Planet-disk interaction in highly inclined systems

TL;DR

This paper investigates how highly inclined proto-planetary disks interact with planets, revealing that inclination damping is often too slow to realign such planets with their disks, influencing planetary system formation theories.

Contribution

It provides a linear regime analysis of planet-disk interactions at high inclinations, including migration and damping timescales, and discusses implications for Hot Jupiters and small planetary systems.

Findings

Inclination damping timescales can exceed disk lifetimes.

Highly inclined planets cannot realign with the disk.

Simulation dependence on gravitational softening is logarithmic.

Abstract

We study the interaction of a proto-planetary disk and a planet on a highly inclined orbit in the linear regime. The evolution of the planet is dominated by dynamical friction for planet masses above several Earth-masses. Smaller planets are dominated by aerodynamic drag, especially for very high inclinations and retrograde orbits. The time-scales associated with migration and inclination damping are calculated. For certain values of the inclination, the inclination damping time-scale is longer than the migration time-scale and the disk lifetime. This result shows that highly inclined planets can not (re-)align with the proto-planetary disk. We discuss the dependence of numerical simulations on the gravitational softening parameter. We find only a logarithmic dependence, making global three dimensional simulations of this process computationally feasible. A large fraction of Hot Jupiters is on highly inclined orbits with respect to the rotation axis of the star. On the other hand small-mass planetary systems discovered by the Kepler mission have low mutual inclinations. This shows that there are two distinct formation mechanisms at work. The process that creates inclined Hot Jupiters does not operate on small mass planets because the damping timescales are so long that these systems would still be inclined today.