Turbulent disc viscosity and the bifurcation of planet formation histories

Abstract

ALMA observations of dust ring/gap structures in a minority but growing sample of protoplanetary discs can be explained by the presence of planets at large disc radii - yet the origins of these planets remains debated. We perform planet formation simulations using a semi-analytic model of the HL Tau disc to follow the growth and migration of hundreds of planetary embryos initially distributed throughout the disc, assuming either a high or low turbulent α viscosity. We have discovered that there is a bifurcation in the migration history of forming planets as a consequence of varying the disc viscosity. In our high viscosity discs, inward migration prevails and yields compact planetary systems, tempered only by planet trapping at the water iceline around 5 au. In our lower viscosity models however, low mass planets can migrate outward to twice their initial orbital radii, driven by a radially extended region of strong outward-directed corotation torques located near the heat transition (where radiative heating of the disc by the star is comparable to viscous heating) - before eventually migrating inwards. We derive analytic expressions for the planet mass at which the corotation torque dominates, and find that this 'corotation mass' scales as Mp, corot ∼α2/3. If disc winds dominate the corotation torque, the corotation mass scales linearly with wind strength. We propose that the observed bifurcation in disc demographics into a majority of compact dust discs and a minority of extended ring/gap systems is a consequence of a distribution of viscosity across the disc population.