The model is based on a disk of dust rapidly settling out within a low-mass nebula of gas. The solid particles accumulate by condensation and coalescence, forming objects, planetesimals, the size of which was about one kilometre. The later stages were dominated by gravitation: the accretion of more massive bodies, planetoids, occurred through multiple collisions and gravitational interactions. The young Sun then passed though a phase of intense activity during which the extremely intense solar wind stripped the protosolar disk of its gas and remaining dust.
In this section we summarize the principal characteristics of the standard model.
Modern observations show that stars of the solar type form collectively, through the propagation of an internal instability, deep within molecular clouds that are 2-5 pc in diameter (see Chap. 6). The collapse phase lasts between 105 and 106 years. A protostar forms at the centre of an accretion disk. A fraction of the material falling onto the star is ejected in jets that are observed following the protostar's axis of rotation (Figs. 4.6 and 5.9). The disk subsequently cools over a few hundred thousand years (Fig. 5.19), which causes material within the disk to condense, beginning with refractory elements.
This section describes the simple case of a dynamically static, cooling nebula. A more realistic case would be a dynamically evolving nebula (an accretion disk with radial mass transport). The sequence in which the elements in the protoplanetary
disk condensed is known through laboratory studies of meteorites. Close to the Sun, metals condensed first (between 4.56 and 4.55 thousand million years ago), then, at a greater heliocentric distance, silicate materials (between 4.55 and 4 thousand million years ago). Beyond a certain distance from the Sun, the temperature was sufficiently low to allow the condensation of ices, starting with that of water. The most abundant elements (O, C, N, etc.) were also present in solid form, which considerably increased the solid:gas ratio in the disk. We shall see that this limit, known as the 'ice line', does in fact represent the boundary between the region in which the terrestrial planets formed and that in which the giant planets arose. According to the standard model, this boundary lies at a distance of about 4-5 AU.
The condensation sequence (Fig. 2.8) may be calculated, assuming chemical equilibrium, from the measured abundances of elements in the Sun, based on a series of equations that incorporate factors governing all the condensation products that are capable of being formed from a given molecular gas. The calculations show that the condensation sequence revealed depends exceptionally closely on the state of oxidation of the initial gas. The results of the work of Grossman and Larimer (1974) are shown in Fig. 4.7, for a C:O value of 0.55, in accordance with the solar abundances of C and O, measured at that time (Prinn and Owen, 1976; however, it should be noted that these measurements are still the subject of debate).
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