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

Nature Astronomy volume 1, Article number: 0131 (2017) Cite this article

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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 point1,2. 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 point3,4. 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.

To demonstrate magnetic effects on the winds of HAT-P-7 b, I simulate the atmosphere of a hot giant exoplanet with parameters similar to HAT-P-7 b using a spherical, three-dimensional, anelastic magnetohydrodynamic (MHD) code6,8. I start with a hydrodynamic simulation of HD209458 b in terms of gravity, radius and rotation, but with the mean temperature (2,200 K) and day–night temperature differential (1,000 K) of HAT-P-7 b (the temperature and magnetic diffusivity profiles are shown in Supplementary Fig. 1). The strong day–night temperature differential drives strong eastwards atmospheric winds, consistent with previous simulations9,10. This simulation is run for ∼100 rotation periods before a magnetic field is added, after which both the hydrodynamic and MHD simulations are run for an additional 280 rotation periods. Details of the numerical code and simulation can be found in the Methods.

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Figure 1: Magnetic field lines in the atmosphere of a hot giant exoplanet.
Figure 2: Atmospheric dynamics of a simulated hot giant exoplanet.
Figure 3: Hotspot displacement of simulated HAT-P-7b

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Acknowledgements

T.M.R. thanks J. McElwaine and G. Glatzmaier for helpful discussions leading to this paper and J. Vriesema for help with the graphics. Figure 1 was produced using VAPOR. Funding for this work was provided by NASA (National Aeronautics and Space Administration) grant NNX13AG80G and the computing was carried out on Pleiades at NASA Ames.

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  1. Department of Mathematics and Statistics, Newcastle University, Newcastle, NE1 7EU, UK

    T. M. Rogers

  2. Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, 85721, Arizona, USA

    T. M. Rogers

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Supplementary Information

Supplementary Video 1 caption, Supplementary Figures 1,2 (PDF 134 kb)

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Supplementary Video 1 (MOV 126484 kb)

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Rogers, T. Constraints on the magnetic field strength of HAT-P-7 b and other hot giant exoplanets. Nat Astron 1, 0131 (2017). https://doi.org/10.1038/s41550-017-0131

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