Pertinent observational constraints on the formation of the solar system include

1. The solar abundance of elements has been determined spectroscopically down to relative to H.

2. Elemental abundances in meteorites are measured. Those of the C1 carbonaceous chondrites resemble solar material with volatile elements partially removed.

3. The terrestrial bulk abundances are inferred from crustal abundances and earth models. These abundances are roughly consistent with solar ratios for the major rock-forming minerals.

4. The planetary bulk densities show a monotonic decrease going outward in the solar system.

5. Crystallization ages from radioactive dating indicate contemporaneous formation years ago.

6. Abundances imply that radioactive nuclei now extinct, were present at the time of formation and decayed via the reactions

This suggests that a supernova occurred shortly before crystallization.

7. The ratios of stable isotopes ate generally the same throughout the solar system, implying it was initially well-mixed.

8. High temperature assemblages of the elements Ca, Al, and Ti in meteorite inclusions suggest that condensation occurred early from a solar composition gas.

The conventional model for solar system formation, called the solar nebula hypothesis, holds that the solar nebula cooled from T 2000 K with an initially uniform solar composition. The condensation was then quenched at a certain temperature (which is a function of distance from the sun), and the remaining gas was blown away by a high luminosity early sun. Most of the dense material then aggregated to form planets. A fraction of this material was scattered by gravitational encounters with other protoplanets, forming the comets and chondric meteorites. Some of the volatile material was swept up by planets and incorporated into planetary atmospheres. Some large, differentiated bodies were fragmented through collisions, producing stoney-iron and iron meteorites.

However, there are a number of problems with this scenario.

1. Dynamical: it is difficult to construct a model which would correctly quench the process, blow off the volatiles, aggregate the planets, and scatter the correct amount of cometary material.

2. Initial conditions: it is difficult to constrain T(r), P(r), the homogeneity of the nebula and timing of events.

3. Indeterminables: turbulence in the nebula, timing of the solar and nebular events, and close encounters between objects complicate the process.

4. Additional complications: isotopic anomalies in ¹⁶O, ¹⁷O, and ¹⁸O show that it does not follow mass-dependent fractionation. Furthermore, nuclear anomalies such as ²⁶Al imply a late, secondary injection.

The Safronov (1972) model considers systematic condensation of the refractory minerals close to the Sun, followed by condensation of more volatile minerals at farther orbital distances. At 1900 K, the metals W, Re, Ir, and Os condense. At 1600-1750 K, Al₂O₃, CaTiO₃, Ca₂Al₂Si₂O₇, and refractory metals such as Pt condense. At 1470 K, Te condenses. At the cooler temperatures, Fe, feldspars, forsterite, enstatite, and ferrosilite condense. At K, FeS and hydrous minerals condense (Anderson 1989, p. 15).

The cold accretion hypothesis holds that the Earth is made up entirely of cold carbonaceous chondrite material. The Homogeneous accretion hypothesis assumes that volatile-rich material came as a late veneer, causing planets to form in a series of layers, with the highest temperature condensates at the center (Turkerian and Clark 1969).

Milky Way Galaxy, Sun