  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.
