Overview of the Solar System

The Earth belongs to a group of nine planets, orbiting the Sun, called the Solar System. Each object follows its own orbit about the Sun. All of the planets orbit in the same direction. As large as the Earth seems to us, it is small compared to the distances between objects in the Solar System. This is true of the other planets, even those much larger than the Earth. For all practical purposes, the Solar System is vast emptiness, with a few small island oases.

If we could look at a side view of the Solar System, we would notice that the orbits are not very tilted with respect to that of the Earth. So, in a side view from the outside, the Solar System would look like a very thin disk. We call the plane of the Earth's orbit the ecliptic. The motion of the Earth around the Sun causes the Sun to appear to move against the background of fixed stars. That path is just the projection of the ecliptic onto the sky. The Earth's rotation axis is tilted (by 23.5°) so that the ecliptic does not line up with the Earth's equator.

We begin a brief tour of the Solar System by looking at the planets. A photograph of each planet is shown in Fig. 22.1. It is convenient to divide the planets into two groups, the inner planets and the outer planets. The inner four planets are Mercury, Venus, Earth and Mars. The giant outer planets are Jupiter, Saturn, Uranus and Neptune. The outer planets are much more massive than the inner planets. They also have very different compositions. The outermost planet is Pluto (though it does spend part of its orbit closer to the Sun than Neptune). Pluto is small, like the inner planets.

Most of the planets have moons orbiting them. Of course, the most familiar is our own Moon. Mercury and Venus are the only planets without any known moons. Mars has two small moons. Jupiter has four large moons and numerous smaller moons. Saturn has one large moon, and, like Jupiter, a collection of smaller moons. Uranus has three modest sized moons, and several smaller ones. Neptune has one large and a number of small moons, and, finally, Pluto has one moon that is relatively large compared to the size of the planet. All of these moons add to the diversity of objects that we can study in the Solar System.

In addition to planets and moons, there is a collection of smaller sized objects. Found mostly between the orbits of Mars and Jupiter is a collection of rocky bodies, called asteroids. They are mostly between the orbits of Mars and Jupiter. From time to time we see objects appear faintly in our sky that then brighten, and develop a tail, as shown in Fig. 22.2. These are comets. Occasionally the Earth runs into small debris left in its orbit. The material falls through the atmosphere and is heated. We see the glowing trail as a meteor. Occasionally the Earth suffers large impacts from these.

One of the goals in studying the Solar System is to find clues to its origin, and to put together a picture of that origin. We will defer that discussion until Chapter 27, after we have discussed all the material in the Solar System. For now, we note that we expect the Solar System to form as a biproduct of the formation of the Sun. The Solar System should have formed out of the disk that was part of the late stages of the formation of the

and disk, as discussed in Chapter 15. Differences among the planets should be explainable by differences in temperature, density and composition as one goes farther out in the protosolar nebula.

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