How did the Sun and planets form? In other words, where did the solar system come from? This question has tantalized astronomers for centuries. Our goal in this chapter is to examine our current understanding of how the solar system came to be—that is, our current best theory of the origin of the solar system.
Recall from Section 1-1 that a theory is not merely a set of wild speculations, but a self-consistent collection of ideas that must pass the test of providing an accurate description of the real world. Since no humans were present to witness the formation of the planets, scientists must base their theories of solar system origins on their observations of the present-day solar system. (In an analogous way, paleontologists base their understanding of the lives of dinosaurs on the evidence provided by fossils that have survived to the present day.) In so doing, they are following the steps of the scientific method that we described in Section 1-1.
What key attributes of the solar system should guide us in building a theory of solar system origins? Among the many properties of the planets that we discussed in Chapter 7, three of the most important are listed in Table 8-1. Any theory that attempts to describe the origin of the solar system must be able to explain how these attributes came to be. We begin by considering what Property 1 tells us; we will return to Properties 2 and 3 and the orbits of the planets later in this chapter.
| Any theory of the origin of the solar system must be able to account for these properties of the planets. | |
| Property 1: Sizes and compositions of terrestrial planets versus Jovian planets | The terrestrial planets, which are composed primarily of rocky substances, are relatively small, while the Jovian planets, which are composed primarily of hydrogen and helium, are relatively large (see Section 7-1 and Section 7-4). |
| Property 2: Directions and orientations of planetary orbits | All of the planets orbit the Sun in the same direction, and all of their orbits are in nearly the same plane (see Section 7-1). |
| Property 3: Sizes of terrestrial planet orbits versus Jovian planet orbits | The terrestrial planets orbit close to the Sun, while the Jovian planets orbit far from the Sun orbits versus Jovian planet orbits (see Section 7-1). |
Is Earth an exception to any of the three key properties of our solar system in Table 8-1?