| The Solar System consists of one star (the Sun), the nine principal planets, their satellites and lesser bodies such as asteroids, comets and meteoroids, plus a vast amount of thinly-spread interplanetary matter. The Sun contains more than 99% of the mass of the system, and Jupiter is more massive than all the other planets combined. The centre of gravity of the Solar System lies just outside the surface of the Sun, due mainly to the mass of Jupiter.
The Solar System is divided into two parts. There are four relatively small, rocky planets (Mercury, Venus, the Earth and Mars), beyond which come the asteroids, of which only one (Ceres) is over 900 km in diameter. Next come the four giants (Jupiter, Saturn, Uranus and Neptune), plus Pluto, which is smaller than our Moon and has an unusual orbit which brings it at times closer in than Neptune. Pluto may not be worthy of true planetary status, and may be only the largest member of the ‘Kuiper Belt’ swarm of asteroidalsized bodies moving in the far reaches of the Solar System. However, Pluto does seem to be in a class of its own, and in size is intermediate between the smallest principal planet (Mercury) and the largest asteroid (Ceres). Planetary data are given in Table 1.1.
It now seems that the distinctions between the various classes of bodies in the Solar System are less clear-cut than has been previously thought. For example, it is quite probable that some ‘near-Earth asteroids’, which swing away from the main swarm, are ex-comets which have lost their volatiles; and some of the smaller satellites of the giant planets are almost certainly ex-members of the asteroid belt which were captured long ago. |
