The formation of the sun and planets

Abstract

The evidence concerning the presence of the products of extinct radioactivities in meteorites and in the atmosphere is reviewed and analyzed. Such radioactivities are assumed to be present in the interstellar medium when condensation to form the solar system begins. Determinations of the anomalous abundances of their product isotopes give a measure of the time interval between the start of the condensation and the cessation of chemical fractionation in the system in which they are found. The radioactive content of the interstellar medium depends on the history of galactic nucleosynthesis. Some mechanisms of nucleosynthesis are reviewed, and it is shown that the elements can be divided into products of primary and secondary processes. The extinct radioactivities are secondary products. A model of the stellar activity throughout galactic history is devised, and its parameters are determined by the abundances of the uranium and thorium isotopes. In this model stellar activity decreases exponentially with time, but there is a localized increase immediately prior to formation of the solar system. The following chronology is then deduced for events associated with the early history of the solar system: If the main heat source required to melt iron in meteorite parent bodies is Al²⁶ (there is some doubt about this), the time interval between the start of condensation and the formation and thermal insulation of the meteorite bodies is less than 6 × 10⁶ years. From the anomalous silver composition in iron meteorites it is deduced that the time interval to the solidification of iron in meteorite parent bodies is 2 to 4 × 10⁷ years. From the anomalous xenon composition of meteorites it is concluded that the time interval to the cessation of xenon diffusion in the meteorite parent body is about 1.5 × 10⁸ years. From the anomalous xenon composition of the atmosphere it is deduced that no xenon was retained in the atmosphere until about 10⁸ years after xenon diffusion ceased in the meteorite parent bodies, and that about 30% of the atmospheric xenon was once contained in the sun. Some physical processes probably associated with the formation of the solar system are discussed. It appears that an interstellar cloud must be fairly dense (∼10³ hydrogen atoms/cm³) and fairly massive (∼10³ M⊙) before it can undergo gravitational collapse. It appears likely that it will attain instability through the action of an unusually large external pressure, possibly by being surrounded by an HII region or bombarded by part of the interstellar medium accelerated by the passage of a supernova shell. The collapse and fragmentation of the cloud requires of the order of 10⁶ years. The subsequent evolution of the protostars is very rapid. When the central temperature becomes 1800 °K, they have probably become rotationally unstable at the equator and are on the verge of a dynamical instability associated with dissociation of hydrogen molecules and ionization of hydrogen and helium. In the collapse most or all of their mass goes into the formation of a nebular disk. The evolution and dissipation of this disk probably takes only a few million years at most. The time scale for this history appears to be consistent with the chronology deduced from the evidence of the extinct radioactivities.