KT4

Over the past 15 years, our understanding of the processes that sculpted the inner Solar System has been in a state of flux. The 'classical model', -- which assumes that the rocky planets accreted in an orderly way from a continuous disk of rocky planetesimals -- systematically fails to match the small mass of Mars relative to Earth and the total mass and structure of the asteroid belt. New dynamical models have invoked different processes. In the Grand Tack model, Jupiter's large-scale migration clears out Mars' feeding zone. In the Early Instability model, the same zone is depleted by an early dynamical instability among the giant planets. In contrast, the Low-mass Asteroid belt model assumes that few planetesimals ever formed in the present-day asteroid belt or Mars region. Each model can reliably match the inner Solar System. Two new models invoke pebble accretion (rather than planetesimal accretion) as the dominant pathway for terrestrial planet growth. What remains to be understood is whether these models are consistent with all of the relevant physics of planet-forming disks and how they fit into the broader view of planet formation constrained by astronomical and cosmochemical measurements.