Hot Jupiters are asynchronous rotators

TL;DR

This paper challenges the common assumption that hot Jupiters are tidally locked, showing that magnetic torques can cause them to rotate asynchronously, especially if they are hot or strongly magnetized.

Contribution

It introduces a toy tidal-ohmic model demonstrating that magnetic torques can induce significant asynchronous rotation in hot Jupiters, supported by atmospheric simulations.

Findings

Magnetic torque does not cancel out over the atmosphere.

Asynchronous rotation can be substantial at tidal-ohmic equilibrium.

Hot and magnetized hot Jupiters are more likely to be asynchronous.

Abstract

Hot Jupiters are typically assumed to be synchronously rotating, from tidal locking. Their thermally-driven atmospheric winds experience Lorentz drag on the planetary magnetic field anchored at depth. We find that the magnetic torque does not integrate to zero over the entire atmosphere. The resulting angular momentum feedback on the bulk interior can thus drive the planet away from synchronous rotation. Using a toy tidal-ohmic model and atmospheric GCM outputs for HD189733b, HD209458b and Kepler7b, we establish that off-synchronous rotation can be substantial at tidal-ohmic equilibrium for sufficiently hot and/or magnetized hot Jupiters. Potential consequences of asynchronous rotation for hot Jupiter phenomenology motivate follow-up work on the tidal-ohmic scenario with approaches that go beyond our toy model.