Primordial giant planet obliquity driven by a circumplanetary disk

TL;DR

This paper investigates how a tilted circumplanetary disk can cause a giant planet's spin axis to tilt, potentially explaining primordial obliquities in exoplanets, through a model of warp evolution influenced by tidal forces and planetary oblateness.

Contribution

It introduces a model of disk-planet interaction showing how a massive, tilted circumplanetary disk can generate significant planetary obliquity.

Findings

Planetary spin aligns with the disk for large disk masses.

Tilt of the planetary spin axis increases on the disk's timescale.

Mechanism could explain primordial obliquities in giant exoplanets.

Abstract

Detached circumplanetary disks are unstable to tilting as a result of the stellar tidal potential. We examine how a tilted circumplanetary disk affects the evolution of the spin axis of an oblate planet. The disk is evolved using time-dependent equations for linear wave-like warp evolution, including terms representing the effect of the tidal potential and planetary oblateness. For a disk with a sufficiently large mass, we find that the planet spin quickly aligns to the misaligned disk. The tilt of the planetary spin axis then increases on the same timescale as the disk. This can be an efficient mechanism for generating primordial obliquity in giant planets. We suggest that directly imaged exoplanets at large orbital radii, where the disk mass criterion is more likely to be satisfied, could have significant obliquities due to the tilt instability of their circumplanetary disks.