Origin Of The Solar System 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 orbits 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 in a regular manner. In modern times the slow rotation of the Sun has been explained as resulting from the deceleration of its angular motion through its magnetic interaction with the outflowing solar wind, so that this feature should not have been considered a constraint on theories of the origin of the solar system.
The many 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. A common feature of dualistic theories is that another star once passed close to the 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 generally 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 nebula. 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 conditions 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 that 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 planets, 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 the collapsed remnant of a fragment of an interstellar cloud. 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 a giant gaseous protoplanet. From the evolution of such protoplanets there could arise, in the outer solar system, the giant planets that are observed today. In the inner solar system, the possibility exists that giant gaseous protoplanets 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 accumulations of material to form the planets. It must be stressed that all theories of the origin of the solar system currently being formulated respond to and are limited by the rapid accumulation of facts about planetary bodies within the solar system. Because of the rapid rate of progress in such studies, it is generally recognized that such theories are preliminary and simplified, so that ideas and theories in this area of research can be expected to continue to evolve rapidly.