The Giant Outer Planets

The giant outer planets—Jupiter, Saturn, Uranus, and Neptune (Figure 1.1)—are by far the largest planetary bodies in the solar system and together comprise 99.56% of the planetary mass. Although very far from the Earth, the enormous physical size of Jupiter and Saturn meant that these planets were easily visible to the ancients. However, the other two "giants", Uranus and Neptune, are significantly smaller and so much farther from the Earth that they were unknown before the advent of telescopes, although Uranus is in fact just visible to the naked eye. Uranus was discovered by accident in 1781 by William Herschel (1738-1822) (later Sir William Herschel). Perturbations in the observed orbit of Uranus led John Couch Adams (1819-1892) and Urbain Jean Joseph Le Verrier (1811-1877) to independently predict the presence of a further planet, and Neptune was subsequently discovered close to its predicted position by Johann Gottfried Galle (1812-1910) in 1846. The mean observable properties of the outer planets are listed in Table 1.1.

All the giant planets are observed to rotate very rapidly and the shape of the planets distort under the centrifugal forces that arise. Hence, all the giant planets are noticeably oblate, especially Jupiter and Saturn, with the pole-to-pole diameter being significantly less than the equatorial diameter. Another key difference between the inner terrestrial planets, such as the Earth, and the giant planets is that the latter have surprisingly low densities, roughly equivalent to water and similar to that of the Sun, which has a density of 1.41 gcm~3. Hence, while we know that the Earth is a rocky body, the outer planets must be composed predominantly of much lighter materials. In fact the giant planets are now known not to have a solid rocky "surface" at all, but instead are gaseous in nature throughout.

Considering the mass, radius, and density of the giant planets they can be seen to divide naturally into two pairs: Jupiter and Saturn, composed primarily of hydrogen and helium and sometimes known as the "gas giants", and Uranus and Neptune,

Figure 1.1. The giant planets as observed by the Voyager spacecraft together with the Earth for comparison. Courtesy of NASA.

composed primarily of ices such as water and methane and sometimes known as the "ice giants". Jupiter and Saturn both have a radius in the range 60,000km to 72,000 km, while Uranus and Neptune are somewhat smaller with a radius of approximately 25,000 km. These have respectively ten times, and four times the radius of the Earth. By comparison, the radius of the Sun is ten times larger than

Table 1.1. Observed properties of the giant planets and Earth.

Property

Earth

Jupiter

Saturn

Uranus

Neptune

Solar distance (AU)a

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