  The multiple-planet systems discovered by the Kepler mission exhibit the
following feature: planet pairs near first-order mean-motion resonances prefer
orbits just outside the nominal resonance, while avoiding those just inside the
resonance. We explore an extremely simple dynamical model for planet formation,
in which planets grow in mass at a prescribed rate without orbital migration or
dissipation. We develop an analytic version of this model for two-planet
systems in two limiting cases: the planet mass grows quickly or slowly relative
to the characteristic resonant libration time. In both cases the distribution
of systems in period ratio develops a characteristic asymmetric peak-trough
structure around the resonance, qualitatively similar to that observed in the
Kepler sample. We verify this result with numerical integrations of the
restricted three-body problem. We show that for the 3:2 resonance, where the
observed peak-trough structure is strongest, our simple model is consistent
with the observations for a range of mean planet masses 20-100M_{\oplus}. This
mass range is higher than expected, by at least a factor of three, from the few
Kepler planets with measured masses, but part of this discrepancy could be due
to oversimplifications in the dynamical model or uncertainties in the planetary
mass-radius relation.
