Page 4 - Curriculum Visions Dynamic Book

The planets in our solar system are not all the same. They can be divided into two kinds: those that are mostly made of rock, are fairly small, and have thin atmospheres, and those that are mainly made of gases and liquids, are very large, and have very thick atmospheres. This book is concerned with the second kind, the gas giant planets, also known as the Jovian planets.
What the gas giants have in common
The gas giant planets are Jupiter, Saturn, Uranus, and Neptune. Each of these planets has an orbit that
is roughly circular (they are only slightly flattened, or elliptical).
Like all the planets, the Jovian planets orbit the Sun in very regular ways, separated by enormous distances in which there is very little matter at all.
The planets all lie in the same plane—that is, if looked at from the edge of their orbits, the planets all appear to be more or less in a straight line centreed on the Sun. The planets also all revolve in the same direction as the Sun. It is as though they were all formed from the same flat disk of dustlike material, or nebula. As we shall see, astronomers think this happened over four billion years ago.
The outer planets are separated by much vaster distances than the inner planets. Except for Pluto, which is a rocky planet, each of these bodies is composed mainly of gas or ice.
Although the composition of the Jovian planets is not the same as the average for the universe, they are much closer to one another in their makeup than the rocky planets are—and the largest ones, Jupiter and Saturn, are closest of all. So it seems that these planets condensed out of an early gas cloud.
Neptune
(see pages 52–57)
Uranus
(see pages 48–51)
 The gas giant planets: (left to right) Neptune, Uranus, Saturn, and Jupiter, shown in proportion but not at their correct distances apart.
To explore the inner planets and Pluto, see Volume 4: Rocky planets.
 The swirling gases of Jupiter.
The Great Red Spot (top) and the white ovals are long-lasting features of the planet.
Why such features should survive in a constantly churning ball of gas remains a mystery. The Great Red Spot is three times as large as the Earth.
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