Saving Planetary Systems: Dead Zones and Planetary Migration

The tidal interaction between a disk and a planet leading to a planet's migration is widely believed to be the mechanism that explains the variety of orbital radii of extrasolar planets. A long-standing question is what stops the migration before planets plunge into their central stars. We propose a new, simple mechanism to significantly slow down planet migration and test it using a hybrid numerical integrator to simulate disk-planet interaction. Key to this scenario are the low-viscosity regions in protostellar disks known as dead zones. Low viscosity affects planetary migration in two ways. First, it allows a smaller mass planet to open a gap, and hence trade the faster type I migration (pre-gap-opening migration) for the slower type II migration (post-gap-opening migration). Second, low viscosity slows down type II migration itself, because type II migration varies directly with viscosity. We present numerical simulations of planetary migration in disks using a hybrid symplectic integrator-gas dynamics code. Assuming that the disk viscosity parameter inside the dead zone is α = 10-4 to 10-5, we find that, when a low-mass planet (1-10 M⊕) migrates from outside the dead zone, it is stopped by mass accumulation inside the dead zone. When a low-mass planet migrates from inside the dead zone, it opens a gap, slowing its migration. A massive planet like Jupiter, in contrast, opens a gap and slows down inside the dead zone, independent of its initial orbital radius. The final orbital radius of a Jupiter-mass planet depends on the dead zone's viscosity. For the range of α-values noted above, this can vary from 7 AU to an orbital radius of 0.1 AU, which is characteristic of the hot Jupiters.