7-1 The solar system has two broad categories of planets: Earthlike and Jupiterlike

Each of the planets that orbit the Sun is unique. Only Earth has liquid water and an atmosphere that humans can breathe; only Venus has a perpetual cloud layer made of sulfuric acid droplets; and only Jupiter has immense storm systems that persist for centuries. But there are also striking similarities among planets. Volcanoes are found not only on Earth but also on Venus and Mars; rings encircle Jupiter, Saturn, Uranus, and Neptune; and impact 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 first step is to organize our knowledge of the planets in a systematic way. We can organize this information in two ways. First, we can contrast the orbits of different planets around the Sun; and second, we can compare the planets’ physical properties such as size, mass, average density, and chemical composition.

Comparing the Planets: Orbits

A planet falls naturally into one of two categories according to the size of its orbit. As Figure 7-1 shows, the orbits of the four inner planets (Mercury, Venus, Earth, and Mars) are crowded in close to the Sun. In contrast, the orbits of the next four planets (Jupiter, Saturn, Uranus, and Neptune) are widely spaced at great distances from the Sun. Table 7-1 lists the orbital characteristics of these eight planets.

Figure 7-1: 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.

TABLE 7-1 Characteristics of the Planets

Most of the planets have orbits that are nearly circular. As we learned in Section 4-4, Kepler discovered in the seventeenth century that these orbits are actually ellipses. Astronomers denote the elongation of an ellipse by its eccentricity (see Figure 4-10b). The eccentricity of a circle is zero, and indeed most of the eight planets (with the notable exception of Mercury) have orbital eccentricities that are very close to zero.

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, the orbits of the eight planets all lie in nearly the same plane. 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 (see Section 2-5). What’s more, the plane of the Sun’s equator is very closely aligned with the orbital planes of the planets. As we will see in Chapter 8, these near-alignments are not a coincidence. They provide important clues about the origin of the solar system.

Not included in Figure 7-1 or Table 7-1 is Pluto, which has an orbit that reaches beyond Neptune. Until the late 1990s, Pluto was generally regarded as the ninth planet. But in light of recent discoveries most astronomers now consider Pluto to be simply one member of a large collection of trans-Neptunian objects that orbit far from the Sun. Pluto is not even the largest of this new class of objects! Trans-Neptunian objects orbit the Sun in the same counterclockwise direction as the eight planets, though many of them have orbits that are steeply inclined to the plane of the ecliptic and have high eccentricities (that is, the orbits are quite elongated and noncircular). We will discuss trans-Neptunian objects, along with other small bodies that orbit the Sun, in Section 7-5.

Comparing the Planets: Physical Properties

When we compare the physical properties of the planets, we again find that they fall naturally into two classes—four small inner planets and four large outer ones. The four small inner planets are called terrestrial planets because they resemble Earth (in Latin, terra). They all have hard, rocky surfaces with mountains, craters, valleys, and volcanoes. 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 large outer planets are called Jovian planets because they resemble Jupiter. (Jove was another name for the Roman god Jupiter.) 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 7-2 show the distinctive appearances of the two classes of planets.

Figure 7-2: R I V U X G
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 the Jovian planets are the next four planets from the Sun.

(Calvin J. Hamilton and NASA/JPL)

The most apparent difference between the terrestrial and Jovian planets is their diameters. Earth, with its diameter of about 12,756 km (7926 mi), is the largest of the four inner, terrestrial planets. In sharp contrast, the four outer, Jovian planets are much larger than the terrestrial planets. First place goes to Jupiter, whose equatorial diameter 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 7-2 shows the Sun and the planets drawn to the same scale. The diameters of the planets are given in Table 7-1.

The masses of the terrestrial and Jovian planets are also dramatically different. If a planet has a moon, you can calculate the planet’s mass from the moon’s period and semimajor axis by using Newton’s form of Kepler’s third law (see Section 4-7 and Box 4-4). Astronomers have also measured the mass of each planet by sending a spacecraft to pass near the planet. The planet’s gravitational pull (which is proportional to its mass) 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 range from tens to 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.

Once we know the diameter and mass of a planet, we can learn something about what that planet is made of. 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. Box 7-1 describes some applications of the idea of average density to everyday phenomena on Earth.

The four inner, terrestrial planets have very high average densities (see Table 7-1); 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. 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 Jovian planets probably have large cores of mixed rock and highly compressed water that are 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 terrestrial planets are made of rocky materials and have dense iron cores. These planets have solid surfaces and high average densities. The Jovian planets are composed primarily of light elements such as hydrogen and helium, resulting in low average densities. These planets have interiors made mostly of gas and liquid, and have no solid surface.