These are the bodies that dominate the Solar System – provided you think it is size that matters, and are willing to overlook the Sun itself. The four giant planets are illustrated to scale in the lower half of Figure 3, showing how comprehensively their size overshadows the terrestrial planets. The view of Uranus is from the Hubble Space Telescope in orbit about the Earth, whereas the other giant planets are as seen by visiting spacecraft. Their domination by mass is not quite so overwhelming, because they are less dense than the terrestrial planets. Jupiter’s density is only24% of the Earth’s, and Saturn is even less dense and would fl oat if dropped into a suffi ciently large (and purely hypothetical)bucket of water. All of them have rings in their equatorial plane,though only those of Saturn and Uranus are suffi ciently prominent to be visible in Figure 3. Although the rings look solid, they are made of myriads of orbiting particles and are extremely insubstantial. They are discussed, along with the giant planets’satellites, in the next chapter.
By convention, the size of a giant planet is measured from the top of its clouds. These occur in the planet’s troposphere, above which are largely transparent and progressively less dense layers classifi able in the same way as for the Earth’s atmosphere. The base of a giant planet’s troposphere is hard to defi ne and has never been explored even in the case of Jupiter, where in 1995 an entry probe released by the Galileo spacecraft reached a depth of 160kilometres below the cloud-tops before pressure (22 atmospheres)and temperature (153 °C) put paid to it. Probably, the troposphere of each giant planet merges seamlessly into a fl uid interior at temperatures and pressures so high that there is no distinction between gas and liquid. Certainly, there is no solid surface that a human could ever stand upon.
Basic data for the giant planets are given in Table 5 . The polar diameters quoted there are less than equatorial diameters,because rapid rate of rotation (see Table 2) fl attens their shapes.Jupiter’s polar diameter is 6.5% less, and Saturn’s 10% less, than its polar diameter. The difference is only about 2% for the less gassy and more slowly rotating Uranus and Neptune (and is less than 1% for each terrestrial planet).