The gravitational interaction between planets on inclined orbits and protoplanetary disks as the origin of primordial spin--orbit misalignments

TL;DR

This paper models how a giant planet can induce primordial spin--orbit misalignments in protoplanetary disks through gravitational interactions, explaining observed exoplanet misalignments.

Contribution

It introduces a model where a giant planet causes differential disk misalignment, including secular growth of the spin--orbit angle, expanding understanding of primordial misalignment mechanisms.

Findings

Inner disk misaligns due to planetary torque

Secular growth of misalignment from disk mass depletion

Possible formation of retrograde configurations

Abstract

Many of the observed spin--orbit alignment properties of exoplanets can be explained in the context of the primordial disk misalignment model, in which an initially aligned protoplanetary disk is torqued by a distant stellar companion on a misaligned orbit, resulting in a precessional motion that can lead to large-amplitude oscillations of the spin--orbit angle. We consider a variant of this model in which the companion is a giant planet with an orbital radius of a few au. Guided by the results of published numerical simulations, we model the dynamical evolution of this system by dividing the disk into inner and outer parts---separated at the location of the planet---that behave as distinct, rigid disks. We show that the planet misaligns the inner disk even as the orientation of the outer disk remains unchanged. In addition to the oscillations induced by the precessional motion, whose amplitude is larger the smaller the initial inner-disk-to-planet mass ratio, the spin--orbit angle also exhibits a secular growth in this case---driven by ongoing mass depletion from the disk---that becomes significant when the inner disk's angular momentum drops below that of the planet. Altogether, these two effects can produce significant misalignment angles for the inner disk, including retrograde configurations. We discuss these results within the framework of the Stranded Hot Jupiter scenario and consider their implications, including to the interpretation of the alignment properties of debris disks.