The origin of the planets and their moons is a complex and as yet incompletely solved puzzle, although the basic outlines of the process are quite well understood. Most of our knowledge of the solar system's formative stages has emerged from studies of interstellar gas clouds, fallen meteorites, and Earth's Moon, as well as from the various planets observed with ground-based telescopes and planetary space probes. Ironically, studies of Earth itself do not help much because information about our planet's early stages eroded away long ago. Meteorites and comets provide perhaps the most useful information, for nearly all have preserved within them traces of solid and gaseous matter from the earliest times.

Despite the recent widely publicized discoveries of planets orbiting other stars (see Interlude 15-1), astronomers still have little detailed information on their properties, and so far there is no firm evidence for planets like Earth anywhere beyond our solar system. For that reason, our theories of planet formation still concentrate on the planetary system in which we live. Bear in mind, however, that no part of the scenario we will describe in the paragraphs that follow is in any way unique to our own system. The same basic processes could have occurred—and, many astronomers believe, probably did occur—during the formative stages of most of the stars in our galaxy. It is part of the job of the planetary scientist to distinguish between those properties of the solar system that are inherent—that is, that were imposed at formation—and those that must have evolved since the solar system formed. In this chapter we draw together all the planetary data we have amassed and show how the regularities—and the irregularities—of the solar system can be explained by a single comprehensive theory.

MODEL REQUIREMENTS

Any theory of the origin and architecture of our planetary system must adhere to the known facts. We know of nine outstanding properties of our solar system as a whole. They may be summarized as follows.

1. Each planet is relatively isolated in space. The planets exist as independent bodies at progressively larger distances from the central Sun; they are not bunched together. In very rough terms, each planet tends to be twice as far from the Sun as its next inward neighbor.
2. The orbits of the planets are nearly circular. In fact, with the exceptions of Mercury and Pluto, which we will argue are special cases, each planetary orbit closely describes a perfect circle.
3. The orbits of the planets all lie in nearly the same plane. The planes swept out by the planets' orbits are accurately aligned to within a few degrees. Again, Mercury and Pluto are slight exceptions.
4. The direction in which the planets orbit the Sun (counterclockwise as viewed from above Earth's North Pole) is the same as the direction in which the Sun's rotates on its axis. Virtually all the large-scale motions in the solar system (other than comet orbits) are in the same plane and in the same sense. The plane is that of the Sun's equator, and the sense is that of the Sun's rotation.
5. The direction in which most planets rotate on their axis is roughly the same as the direction in which the Sun rotates on its axis. This property is less general than the one just described for revolution, as three planets—Venus, Uranus, and Pluto—do not share it.
6. Most of the known moons revolve about their parent planets in the same direction that the planets rotate on their axes.
7. Our planetary system is highly differentiated. The inner terrestrial planets are characterized by high densities, moderate atmospheres, slow rotation rates, and few or no moons. By contrast, the jovian planets, farther from the Sun, have low densities, thick atmospheres, rapid rotation rates, and many moons.
8. The asteroids are very old and exhibit a range of properties not characteristic of either the inner or the outer planets or their moons. The asteroid belt shares, in rough terms, the bulk orbital properties of the planets. However, it appears to be made of primitive, unevolved material, and the meteorites that strike Earth are the oldest rocks known.
9. The comets are primitive, icy fragments that do not orbit in the ecliptic plane and reside primarily at large distances from the Sun.

All these observed facts, taken together, strongly suggest a high degree of order within our solar system. The whole system is not a random assortment of objects spinning or orbiting this way or that. Consequently, it hardly seems possible that our solar system could have formed by the slow accumulation of already-made interstellar "planets" casually captured by our Sun over the course of billions of years. The overall architecture of our solar system is too neat, and the ages of its members too uniform, to be the result of random chaotic events. The overall organization points toward a single formation, an ancient but one-time event, 4.6 billion years ago. A convincing theory that explains all the nine features just listed has been a goal of astronomers for many centuries.

ADDITIONAL CONSIDERATIONS

It is equally important to recognize what our theory of the solar system does not have to explain. There is plenty of scope for planets to evolve after their formation, so circumstances that have developed since the initial state of the solar system was established need not be included in our list. Examples are Mercury's 3:2 spin—orbit coupling, Venus's runaway greenhouse effect, the Moon's synchronous rotation, the emergence of life on Earth and its absence on Mars, the Kirkwood gaps in the asteroid belt, and the rings and atmospheric appearance of the jovian planets. There are many more. Indeed, all the properties of the planets for which we have already provided an evolutionary explanation need not be included as items that our theory must account for at the outset.

In addition to its many regularities, our solar system also has many notable irregularities, some of which we have already mentioned. Far from threatening our theory, however, these irregularities are important facts for us to consider in shaping our explanations. For example, it is necessary that the explanation for the solar system not insist that all planets rotate in the same sense or have only prograde moons, because that is not what we observe. Instead, the theory of the solar system should provide strong reasons for the observed planetary characteristics yet be flexible enough to allow for and explain the deviations, too. And, of course, the existence of the asteroids and comets that tell us so much about our past must be an integral part of the picture. That's quite a tall order, yet many researchers now believe that we are close to that level of understanding.