Extended Heat Deposition in Hot Jupiters: Application to Ohmic Heating

TL;DR

This paper develops an analytical model for heat deposition in hot Jupiters, especially Ohmic heating, explaining their inflated radii and contraction behavior, with implications for planetary evolution and observed exoplanet sizes.

Contribution

It generalizes previous localized heating models to include extended heat sources and applies this to Ohmic heating, providing insights into planetary inflation and contraction timescales.

Findings

Model explains observed radii of most inflated hot Jupiters.

Extended heat sources halt planetary cooling and contraction.

Re-inflation timescales are significantly longer than cooling times.

Abstract

Many giant exoplanets in close orbits have observed radii which exceed theoretical predictions. One suggested explanation for this discrepancy is heat deposited deep inside the atmospheres of these "hot Jupiters". Here, we study extended power sources which distribute heat from the photosphere to the deep interior of the planet. Our analytical treatment is a generalization of a previous analysis of localized "point sources". We model the deposition profile as a power law in the optical depth and find that planetary cooling and contraction halt when the internal luminosity (i.e. cooling rate) of the planet drops below the heat deposited in the planet's convective region. A slowdown in the evolutionary cooling prior to equilibrium is possible only for sources which do not extend to the planet's center. We estimate the Ohmic dissipation resulting from the interaction between the atmospheric winds and the planet's magnetic field, and apply our analytical model to Ohmically heated planets. Our model can account for the observed radii of most inflated planets which have equilibrium temperatures $\approx 1500\textrm{K}-2500\textrm{K}$, and are inflated to a radius $\approx 1.6 R_J$. However, some extremely inflated planets remain unexplained by our model. We also argue that Ohmically inflated planets have already reached their equilibrium phase, and no longer contract. Following Wu & Lithwick who argued that Ohmic heating could only suspend and not reverse contraction, we calculate the time it takes Ohmic heating to re-inflate a cold planet to its equilibrium configuration. We find that while it is possible to re-inflate a cold planet, the re-inflation timescales are longer by a factor of $\approx 30$ than the cooling time.