SCALE OF THE SOLAR SYSTEM:
PLANETARY ORBITS
The primary, standard, measuring scale of the solar system is based on the earth's average distance from the sun, which is known as the astronomical unit (1 astronomical unit is about 150 million kilometers).' In deriving the scale of the solar system, astronomers have employed several independent techniques, of which the most accurate is that of timing the round trip of a pulsed radio signal reflected from a planet. Combining this information with the planets' distances in astronomical units, from Kepler's third law, leads to the absolute size of the solar system in kilometers.
A German astronomer, Johann Bode (1747-1826), called attention in 1772 to a numerical scheme, or rule (originally discovered by Johann Titius (1729-1796) in 1766), that seemed to predict the mean distances of the then-known planets from the sun. Although not a physical law in the same sense as Newton's laws, it is known as Bode's law; both Uranus, discovered in 1781, and the first asteroid, Ceres, found in 1801, adhered fairly well to this rule, but it broke down later when Neptune and Pluto were discovered. Despite the rule's having no unique physical basis, a similar rule relating the separations between planets seems to be characteristic of the formation of bodies in the gravitational field of a star.
All the planets are much alike in orbital characteristics. They revolve around the sun in the same direction in roughly circular orbits that lie nearly in the same plane. Mercury, the innermost planet, and Pluto, the outermost planet, depart most from this regularity. Between the terrestrial planets (Mercury, Venus, the earth, and Mars) the average spacing is much smaller than that separating the Jovian planets, Jupiter, Saturn, Uranus, and Neptune. The planets orbit at mean distances ranging from 40 percent of the earth's distance from the sun to 40 times earth's distance, with orbital periods between a quarter of a year and 248 years.
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.
THE SATElLITES
Of the 49 or so satellites all but four belong to the Jovian planets. It seems likely that more will be discovered in the future; and in fact many more may eventually be found since Jupiter and Saturn could gravitationally bind a lot of small bodies. Very small and faint satellites of the Jovian planets cou Id easily escape detection by our present technology. Two of Jupiter's satellites, Ganymede and Callisto, and one of Saturn's satellites, Titan, are as large as, or larger than, Mercu ry.
Those satellites that are reasonably near their parent planet move in nearly circular orbits in the plane of their planet's equator and in the same direction as their planet rotates. The outer satellites usually have more eccentric orbits, which are more highly inclined to the equatorial plane of their planet. The four outer satellites of Jupiter, the most distant satellite of Saturn, and the inner satellite of Neptune have orbits that are reversed from the direction of their planet's rotation. It is possible that the reason for these differences is that the outer satellites were captured by the primaries after the planets and their inner satellite systems were formed.
RING SYSTEMS
One of the most exciting developments in planetary research in recent years has been the discovery of ring systems for Uranus and jupiter. (Saturn's rings had been discovered with the introduction of the telescope into astronomy.) Rings are actually individual, small solid bodies in orbit about a planet in its equatorial plane. They are thus very small satellites of the planet. Uranus's and jupiter's rings do not contain so many tiny satellites as do Saturn's; so they are much fainter than the ring system about Satu rn and have managed to escape detection until recently. Although no ring system has yet been found for Neptune, it is possible that it also has a faint set of rings similar to those around the other three Jovian planets.
MINOR MEMBERS
The asteroids, or minor planets, are rocky bodies whose diameter vary from a few between 150 and 1000 kilometers down to thousands less tha a kilomter across. Most asteroids are found between Mars and Jupiter, traveling around the sun in the same direction as the planets. However, many of them orbit the sun in the vicinity of the earth's orbit, with some in fairly elliptic orbits. In recent years the term "asteroid" has been expanded to include small objects, which are presumably not comets, located in the outer portion of the solar system. It is unlikely that their physical makeup is like that of those in the inner part of the solar system.
The meteoroids range in size from irregular solid bodies, called meteorites when they strike the ground, to tiny particles, called meteors if they merely flash through the atmosphere. As we go down the scale in size, the number of meteoroids increases very rapidly. They are composed of rocky material and are apparently related to asteroids and comets. All the meteoroids are satellites of the sun and are apparently moving in a wide variety of orbits, as best as we can determine.
Unlike the planetary bodies, most comets move around the sun in highly eccentric orbits with very long periods of revolution and at all angles of inclination. Some comets with short periods are regular visitors to the vicinity of the earth. The small masses of comets mean that it is possible for the larger planets, jupiter in particular, to alter their orbits. Astronomers believe a comet to be a "dirty iceball" that is a conglomerate of icy materials mixed with rocky matter, while most of the asteroids and meteoroids are composed of a rocky material. The cometary composition is apparently characteristic of many bodies in the outer solar system. We shall have more to say in later chapters about the relationships of asteroids, meteoroids, comets, satellites, and planets to each other.
The interplanetary medium is primarily gas particles-mostly protons and electrons-that are ejected from the sun's atmosphere at several hundred kilometers per second. These subatomic particles form the solar wind. Some dust is there too, most of it being cometary debris. Despite huge numbers of gas and dust particles, interplanetary space has fewer bits of matter and is a better vacuum than can be made in a terrestrial laboratory.
