On large scales, the solar system presents us with a sense of orderly motion. The planets move nearly in a plane, on almost concentric (and nearly circular) elliptical paths, in the same direction around the Sun, at steadily increasing orbital intervals. However, the individual properties of the planets themselves are much less regular.

Figure 6.7 compares the major planets with one another and with the Sun. A clear distinction can be drawn between the inner and the outer members of our planetary system based on densities and other physical properties. The inner planets—Mercury, Venus, Earth, and Mars—are small, dense, and rocky in composition. The outer worlds—Jupiter, Saturn, Uranus, and Neptune (but not Pluto)—are large, of low density, and gaseous.

Figure 6.7 Diagram, drawn to scale, of the relative sizes of the planets and our Sun. Notice how much larger the joviian planets are than Earth and the other terrestrials and how much larger still is the Sun.


Terrestrial Planets I
Jovian Planets


Because the physical and chemical properties of Mercury, Venus, and Mars are somewhat similar to Earth's, the four innermost planets are called the terrestrial planets. (The word terrestrial derives from the Latin word terra, meaning "land" or "earth.") The larger outer planets—Jupiter, Saturn, Uranus, and Neptune—are all similar to one another chemically and physically (and very different from the terrestrial worlds). They are labeled the jovian planets, after Jupiter, the largest member of the group. (The word jovian comes from Jove, another name for the Roman god Jupiter.) The jovian worlds are all much larger than the terrestrial planets, and quite different from them in both composition and structure.

The four terrestrial planets all lie within about 1.5 A.U. of the Sun. All are small and of relatively low mass, and all have generally rocky composition and solid surfaces. Beyond that, however, the similarities end. When we take into account how the weight of overlying layers compresses the interiors of the planets to different extents (greatest for Earth, least for Mercury), we find that the average uncompressed densities of the terrestrial worlds—that is, the densities they would have in the absence of any compression—decrease steadily as we move farther from the Sun. This decrease in density indicates that the overall compositions of these planets differ significantly one from the other.

There are many more differences among the terrestrial worlds. All have atmospheres, but the atmospheres are about as dissimilar as we could imagine, ranging from a near-vacuum on Mercury to a hot, dense inferno on Venus. Earth alone has oxygen in its atmosphere (as well as liquid water on its surface). The present-day conditions on the surfaces of the four planets are also quite distinct from one another. Earth and Mars spin at roughly the same rate—one rotation every 24 (Earth) hours—but Mercury and Venus both take months to rotate just once, and Venus rotates in the opposite sense from the others. Earth and Mars have moons, but Mercury and Venus do not. Earth and Mercury have measurable magnetic fields, of very different strengths, whereas Venus and Mars have none. Finding the common threads in the evolution of four such diverse worlds is no simple task! Comparative planetology will be our indispensable guide as we proceed through the coming chapters.


Yet for all their differences, the terrestrial worlds still seem very similar when compared with the jovian planets. Perhaps the simplest way to express the major differences between the terrestrial and jovian worlds is to say that the jovian planets are everything the terrestrial planets are not. Table 6.2 compares and contrasts some key properties of these two planetary classes.

 TABLE 6.2  Comparison of the Terrestrial and Jovian Planets
close to the Sun far from the Sun
closely spaced orbits widely spaced orbits
small masses large masses
small radii large radii
predominantly rocky predominantly gaseous
solid surface no solid surface
high density low density
slower rotation faster rotation
weak magnetic fields strong magnetic fields
few moons many moons
no rings many rings

The terrestrial worlds lie close together, near the Sun; the jovian worlds are widely spaced through the outer solar system. The terrestrial worlds are small, dense, and rocky; the jovian worlds are large and gaseous, being made up predominantly of hydrogen and helium (the lightest elements), which are rare on the inner planets. The terrestrial worlds have solid surfaces; the jovian worlds have none (their dense atmospheres thicken with depth, eventually merging with their liquid interiors). The terrestrial worlds have weak magnetic fields, if any; the jovian worlds all have strong magnetic fields. The terrestrial worlds have only three moons among them; the jovian worlds have many moons each, no two of them alike and none of them like our own. Furthermore, all the jovian planets have rings, a feature unknown on the terrestrial planets. Despite their greater size, the jovian worlds all rotate much faster than any terrestrial planet.

Finally, beyond the outermost jovian planet, Neptune, lies one more small world, frozen and mysterious. Pluto doesn't fit well into either planetary category. Indeed, there is debate among planetary scientists as to whether it should be classified as a planet at all. In both mass and composition, it has much more in common with the icy jovian moons than with any terrestrial or jovian planet. Astronomers speculate that it may in fact be the largest member of a newly recognized class of solar system objects that reside beyond the jovian worlds.