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 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. This type of theory has dominated thinking about the origin of the solar system since World War II. Photographs taken of nearby stars, such as Beta Pictoris, appear to show systems forming in this way from disks of surrounding materials.
The large amount of activity that has taken place in more recent 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. Discoveries of anomalies in the isotopic compositions of the elements in certain mineral phases in meteorites may provide 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 science of comparative planetology, in which the differences observed among the planets pose precise questions concerning the mechanisms by which they may have been formed.
Studies of the stars within our Galaxy have shown that the galaxy's age is much greater than the age of the solar system. Therefore, processes observed in the current formation of stars within our galaxy 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 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 system, 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 accumulations of material to form the planets. Computer-model studies conducted in the final years of the 20th century suggested that the arrangement of planets in the solar system as it now exists is dramatically different from the configuration of the system when it first formed.