With isotopic analyses of Earth and meteorites, we demonstrated that our planet accreted most of its mass (~95%) from local material, the rest being accreted from carbonaceous material formed in the outer Solar system. But, for a volatile element like Zn, Earth accreted 30% of its Zn budget from carbonaceous material, because the latter is much reacher in Zn than the local, volatile depleted material. Mars accreted basically no carbonaceous material, because no carbonaceous signature is detected in martian Zn. Together, these results prove that the terrestrial planets did not form by accreting dust drifting radially towards the Sun from the outer part of the protoplanetary disk (a process dubbed pebble accretion), because this model would predict a large fraction of carbonaceous material in all terrestrial planets.We have shown that pebble accretion was inefficient for the terrestrial planets because dust particles were small and the flux of dust from the outer part of the disk was blocked by the presence of Jupiter
We have studied the dynamics of dust in the early phases of the protoplanetary disk, characterized by a viscous radial expansion as the disk was accreting material from the protosolar cloud. We have shown that radial expansion of the disk favors the accumulation of dust at the snowline (where water condenses) and at the silicate line (where silicates condense). Dust accumulation at these two locations favors the formation of planetesimals. The ring of icy planetesimals is favorable to start forming the core of Jupiter, while the ring of silicate planetesimals is potentially the birthplace of terrestrial planets.
Thus, we have modeled in detail the process of terrestrial planet formation, starting from a ring of 100km planetesimals placed at 1 AU. We have shown that this ring generates rapidly a system of planetary embryos of masses comparable to the mass of Mars. This system tends to spread and migrate, unless the surface density distribution of the gas of the protoplanetary disk also peaks at about 1 AU. In this case, the system of planetary embryos remains confined and embryos tend to coagulate at the center of the ring, forming protoplanets of 3 - 4 Mars masses by the time the gas is removed from the system. The subsequent evolution of the system leads to the final formation of Venus and Earth, i.e. forming a planetary system that reproduces the observed mass distribution of the real planets (with small Mercury and Mars at the extremes and large Venus and Earth at the center). The timing of the giant planet instability has a weak impact on terrestrial planet formation. The simulations show a large range of possible formation timescales for Earth, ranging from ~20 to 150 My. But a formation timescale of ~ 60 My is preferred because it allows explaining the mass of material accreted by our planet after the Moon-forming event (called the Late Veneer) and the dynamical excitation of the terrestrial planet system (quantified by the so-called " angular momentum deficit" ).
