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We assess the multi-planet systems discovered by the Kepler satellite interms of current ideas about orbital migration and eccentricity damping due toplanet-disk interactions. Our primary focus is on mean motion resonances. Onlya few percent of planet pairs are in close proximity to a resonance. However,predicted migration rates (parameterized by $\tau_n=n/{|\dot n|}$) imply thatduring convergent migration most planets would have been captured into firstorder resonances. Eccentricity damping (parameterized by $\tau_e=e/{|\dot e|}$)offers a plausible resolution. Estimates suggest $\tau_e/\tau_n\sim (h/a)^2\sim10^{-2}$, where $h/a$ is the ratio of disk thickness to radius. Together,eccentricity damping and orbital migration give rise to an equilibriumeccentricity, $e_{eq}\sim(\tau_e/\tau_n)^{1/2}$. Capture is permanent provided$e_{eq}\lesssim \mu^{1/3}$, where $\mu$ denotes the planet to star mass ratio.But for $e_{eq}\gtrsim \mu^{1/3}$, capture is only temporary because librationsaround equilibrium are overstable and lead to passage through resonance ontimescale $\tau_e$. Most Kepler planet pairs have $e_{eq}>\mu^{1/3}$. Since$\tau_n>> \tau_e$ is the timescale for migration between neighboringresonances, only a modest percentage of pairs end up trapped in resonancesafter the disk disappears. Planet pairs close to a mean motion resonancetypically exhibit period ratios 1-2% larger than those for exact resonance. Thedirection of this shift undoubtedly reflects the same asymmetry that requiresconvergent migration for resonance capture. Permanent resonance capture atthese separations from exact resonance would demand $\mu(\tau_n/\tau_e)^{1/2}\gtrsim 0.01$, a value that estimates of $\mu$ fromtransit data and $(\tau_e/\tau_n)^{1/2}$ from theory are insufficient to match.Plausible alternatives involve eccentricity damping during or after diskdispersal. (Abridged)

Overstable Librations can account for the Paucity of Mean Motion Resonances among Exoplanet Pairs