Since hot Jupiter exoplanets are “phase locked” by tidal interactions (that is, the same side always faces the host star, just as the same side of our moon always faces us), there will be a large flow of heat from the highly irradiated “day side” to the cooler “night side”. This is thought to result in very strong winds rushing around the planet’s atmosphere.

Taylor Bell and Nicolas Cowan have pointed out that hydrogen will tend to be ionised on the day-side face. After flowing to the cooler face in a wind, it will then tend to recombine into neutral atoms, and thus will enhance the transport of heat.

The result is that either heat redistribution will be more effective than previously thought, helping to explain some observations of hot Jupiters, or the winds need be less strong than thought.

Bell and Cowan calculate the difference for WASP-12b. The plot shows models of the difference in temperature (x axis) against the offset of the “hot spot” caused by heat flow (y axis). The different colour coding shows the wind speed. The plot then shows the difference between models including hydrogen recombination, versus previous models by Schwartz. For a given wind speed, including hydrogen recombination results in a larger offset angle, and thus more redistribution of heat.