  Planet-planet scattering is the leading mechanism to explain the large
eccentricities of the observed exoplanet population. However, scattering has
not been considered important to the production of pairs of planets in mean
motion resonances (MMRs). We present results from a large number of numerical
simulations of dynamical instabilities in 3-planet systems. We show that MMRs
arise naturally in about five percent of cases. The most common resonances we
populate are the 2:1 and 3:1 MMRs, although a wide variety of MMRs can occur,
including high-order MMRs (up to eleventh order). MMRs are generated
preferentially in systems with uneven mass distributions: the smallest planet
is typically ejected after a series of close encounters, leaving the remaining,
more massive planets in resonance. The distribution of resonant planets is
consistent with the phase-space density of resonant orbits, meaning that
planets are randomly thrown into MMRs rather than being slowly pulled into
them. It may be possible to distinguish between MMRs created by scattering vs.
convergent migration in a gaseous disk by considering planetary mass ratios:
resonant pairs of planets beyond ~1 AU with more massive outer planets are
likely to have formed by scattering. In addition, scattering may be responsible
for pairs of planets in high-order MMRs (3:1 and higher) that are not easily
populated by migration. The frequency of MMRs from scattering is comparable to
the expected survival rate of MMRs in turbulent disks. Thus, planet-planet
scattering is likely to be a major contributor to the population of resonant
planets.
