We investigate the current sample of exoplanet spin-orbit measurements to
determine whether a dominant planet migration channel can be identified, and at
what confidence. We use the predictions of Kozai migration plus tidal friction
(Fabrycky and Tremaine 2007) and planet-planet scattering (Nagasawa et al.
2008) as our misalignment models, and we allow for a fraction of intrinsically
aligned systems, explainable by disk migration. Bayesian model comparison
demonstrates that the current sample of 32 spin-orbit measurements strongly
favors a two-mode migration scenario combining planet-planet scattering and
disk migration over a single-mode Kozai migration scenario. Our analysis
indicates that between 34% and 76% of close-in planets (95% confidence)
migrated via planet-planet scattering. Separately analyzing the subsample of 12
stars with T_eff > 6250 K---which Winn et al. (2010) predict to be the only
type of stars to maintain their primordial misalignments---we find that the
data favor a single-mode scattering model over Kozai with 81% confidence. We
also assess the number of additional hot star spin-orbit measurements that will
likely be necessary to provide a more confident model selection, finding that
an additional 20-30 measurements has a >50% chance of resulting in a
95%-confident model selection, if the current model selection is correct. While
we test only the predictions of particular Kozai and scattering migration
models in this work, our methods may be used to test the predictions of any
other spin-orbit misaligning mechanism.Comment: 9 pages, 8 figures, ApJ responded to refere
