  An intriguing trend among \kepler's multi-planet systems is an overabundance
of planet pairs with period ratios just wide of a mean motion resonance (MMR)
and a dearth of systems just narrow of them. Traditional planet formation
models are at odds with these observations. They are also in contrast with the
period ratios of radial-velocity-discovered multi-planet systems which tend to
pile up at 2:1 MMR. We propose that gas-disk migration traps planets in a MMR.
After gas dispersal, orbits of these trapped planets are altered through
interaction with a residual planetesimal disk. We study the effects of
planetesimal disk interactions on planet pairs trapped in 2:1 MMR using planets
of mass typical of the Kepler planet candidates (KPC) and explore large ranges
for the mass, and density profile of the planetesimal disk. We find that
planet-planetesimal disk interactions naturally create the observed asymmetry
in period-ratio distribution for large ranges of planetesimal disk and planet
properties. If the planetesimal disk mass is above a threshold of ~0.2x the
planet mass, these interactions typically disrupt MMR. Afterwards, the planets
migrate in such a way that the final period-ratio is slightly higher than the
integer ratio corresponding to the initial MMR. Below this threshold these
interactions typically cannot disrupt the resonance and the period ratio stays
close to the integer ratio. The threshold explains why the more massive planet
pairs found by RV surveys are still in resonance. We encourage future research
to explore how significantly the associated accretion would change the planets'
atmospheric and surface properties.
