Constraints on the magnetic field strength of HAT-P-7 b and other hot giant exoplanets

Abstract

Observations of the infrared and optical light curves of hot giant exoplanets have demonstrated that the peak brightness is generally offset eastwards from the substellar point^1,2. This observation is consistent with hydrodynamic numerical simulations producing fast, eastwards directed winds that advect the hottest point in the atmosphere eastwards of the substellar point^3,4. However, recent continuous Kepler measurements of HAT-P-7 b show that its peak brightness offset varies considerably over time, with excursions such that the brightest point is sometimes westwards of the substellar point⁵. These variations in brightness offset require wind variability, with or without the presence of clouds. While such wind variability has not been seen in hydrodynamic simulations of hot giant exoplanet atmospheres, it has been seen in magnetohydrodynamic simulations⁶. Here I show that magnetohydrodynamic simulations of HAT-P-7 b indeed display variable winds and a corresponding variability in the position of the hottest point in the atmosphere. Assuming that the observed variability in HAT-P-7 b is due to magnetism, I constrain its minimum magnetic field strength to be 6 G. Similar observations of wind variability on hot giant exoplanets, or the lack thereof, could help constrain their magnetic field strengths. As dynamo simulations of these planets do not exist and theoretical scaling relations⁷ may not apply, such observational constraints could prove immensely useful.