Thinkshop 2020

The classic view of a viscous disk, where viscosity is generated by strong turbulence driven by the magneto rotational instability, is challenged by modern magneto-hydrodynamic simulations. Disks are probably much less viscous than previously thought. Nevertheless, disks cannot be in-viscid, a minimum viscosity is set for example by the so-called vertical shear instability (VSI). In addition, disk winds remove angular momentum from thin surface layers of the proto-planetary disk, promoting fast radial transport of gas towards the central star in these layers. This radial transport accounts for the observed stars' accretion rate.
In a classical viscous disk with radial transport corresponding to observed stellar accretion rate, giant planets migrate towards the star and easily become hot Jupiters with short orbital period. However, the majority of observed giant planets have distances of 1-3AU from their parent star.
This contradiction has been investigated looking at a variety of migrations mechanisms, but no general mechanism to reduce planet migration has been found.
The new paradigm of disks with small bulk viscosity and fast radial advection in surface offers a different perspective of the problem.
Consequently, we perform 3D numerical simulations using the FARGOCA code. We simulate the effect of disk wind by imposing a loss of angular momentum generating a desired mass flux in a thin surface layer.
We show that planets migration is only marginally affected by the fast gas in the thin layers and that the migration speed is mainly regulated by the bulk viscosity of the disk. However, the migration rate measured with a bulk viscosity of alpha=1.e-4 (typically of the order of that generated by the VSI in the outer disk) is still too fast to understand the observed radial distribution of extra-solar planets. Decreasing further the viscosity seems necessary for the understanding of the observations.