On the Terminal Rotation Rates of Giant Planets

TL;DR

This paper proposes that magnetic coupling and hydrodynamic processes during giant planet formation efficiently brake planetary rotation, explaining why observed giant planets rotate below breakup speeds.

Contribution

It introduces a mechanism involving magnetic coupling and hydrodynamic circulation that accounts for the slower rotation rates of giant planets compared to theoretical expectations.

Findings

Magnetic fields generated by young giant planets facilitate angular momentum loss.

Hydrodynamic circulation within the Hill sphere expels spin angular momentum.

The model aligns giant planet rotation evolution with stellar rotation processes.

Abstract

Within the general framework of core-nucleated accretion theory of giant planet formation, the conglomeration of massive gaseous envelopes is facilitated by a transient period of rapid accumulation of nebular material. While the concurrent buildup of angular momentum is expected to leave newly formed planets spinning at near-breakup velocities, Jupiter and Saturn, as well as super-Jovian long-period extrasolar planets, are observed to rotate well below criticality. In this work, we demonstrate that the large luminosity of a young giant planet simultaneously leads to the generation of a strong planetary magnetic field, as well as thermal ionization of the circumplanetary disk. The ensuing magnetic coupling between the planetary interior and the quasi-Keplerian motion of the disk results in efficient braking of planetary rotation, with hydrodynamic circulation of gas within the Hill sphere playing the key role of expelling spin angular momentum to the circumstellar nebula. Our results place early-stage giant planet and stellar rotation within the same evolutionary framework, and motivate further exploration of magnetohydrodynamic phenomena in the context of the final stages of giant planet formation.