The inner planets and their satellites, described in the previous chapter, comprise less than half of one per cent of the planetary mass in the Solar System: more than 99.5 per cent of the planetary matter is found beyond the orbit of Mars. This region includes most of the asteroids and comets, the giant planets Jupiter, Saturn, Uranus and Neptune and their 30 or so satellites, plus the planet Pluto, a body more like a detached satellite than a companion planet to the giants. The giant planets differ markedly from the bodies of the inner Solar System. They have mean densities between 704 and 1600kgm~3, much less than the Earth’s value of 5518kgmr3. They are comparatively large, with radii between 3.7 and 11.2 times that of the Earth, which is itself the largest terrestrial planet. All four rotate rapidly, with periods- ranging from just under 10 up to 15 hours. This fast rotation produces a pronounced flattening, particularly of Jupiter and Saturn. Orbital elements for the outer planets have already been given .

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