Atmospheric Circulations of Hot Jupiters as Planetary Heat Engines

TL;DR

This paper develops a planetary heat engine model to explain the high wind speeds of hot Jupiters, compares predictions with simulations and observations, and explores different drag mechanisms affecting these winds.

Contribution

It introduces a theoretical framework modeling hot Jupiters as heat engines and evaluates various drag mechanisms against observational data.

Findings

Predicted wind speeds match 3D simulation results across parameters.

Magnetic drag may be too weak for some planets but fits others.

Different drag mechanisms predict distinct wind trends for hot Jupiters.

Abstract

Because of their intense incident stellar irradiation and likely tidally locked spin states, hot Jupiters are expected to have wind speeds that approach or exceed the speed of sound. In this work we develop a theory to explain the magnitude of these winds. We model hot Jupiters as planetary heat engines and show that hot Jupiters are always less efficient than an ideal Carnot engine. Next, we demonstrate that our predicted wind speeds match those from three-dimensional numerical simulations over a broad range of parameters. Finally, we use our theory to evaluate how well different drag mechanisms can match the wind speeds observed with Doppler spectroscopy for HD 189733b and HD 209458b. We find that magnetic drag is potentially too weak to match the observations for HD 189733b, but is compatible with the observations for HD 209458b. In contrast, shear instabilities and/or shocks are compatible with both observations. Furthermore, the two mechanisms predict different wind speed trends for hotter and colder planets than currently observed. As a result, we propose that a wider range of Doppler observations could reveal multiple drag mechanisms at play across different hot Jupiters.