  It is often assumed that the "Kepler dichotomy" -- the apparent excess of
planetary systems with a single detected transiting planet in the Kepler
catalog -- reflects an intrinsic bimodality in the mutual inclinations of
planetary orbits. After conducting 600 simulations of planet formation followed
by simulated Kepler observations, we instead propose that the apparent
dichotomy reflects a divergence in the amount of migration and the separation
of planetary semimajor axes into distinct "clusters". We find that our
simulated high-mass systems migrate rapidly, bringing more planets into orbital
periods of less than 200 days. The outer planets are often caught in a
migration trap -- a range of planet masses and locations in which a dominant
co-rotation torque prevents inward migration -- which splits the system into
two clusters. If clusters are sufficiently separated, the inner cluster remains
dynamically cold, leading to low mutual inclinations and a higher probability
of detecting multiple transiting planets. Conversely, our simulated low-mass
systems typically bring fewer planets inside 200 days, forming a single cluster
that quickly becomes dynamically unstable, leading to collisions and high
mutual inclinations. We propose an alternative explanation for the apparent
Kepler dichotomy in which migration traps during formation lead to fewer
planets inside the Kepler detection window, and where mutual inclinations play
only a secondary role. If our scenario is correct, then Kepler's STIPs (Systems
with Tightly-packed Inner Planets) are a sample of planets that escaped capture
by co-rotation traps, and their sizes may be a valuable probe into the
structure of protoplanetary discs.
