  Over the course of the last two decades, traditional models of planet
formation have been repeatedly challenged by the emerging census of extrasolar
planets. Key among them is the orbital architecture problem: while standard
models of orbital migration predict resonant orbits for short-period objects,
most planets do not appear to lie in orbital resonances. Here we show that the
four-planet system Kepler-221, not previously recognized to have active orbital
resonances, has a three-body commensurability relation unique within the Kepler
sample. Using a suite of numerical experiments as well as a perturbative
analysis, we demonstrate that this system likely began as a resonant chain and
proceeded to undergo large-scale divergence away from resonance, under the
action of tidal dissipation. Our results further indicate that obliquity tides,
driven by a secular spin-orbit resonance and mutual inclination, are an
excellent candidate for driving this orbital divergence, and that the high
tidal luminosity may also explain the anomalous size of planet b, which lies
within the Fulton radius gap.
