By earthly standards, the solar system is immense. The distance from the Sun to Pluto is about 40 A.U., almost a million times the Earth's radius and roughly 15,000 times the distance from Earth to the Moon. Yet, despite the solar system's vast extent, the planets all lie very close to the Sun, astronomically speaking. Even the diameter of the largest orbit, that of Pluto, is less than 1/1000 of a light year, whereas the next nearest star is several light years distant.

The planet closest to the Sun is Mercury. Moving outward, we encounter in turn Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. In Chapter 2 we saw the basic properties of the planets' orbits. Their paths are all ellipses, with the Sun at (or very near) one focus. (Sec. 2.4)Most planetary orbits have low eccentricities. The exceptions are the innermost and the outermost worlds, Mercury and Pluto. Accordingly, we can reasonably think of most planets' orbits as circles centered on the Sun. The orbits of the major bodies in the solar system are illustrated in Figure 6.5.

Figure 6.5 Might future space voyagers travel far enough from Earth to gain this perspective on our solar system Except for Mercury and Pluto, the orbits of the planets lie nearly in the same plane. As we move out from the Sun, the distance between the orbits of the planets increases. The entire solar system spans nearly 80 A.U.

Astronomical Ruler

All the planets orbit the Sun in the same sense—counterclockwise as seen from above Earth's North Pole—and in nearly the same plane as Earth (that is, the ecliptic plane). Mercury and Pluto deviate somewhat from this rule. Their orbital planes lie at 7° and 17° to the ecliptic, respectively. Still, we can think of the solar system as being quite flat. Its "thickness" perpendicular to the plane of the ecliptic is less than 1/50 the diameter of Pluto's orbit. If we were to view the planets' orbits from a vantage point in the ecliptic plane about 50 A.U. from the Sun, only Pluto's orbit would be noticeably tilted. Figure 6.6 is a photograph of planets Mercury, Venus, Mars, and Jupiter taken during the July 1991 solar eclipse. These four planets happen to be visible in this one photograph in large part because their orbits lie nearly in the same plane.

Figure 6.6 Taken from Hawaii during the July 1991 eclipse of the Sun, this single photograph shows four planetsMercury, Venus, Mars, and Jupiter. Because they all orbit in nearly the same plane, it is possible for them all to appear (by chance) in the same region of the sky, as seen from Earth.

The planetary orbits are not evenly spaced: the orbits get farther and farther apart as we move farther out from the Sun. Nevertheless, there is a certain regularity in their spacing. In the eighteenth century a fairly simple rule, now known as the Titius-Bode law (Interlude 6-1), seemed to "predict" the radii of the planetary orbits remarkably well. Even the asteroid belt between Mars and Jupiter appeared to have a place in the scheme, which excited great interest among astronomers and numerologists alike. There is apparently no simple explanation for this empirical "law." Today, it is regarded more as a curiosity than as a fundamental property of the solar system, although many theorists still feel that it may hold some deep, but as yet undiscovered, insight into the way the solar system formed.