THE SUN
What is it about the sun that forces planets, asteroids, comets, and other bodies of the solar system to orbit around it? It is its immense gravitational reach since the sun contains 99.86 percent of the solar system's mass. (The natural satellites of the planets, on the other hand, are gravitationally bound to their parent planets because of their closeness to them.)
Like other stars, the sun is a gas from center to surface and generates its radiant energy deep within its hot interior. This giant gaseous sphere has a radius 109 times greater and a mass 333,000 times greater than that of the earth. The sun's family of planets thus intercepts only a minute amount of the radiation that streams from the sun, flooding the solar system.
In addition to the steady emission of radiant energy there are numerous transient phenomena occurring in the sun's outer layers, such as sunspots, plages, flares, prominences, and coronal holes. Associated with these is a flow of protons, electrons, other atomic particles, and magnetic fields out through the orbital planes of the planets. These so-called solar-wind par- ticles and magnetic fields impinge upon the planets and their magnetic fields, producing a variety of phenomena, such as the earth's aurora (northern and southern lights).
THE PLANETS
Outside the sun the nine planets contain the next important share of the mass of the solar system. Although the planets had a common origin, they have significant chemical, physical, and geological differences. Such diversity stems mostly from their different masses and distances from the sun. At the time of the planets' formation these factors determined the ability to retain matter and defined the chemical composition of that matter.
The matter composing the planets, their satellites, and the minor members of the solar system can be roughly divided into three broad classes on the basis of the ease with which it will vaporize. Those materials that are solids for temperatures less than about 2000 K are the rocky materials, such as iron, magnesium, and their oxides and silicates. The second class is the icy materials, which can remain solid only up to temperatures of a few hundred degrees Kelvin. Examples are the ices of water, carbon dioxide, ammonia, and methane. The remaining class consists of those molecules that are gases down to almost absolute zero, such as hydrogen and helium.
On the other hand, we know of sufficient chemical and physical similarities among the planets to enable us to divide them into two well-defined categories.
One category is an inner group composed mostly of rocky material, the terrestrial planets: Mercury, Venus, the earth, and Mars. Since the moon is not significantly smaller than Mercury, many astronomers include the moon among the terrestrial planets. Though like the moon in size, Pluto should have an icy composition more like that of comets. Thus it does not belong to the terrestrials, and it is not like the second group either.
The second group, the Jovian planets-Jupiter, Saturn, Uranus, and Neptune-are farther from the sun; these planets are larger and consist mainly of the lighter elements, primarily hydrogen and helium, the most abundant elements in the universe. Jupiter and Saturn apparently have the same chemical composition as the sun. Uranus and Neptune seem to have less hydrogen and helium and presumably more of the icy materials (frozen gases such as water, ammonia, methane, and carbon dioxide). Table 6.4 contains a summary of the specific physical properties of the planets.
