1930, Pluto was regarded as a true planet, but in August 2006 the International Astronomical Union, the controlling body of world astronomy, introduced a new scheme of classification, as follows:

A planet is any body in orbit round the Sun which is massive enough to assume a spherical shape, and has cleared its immediate neighbourhood of all smaller objects. All these criteria are met by the eight familiar planets, from Mercury to Neptune.