  A commonly noted feature of the population of multi-planet extrasolar systems
is the rarity of planet pairs in low-order mean-motion resonances. We revisit
the physics of resonance capture via convergent disk-driven migration. We point
out that for planet spacings typical of stable configurations for Kepler
systems, the planets can routinely maintain a small but nonzero eccentricity
due to gravitational perturbations from their neighbors. Together with the
upper limit on the migration rate needed for capture, the finite eccentricity
can make resonance capture difficult or impossible in Sun-like systems for
planets smaller than ~Neptune-sized. This mass limit on efficient capture is
broadly consistent with observed exoplanet pairs that have mass determinations:
of pairs with the heavier planet exterior to the lighter planet -- which would
have been undergoing convergent migration in their disks -- those in or nearly
in resonance are much more likely to have total mass greater than two Neptune
masses than to have smaller masses. The agreement suggests that the observed
paucity of resonant pairs around sun-like stars may simply arise from a small
resonance capture probability for lower-mass planets. Planet pairs that thereby
avoid resonance capture are much less likely to collide in an eventual close
approach than to simply migrate past one another to become a divergently
migrating pair with the lighter planet exterior. For systems around M stars we
expect resonant pairs to be much more common, since there the minimum mass
threshhold for efficient capture is about an Earth mass.
