ets formed through direct accretion after the dissipation of the primordial nebula (Lissauer, 1993; Kokubo and Ida, A model for Epstein drag forces in the Solar System's primordial dust cloud is proposed; this model is consistent with the 1996; Namouni et al., 1996). As a result, the orbits of these asteroid belt forming in a cold nebula. Small bodies are assumed planets were formed in the absence of a significant drag to move in exact Keplerian orbits in the absence of drag; the force. There is, however, strong evidence that the formaorbital average of the collisional effects leads to circularization tion of the outer planets took place at an earlier stage, of orbits, for any density profile of the nebula. An increase in when a substantial part of the gas and dust in the solar density near Jupiter's orbit is assumed. In the presence of such nebula was still present. If this is the case, the effect of drag a density gradient but in the absence of a massive perturber, forces is probably quite significant for the understanding of orbital radii will increase or decrease according to whether the the present orbits of the outer planets, as well as for the relative outward gradient is larger or smaller than a critical present distribution of mass in the asteroid belt. The occurvalue. Given both a density gradient and a massive Jupiter, analysis of the planar restricted three-body problem so modified rence of orbital near-resonances between the outer planets leads to a condition on the relative density gradient for the and the strong correlation or anticorrelation between the (initially small) eccentricity of an asteroid near a given resodistribution of the asteroids and resonances with Jupiter nance with the perturber to increase or decrease. These analytisupport a gas-rich scenario for the formation of the outer cal results on the evolution of asteroid eccentricities in the Solar System (Beauge Β΄and Ferraz-Mello, 1993).
combined presence of mean-motion resonances and solar-neb-These arguments have motivated extensive research on ula drag effects are found to be in good agreement with those the effect of drag on the orbits of planetesimals, in the obtained by a seminumerical averaging scheme. Based on these presence of the gravitational perturbation by a massive analytical and seminumerical results, we formulate a self-conplanet, namely Jupiter's embryo. Weidenschilling and sistent hypothesis for the general pattern of gaps and groups Davis (1985), followed by Patterson (1987) and Beauge Γn the asteroid belt.