Introduction to the Solar System
The solar system is the group of celestial bodies, including the Earth, orbiting around and gravitationally bound by the star known as the Sun, one of at least a hundred billion stars in our galaxy. The Sun's retinue includes nine planets, about 50 satell ites, more than 1,000 observed comets, and thousands of lesser bodies known as minor planets (asteroids) and meteoroids. All of these bodies are immersed in a tenuous sea of fragile and rocky interplanetary dust particles, perhaps ejected from comets at t he time of their passage through the inner solar system or resulting from minor planet collisions. The Sun is the only star known to be accompanied by such an extensive planetary system. A few nearby stars are now known to be encircled by swarms of partic les of undetermined size, however, and evidence indicates that a number of stars are accompanied by giant planetlike objects. Thus the possibility of a universe filled with many solar systems remains strong, though as yet unproved.
Since primitive times humanity has been aware that certain of the stars in the sky are not fixed but wander slowly across the heavens. The Greeks gave these moving stars the name planets, or "wanderers." They were the first to predict with accuracy the po sitions of the planets in the sky, and they devised elaborate theoretical models in which the planets moved around combinations of circles that in turn circled the Earth. The Greek mathematician Claudius Ptolemy systematized an elaborate geocentric scheme of this kind in the 2d century AD, which passed with minor changes through the Middle Ages and on to the Polish astronomer Nicolaus Copernicus. In his work of 1543, Copernicus proposed the idea that planetary motions are centered on the Sun rather than o n the Earth, but he retained the description of planetary motions as being a series of superimposed circular motions, mathematically equivalent to the Ptolemaic theory.
During the 17th century a German mathematician by the name of Johannes Kepler abandoned his forebears' concept of circular motion in favor of an elliptical scheme, in which the motions of the planets describe a simple series of ellipses in which the Sun i s at one of the foci. Basing his work on the observations of Tycho Brahe, his former employer and a renowned astronomer, Kepler found (1609, 1619) three important empirical relationships, concerning the motion of the planetary bodies, now known as Kepler' s Laws. Kepler's labors laid the groundwork for the law of Gravitation (1687) of Sir Isaac Newton, from which it became possible for astronomers to predict with great accuracy the movements and positions of the planets.
Only the planets Mercury, Venus, Mars, Jupiter, and Saturn were known to the ancients. The English astronomer William Herschel accidentally discovered Uranus in 1781 as the result of telescopic observations. Discrepancies between the observed positions of Uranus and those predicted led John Couch Adams and Urbain Jean Joseph Leverrier to propose (1846) that another large planet was exerting a gravitational force on Uranus. In the same year the planet Neptune was found close to its predicted position. In t he 20th century smaller apparent discrepancies in the position of Uranus led to predictions of the existence of yet another planet. In 1930, Clyde Tombaugh discovered Pluto close to one of the areas of prediction. Pluto's mass, however, is so small that t he discovery was accidental, resulting from intense scrutiny of that part of the sky to which predictions had called attention. It was theorized that a further planet may exist, although recent corrections in the calculated mass of Uranus leave this in do ubt.
Galileo was in 1609 the first to use the telescope for astronomical purposes, and it has since become an essential tool in planetary studies. In the 19th century planetary astronomy flourished, thanks to the construction of large telescopes and their syst ematic use for planetary observations. Two new tools, the spectroscope and the photographic plate, were also developed in the 19th century and gave rise to the new science of astrophysics. For the first time it became possible to determine not only the or bits and masses of objects in the solar system, but also their temperatures, compositions, and structures. During the early years of the 20th century great advancements took place in the understanding of the physics and chemistry of the planets in the sol ar system, and during the middle years of the century important further advances were derived from radio astronomy and radar astronomy.
