From Tides to Currents: Unraveling the Mechanism That Powers WASP-107b's Internal Heat Flux

TL;DR

This paper investigates the internal heat source of exoplanet WASP-107b, proposing that Ohmic dissipation, rather than tidal heating, explains its high internal heat flux and inflated radius.

Contribution

It introduces Ohmic dissipation as a plausible mechanism for maintaining WASP-107b's thermal state, challenging the tidal heating explanation and providing a new perspective on exoplanet internal heat sources.

Findings

Ohmic dissipation can account for the planet's heat flux and radius.

Tidal heating requires an unrealistically low tidal quality factor.

Secular excitation by the outer companion cannot sustain the eccentricity.

Abstract

The sub-Jovian exoplanet WASP-107b ranks among the best-characterized low-density worlds, featuring a Jupiter-like radius and a mass that lies firmly in the sub-Saturn range. Recently obtained JWST spectra reveal significant methane depletion in the atmosphere, indicating that WASP-107b's envelope has both a high metallicity and an elevated internal heat flux. Together with a detected non-zero orbital eccentricity, these data have been interpreted as evidence of tidal heating. However, explaining the observed luminosity with tidal dissipation requires an unusually low tidal quality factor of $Q \sim 100$. Moreover, we find that secular excitation by the RV-detected outer companion WASP-107c, generally cannot sustain WASP-107b's eccentricity in steady state against tidal circularization. As an alternative explanation, we propose that Ohmic dissipation -- generated by interactions between zonal flows and the planetary magnetic field in a partially ionized atmosphere -- maintains the observed thermal state. Under nominal assumptions for the field strength, atmospheric circulation, and ionization chemistry, we show that Ohmic heating readily accounts for WASP-107b's inflated radius and anomalously large internal entropy.