4-1 The solar system has two broad categories of planets orbiting the Sun: terrestrial (Earthlike) and Jovian (Jupiterlike)

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What do you think of when someone says “solar system”? Do you think of planets, like Earth and Saturn, or of millions of stars and enormous glowing clouds of interstellar gas? When astronomers speak of our solar system, they are specifically talking about the system of planets and other debris that orbit around just a single star, our Sun. Other stars in the night sky also might have planets orbiting them, but those stars are far, far beyond our solar system.

One should pause for a moment and consider, “What is a planet?” You probably have heard that there has been considerable debate about whether or not the object called Pluto is a planet. We certainly won’t resolve that debate here, but for now, let’s consider a planet being an object that has at least two important characteristics. The first is that it is an object that orbits a star. The second is that it is large enough that its own gravity has shaped the object into a sphere. These are defining characteristics nearly all astronomers agree upon, but you can imagine that there are some other characteristics a planet might have that are less agreed upon.

By international agreement, our solar system is defined as currently having eight orbiting planets, with Pluto no longer being officially designated as a planet. Each of the planets orbiting our Sun is unique. Only Earth has liquid water spread widely across its surface and, at the same time, a nitrogen- and oxygen-dominated atmosphere breathable by humans. Only Venus has a perpetual cloud layer made of sulfuric acid droplets. Only Jupiter has immense storm systems that visibly persist for centuries. But there are also striking similarities among the planets. Volcanoes are found not only on Earth but also on Venus, Mars, and several moons orbiting our giant planets; rings encircle Jupiter, Saturn, Uranus, and Neptune; and craters dot the surfaces of Mercury, Venus, Earth, and Mars, showing that all of these planets have been bombarded by interplanetary debris.

How can we make sense of the many similarities and differences among the planets? An important step is to organize our knowledge of the planets in a systematic way. First, we can compare the orbits of different planets around the Sun. Second, we can compare the physical properties of the planets.

Comparing the Planets: Orbits

The planets fall naturally into two classes according to the sizes of their orbits. As Figure 4-1 shows, the orbits of Mercury, Venus, Earth, and Mars are crowded in quite close to the Sun, and these planets are known as the four inner planets. In contrast, the orbits of the next four planets, Jupiter, Saturn, Uranus, and Neptune, known as the outer planets, are widely spaced from each other and orbit at great distances from the Sun. Table 4-1 lists the orbital characteristics of these eight planets. If you are looking for Pluto, we haven’t forgotten it, but we will need to describe that object later because it has characteristics that make it fundamentally different than these first eight planets.

Figure 4-1: The Solar System to Scale This scale drawing shows the orbits of the planets around the Sun. The four inner planets are crowded in close to the Sun, while the four outer planets orbit the Sun at much greater distances. On the scale of this drawing, the planets themselves would be much smaller than the diameter of a human hair and too small to see.

Most of the planets have orbits that are nearly circular. As we learned in Chapter 3, Johannes Kepler discovered in the seventeenth century that these orbits are actually ellipses. Astronomers denote the elongation of an ellipse by its eccentricity (see Figure 3-14). The eccentricity of a circle is zero, and indeed most of our planets—with the notable exception of Mercury—have orbital eccentricities that appear to be nearly indistinguishable from a perfect circle.

If you could observe the solar system from a point several astronomical units (AU) above Earth’s north pole, you would see that all the planets orbit the Sun in the same counterclockwise direction. Furthermore, their orbital paths all lie in nearly the same plane (Figure 4-1). In other words, these orbits are inclined at only slight angles to the plane of the ecliptic, which is the plane of Earth’s orbit around the Sun. What is more, the plane of the Sun’s equator is very closely aligned with the orbital planes of the planets. As we will see later in this chapter, these near-alignments are not a coincidence. They provide important clues about the origin of the solar system.

Comparing the Planets: Physical Properties

When we compare the physical properties of the planets, we again find that they fall naturally into two classes. All four inner planets have hard, rocky surfaces with mountains, craters, valleys, and volcanoes. These planets are also known as terrestrial, or Earthlike, planets. You could stand on the surface of any one of them, although you would need a protective spacesuit on Mercury, Venus, or Mars. The four outer planets resemble Jupiter and are often referred to as Jovian, or Jupiterlike, planets (Jove is the mythological name for king of the Roman gods). An attempt to land a spacecraft on the surface of any of the Jovian planets would be futile, because the materials of which these planets are made are mostly gaseous or liquid. The visible “surface” features of a Jovian planet are actually cloud formations in the planet’s atmosphere. The photographs in Figure 4-2 show the distinctive appearances of the two classes of planets.

Figure 4-2: RIVUXG The Planets to Scale This figure shows the planets from Mercury to Neptune to the same scale. The four terrestrial planets have orbits nearest the Sun, and, orbiting much farther away, the Jovian planets are the next four planets from the Sun.

The most apparent differences between the inner and outer planets are their diameters. The diameter of a planet is the distance from one side of the planet to the other. The four Jovian planets are much larger, with much greater diameters, than the terrestrial planets. First place goes to Jupiter, whose diameter across its equator is more than 11 times that of Earth. On the other end of the scale, Mercury’s diameter is less than two-fifths that of Earth. Figure 4-2 shows the Sun and the planets drawn to the same scale. The diameters of the planets are given in Table 4-1.

The masses of the two categories of planets are also dramatically different. As we saw in Chapter 3, a planet’s gravitational attraction to other objects is proportional to its mass. Gravitational attraction between a moon and a planet is greater for the most massive of planets. So, if a planet has a moon like our Moon, astronomers can determine the planet’s mass from measurements of how long it takes a moon to orbit the planet for a given distance. In a similar way, astronomers have also measured the mass of each planet by sending a spacecraft to pass near the planet. The planet’s gravitational pull deflects the spacecraft’s path, and the amount of deflection tells us the planet’s mass. Using these techniques, astronomers have found that the four Jovian planets have masses that are tens or hundreds of times greater than the mass of any of the terrestrial planets. Again, first place goes to Jupiter, whose mass is 318 times greater than Earth’s mass.

Once we know the diameter and mass of a planet, we can learn something about what that planet is made of on the inside. The trick is to calculate the planet’s average density, or mass divided by volume, measured in kilograms per cubic meter (kg/m³). The average density of any substance depends in part on that substance’s composition. For example, air near sea level on Earth has an average density of 1.2 kg/m³, water’s average density is 1000 kg/m³, and a piece of concrete has an average density of 2000 kg/m³.

The four inner, terrestrial planets have very high average densities (see Table 4-1); the average density of Earth, for example, is 5515 kg/m³. By contrast, a typical rock found on Earth’s surface has a lower average density, about 3000 kg/m³. Thus, Earth must contain a large amount of material that is denser than rock. The most common material in the solar system denser than Earth’s surface rocks is iron. Thus, for Earth to have a density higher than its surface rocks, its core must be dense. This information provides our first clue that terrestrial planets have dense iron cores.

In sharp contrast, the outer, Jovian planets have quite low densities. Saturn has an average density less than that of water. This information strongly suggests that the giant outer planets are composed primarily of light elements such as hydrogen and helium. All four Jupiterlike planets probably have large cores of mixed rock and highly compressed water buried beneath low-density outer layers tens of thousands of kilometers thick.

We can conclude that the following general rule applies to the planets:

The inner planets are made of rocky materials and have dense iron cores, which give these planets high average densities. The outer planets are composed primarily of light elements such as hydrogen and helium, which gives these planets low average densities.