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

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 nonzero 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 ∼ 100. Moreover, we find that secular excitation by the radial-velocity-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.