  We consider a dynamical shake-up model to explain the low mass of Mars and
the lack of planets in the asteroid belt. In our scenario, a secular resonance
with Jupiter sweeps through the inner solar system as the solar nebula
depletes, pitting resonant excitation against collisional damping in the Sun's
protoplanetary disk. We report the outcome of extensive numerical calculations
of planet formation from planetesimals in the terrestrial zone, with and
without dynamical shake-up. If the Sun's gas disk within the terrestrial zone
depletes in roughly a million years, then the sweeping resonance inhibits
planet formation in the asteroid belt and substantially limits the size of
Mars. This phenomenon likely occurs around other stars with long-period massive
planets, suggesting that asteroid belt analogs are common.