Although most astronomers gradually turned their attention away from the solar system to the study of stars and galaxies, the launch (1957) of the first artificial satellite initiated an age that transformed solar-system studies. Beginning in the 1960s sp acecraft accomplished flyby, orbiting, or landing missions to many of the planets. At the present time the reconnaissance of the planets has been accomplished except for Pluto. The U.S. Mariner and Soviet Venera spacecraft have studied the atmosphere and surface of Venus. Mariners and U.S. Viking spacecraft have extensively photographed Mars from orbit, and Viking landers have carried out important initial measurements of surface properties. The investigation of the Moon progressed through the stages of f lybys, orbiters, and landers both manned (U.S. Apollo) and unmanned (U.S. Ranger, Surveyor, and Lunar Orbiter, and Soviet Luna). Lunar soil samples have also been returned for study from several different landing sites. U.S. Pioneer and Voyager probes hav e returned data and images from the outer planets and their satellites, except for Pluto, and in 1993 the Voyagers may have glimpsed the heliopause--the outer edge of the solar system, where the solar wind ebbs--about 82 to 130 times farther away from the Sun than the Earth is.
For more than 300 years there has been serious scientific discussion of the processes and events that led to the formation of the solar system. For most of this time lack of knowledge about the physical conditions in the solar system prevented a rigorous approach to the problem. Explanations were especially sought for the regularity in the directions of rotation and orbit of objects in the solar system, the slow rotation of the Sun, and the Titius-Bode law, which states that the radii of the planetary orb its increase in a regular fashion throughout the solar system. In a similar fashion, the radii of the orbits of the regular satellites of Jupiter, Saturn, and Uranus increase regularly. In modern times the slow rotation of the Sun has been explained as re sulting from the deceleration of its angular motion through its magnetic interaction with the solar wind. Thus this feature in itself should not have been considered a constraint on theories of the origin of the solar system.
The numerous theories concerning the origin of the solar system that have been advanced during the last three centuries can be classified as either dualistic or monistic. One common feature of dualistic theories is that another star once passed close to t he Sun, and tidal perturbations between the two stars drew out filaments of gas from which the planets condensed. Theories of this type encounter enormous difficulties in trying to account for modern information about the solar system, and they have gener ally been discarded. By contrast, monistic theories envisage a disk of gas and dust, called the primitive solar nebula, that formed around the Sun. Many of these theories speculate that the Sun and the planets formed together from the primeval solar nebul a. This type of theory has dominated thinking about the origin of the solar system since World War II. A photograph taken in 1984 of a nearby star, Beta Pictoris, appears to show a solar system forming in this way from a disk of surrounding material.
The large amount of activity that has taken place in the last 20 years in the renewed exploration of the solar system has also provided a great impetus for renewed studies of the origin of the system. One important component of this research has been the detailed studies of the properties of meteorites that has been made possible by modern laboratory instrumentation. The distribution and abundance of the elements within different meteoritic mineral phases has provided much information on the physical cond itions present at the time the solar system began to form. Recent discoveries of anomalies in the isotopic compositions of the elements in certain mineral phases in meteorites promise to give information about the local galactic interstellar environment t hat led to the formation of the solar system. Investigations of the properties of other planets has led to the new science of comparative planetology, in which the differences observed among the planets not only lead to a better understanding of the plane ts, but also pose precise new questions concerning the mechanisms by which the planets may have been formed.
Studies of the stars within our galaxy have shown that the age of our galaxy is much greater than the age of the solar system. Therefore, processes observed in the formation of stars within our galaxy today are likely to be found relevant to the formation of our solar system. Stars appear to form in groups or associations, as a result of the gravitational collapse of clouds of gas and dust in the interstellar medium. Modern monistic theories envisage the gas and dust in the primitive solar nebula to be th e collapsed remnant of such materials.
There has been much discussion of how the planets might have formed from the primeval solar nebula. In recent years attention has focused on the possibility that two types of gravitational instabilities might have played an important role in this process. One type is a gravitational instability in the gas of the primitive solar nebula, from which there would be formed giant gaseous protoplanets whose evolution could lead, in the outer solar system, to the giant planets observed today. In the inner solar s ystem, giant gaseous protoplanets could have formed rocky cores at their centers, which survived the stripping away of the gaseous envelopes caused by gravitational and thermal forces from the growing Sun.
The other form of gravitational instability involves the condensed materials in the solar nebula. Small dust particles that may have been present in the gas of the solar nebula could be expected to settle toward the midplane of the nebula if the gas were not subject to extensive turbulent churning. Gravitational instabilities acting on a thin dust layer might have formed bodies ranging from tens to hundreds of kilometers in radius. Collisions among these bodies may have played a major role in accumulation s of material to form the planets.