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ALMA observations of dust ring/gap structures in a minority but growingsample of protoplanetary disks can be explained by the presence of planets atlarge disk radii - yet the origins of these planets remains debated. We performplanet formation simulations using a semi-analytic model of the HL Tau disk tofollow the growth and migration of hundreds of planetary embryos initiallydistributed throughout the disk, assuming either a high or low turbulent$\alpha$ viscosity. We have discovered that there is a bifurcation in themigration history of forming planets as a consequence of varying the diskviscosity. In our high viscosity disks, inward migration prevails and yieldscompact planetary systems, tempered only by planet trapping at the watericeline around 5 au. In our lower viscosity models however, low mass planetscan migrate outward to twice their initial orbital radii, driven by a radiallyextended region of strong outward-directed corotation torques located near theheat transition (where radiative heating of the disk by the star is comparableto viscous heating) - before eventually migrating inwards. We derive analyticexpressions for the planet mass at which the corotation torque dominates, andfind that this "corotation mass" scales as $M_{\rm p, corot} \sim\alpha^{2/3}$. If disk winds dominate the corotation torque, the corotationmass scales linearly with wind strength. We propose that the observedbifurcation in disk demographics into a majority of compact dust disks and aminority of extended ring/gap systems is a consequence of a distribution ofviscosity across the disk population.

Turbulent Disk Viscosity and the Bifurcation of Planet Formation Histories