The Origin and Evolution of the Solar System

14 The structure of the Solar SystemFigure 1.7. A Hubble Space Telescope view of Pluto and its satellite Charon.since it was first discovered in 1930. Prior to its discovery it was postulated that aninth planet should exist, of mass Ũ, to explain the departures in the motionsof Uranus and Neptune from those predicted. By 1978 this estimate had beenlowered in several stages to ¼¼Å¨ but the discovery of a satellite of Pluto in1979 (figure 1.7) gave the current estimate of ¼¼¼¾½Å ¨. Since this is one-sixthof the mass of the Moon and gives a density less than one-half that of Mars it isobviously not similar to a terrestrial planet. It is reasonable to suppose that itsorigin might be ascribed to some process, or processes, different to that whichproduced the normal planets. Recent discoveries of trans-Neptunian objects (seesection 1.7.3) make it logical to consider Pluto as a member of such a group.1.4 Satellite systems, rings and planetary spins1.4.1 ClassificationMost of the planets are accompanied by smaller bodies, called satellites, in orbitsaround them. In the cases of the major planets these form regular systems similarto the planetary system itself. Several of the satellites are comparable in size to,or slightly larger than, the planet Mercury.The only satellite known from ancient times is the Moon which, being somassive in relation to its primary, must be classified as irregular. The first satellitesof another planet to be discovered were the four large Galilean satellitesorbiting Jupiter, so named because of their discovery by Galileo Galilei in 1610.With telescope developments over the next three and a half centuries many smallersatellites were discovered and a further major boost to the known satellite populationhas been provided by spacecraft observation.Many of the satellites of the major planets are relatively large and occupynear-circular orbits in the equatorial plane of the primary. These are termed regularsatellites and they are linked to the plausible assumption that they originate aspart of the process of planetary formation. Included in the irregular category ofsatellites there are some that are small but in regular orbits and one of the larger