  An intriguing trend among Kepler's multi-planet systems is an overabundance
of planet pairs with period ratios just wide of mean motion resonances (MMR)
and a dearth of systems just narrow of them. In a recently published paper
Chatterjee & Ford (2015; henceforth CF15) has proposed 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. They found
that for massive enough disks planet-planetesimal disk interactions can break
resonances and naturally create moderate to large positive offsets from the
initial period ratio for large ranges of planetesimal disk and planet
properties. Divergence from resonance only happens if the mass of planetesimals
that interact with the planets is at least a few percent of the total planet
mass. This threshold, above which resonances are broken and the offset from
resonances can grow, naturally explains why the asymmetric large offsets were
not seen in more massive planet pairs found via past radial velocity surveys.
In this article we will highlight some of the key findings of CF15. In
addition, we report preliminary results from an extension of this study, that
investigates the effects of planet-planetesimal disk interactions on initially
non-resonant planet pairs. We find that planetesimal scattering typically
increases period ratios of non-resonant planets. If the initial period ratios
are below and in proximity of a resonance, under certain conditions, this
increment in period ratios can create a deficit of systems with period ratios
just below the exact integer corresponding to the MMR and an excess just above.
From an initially uniform distribution of period ratios just below a 2:1 MMR,
planetesimal interactions can create an asymmetric distribution across this MMR
similar to what is observed for the Kepler planet pairs.
