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Observations of infrared and optical light curves of hot Jupiters havedemonstrated that the peak brightness is generally offset eastward from thesubstellar point [1,2]. This observation is consistent with hydrodynamicnumerical simulations that produce fast, eastward directed winds which advectthe hottest point in the atmosphere eastward of the substellar point [3,4].However, recent continuous Kepler measurements of HAT-P-7 b show that its peakbrightness offset varies significantly in time, with excursions such that thebrightest point is sometimes westward of the substellar point [5]. Thesevariations in brightness offset require wind variability, with or without thepresence of clouds. While such wind variability has not been seen inhydrodynamic simulations of hot Jupiter atmospheres, it has been seen inmagnetohydrodynamic (MHD) simulations [6]. Here we show that MHD simulations ofHAT-P-7 b indeed display variable winds and corresponding variability in theposition of the hottest point in the atmosphere. Assuming the observedvariability in HAT-P-7 b is due to magnetism we constrain its minimum magneticfield strength to be 6\,G. Similar observations of wind variability on hotgiant exoplanets, or lack thereof, could help constrain their magnetic fieldstrengths. Since dynamo simulations of these planets do not exist andtheoretical scaling relations [7] may not apply, such observational constraintscould prove immensely useful.

Constraints on the Magnetic Field Strength of HAT-P-7 b and other Hot Giant Exoplanets