The Greeks and other astronomers of old were aware of the Moon, the stars, and five planets—Mercury, Venus, Mars, Jupiter, and Saturn—in the night sky. (Sec. 2.2) They also knew of two other types of heavenly objects that were clearly neither stars nor planets. Comets appear as long, wispy strands of light in the night sky that remain visible for periods of up to several weeks, then slowly fade from view. Meteors, or "shooting stars" are sudden bright streaks of light that flash across the sky, usually vanishing less than a second after they first appear. These transient phenomena must have been very familiar to ancient astronomers, but their role in the "big picture" of the solar system was not understood until much later.

Human knowledge of the basic content of the solar system remained largely unchanged from ancient times until the early seventeenth century, when the invention of the telescope made more detailed observations possible. Galileo Galilei was the first to capitalize on this new technology (his simple telescope is shown in Figure 6.1). Galileo's discovery of the phases of Venus and of four moons orbiting Jupiter early in the seventeenth century helped change forever humankind's vision of the universe. (Sec. 2.5) As technological advances continued, knowledge of the solar system improved rapidly. Astronomers began discovering objects invisible to the unaided human eye. By the end of the nineteenth century, astronomers had found Saturn's rings (1659), the planets Uranus (1758) and Neptune (1846), many planetary moons, and the first asteroids—"minor planets" orbiting the Sun, mostly in a broad band (called the asteroid belt) lying between Mars and Jupiter. Ceres, the largest asteroid and the first to be sighted, was discovered in 1801. A large telescope of mid-nineteenth-century vintage is shown in Figure 6.2.

Figure 6.1 The telescope with which Galileo made his first observations was simple, but its influence on astronomy was immeasurable.

Figure 6.2 By the mid-nineteenth century, telescopes had improved enormously in both size and quality. Shown here is the telescope built and used by Irish nobleman and amateur astronomer the Earl of Rosse.

The twentieth century brought continued improvements in optical telescopes. One more planet (Pluto) was discovered, along with three more planetary ring systems, dozens of moons, and thousands of asteroids. The century also saw the rise of both nonoptical astronomy—especially radio and infrared—and spacecraft exploration, each of which has made vitally important contributions to the field of planetary science. Astronauts have carried out experiments on the Moon (see Figure 6.3), and numerous unmanned probes have left Earth and traveled to all but one of the other planets. Figure 6.4 shows the Sojourner robot roaming the Martian surface in 1997.

Figure 6.3 An Apollo astronaut doing some lunar geology — prospecting near a huge boulder near the Mare Serenitatus.

Figure 6.4 The rover Sojourner, a robot the size of a suitcase on wheels, is seen here traversing a small dune on Mars near its mother ship, Pathfinder.

As currently explored, our solar system is known to contain 1 star (the Sun), 9 planets, 63 moons (at last count) orbiting those planets, 6 asteroids larger than 300 km in diameter, more than 7000 smaller (but well-studied) asteroids, myriad comets a few kilometers in diameter, and countless meteoroids less than 100 m across. The list will undoubtedly grow as we continue to explore our cosmic neighborhood.

As we proceed through the solar system in the next few chapters, we will seek to understand how each planet compares with our own and what each contributes to our knowledge of the solar system as a whole. We will use the powerful and still emerging perspective of comparative planetology—comparing and contrasting the properties of the diverse worlds we encounter—to understand better the conditions under which planets form and evolve. Our goal is to develop a comprehensive theory of the origin and evolution of our planetary system that explains all, or at least most, of its observed properties. We will try to answer basic questions such as: Why did planet X evolve in one way, while planet Y turned out completely differently? and Why are the planets' orbits so orderly, when their individual properties are not? In addressing these issues we will find many similarities and common features among planets, but each will also present us with new questions and unique insights. Whatever the answers, the comparison enriches our knowledge of the ways planets work. As we unravel the origin of the solar system, we also learn about the nature of planetary systems beyond our own.