t(104 years)

Fig. 4.9 A simulation of the formation of Jupiter through the accretion of solid material (continuous line), and gases (dotted line) (After J.B. Pollack et al., 62, 1996)

tion of ices. (Dynamical models indicate that Jupiter's orbital migration must have remained very limited.) Saturn also benefited, to a lesser extent, from the increased availability of material, as well as the other two giant planets, Uranus and Neptune. All thus acquired a core, primarily consisting of ices, with a sufficient mass (some ten Earth masses) to induce gravitational accretion of the surrounding protosolar gas. According to the model by Pollack et al. (1996), the accretion processes for the solid core and for the gas were not entirely decoupled. The formation of the giant planets took place in three phases: (1) accretion of the solid core; (2) simultaneous accretion of gases and solid material; (3) a surge in the accretion of gas (Fig. 4.9).

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