Atmospheric Winds Could Help Us Study The Magnetic Field Of Exoplanets

The numerical simulation of the effect of a magnetic field on a hot Jupiter's atmosphere. T.M.Roger/PSI

Soon we might able to estimate the strength of a hot exoplanet's magnetic field using its winds. This approach appears to have worked on exoplanet HAT-P-7b and it might be possible to use on other similar objects as well.

Dr Tamara Rogers from the Planetary Science Institute constructed an atmospheric model of the planet, which is 40 percent larger and 80 percent more massive than Jupiter, and analyzed the emissions from the object. She studied how the star was heating the planet and how the winds were moving that heat around in an eastward direction.

Surprisingly, however, in some of the observations, the winds are going the other way round. This didn’t work in the simpler model, but adding a magnetic field can explain it. The study is published in Nature Astronomy.

The astronomers estimate a minimum magnetic field of about 6 Gauss, between 10 and 30 times stronger than Earth's. The key to using winds as a proxy for magnetic field resides in the extreme temperatures found in the object. The day-side of HAT-P-7b is a sweltering 2,000 degree Celsius (3,500 degrees Fahrenheit), enough to change the elements in its atmosphere.

“The extreme temperatures of HAT-P-7b ionizes alkali metals such as lithium, sodium, and potassium, which results in the coupling of the atmosphere to a deep-seated magnetic field," Dr Rogers said in a statement. "Magnetic forces are able to then disrupt the strong eastward winds, leading to variable and even oppositely directed winds.” 

Rogers' magnetohydrodynamics (MHD) model is not tailored exclusively to this object but it could be applied to many different exoplanets of the same hot Jupiter variety. Astronomers would still need several observations of the planets in question, though, to make sure the model is reliable.

“Long timeline or multiple epoch observations of hot giant exoplanet phase curves coupled with MHD models of the atmospheres of these planets, can be used to place constraints on the magnetic field strengths of other hot giant exoplanets,” Rogers added. “This will provide new insights into dynamo theory, planetary evolution, and interpretations of star-planet magnetic interactions.”

These hot Jupiters are gas giant planets that orbit their star in just a handful days and can reach scorching temperatures. Their size and location make them interesting targets to observe, and even though we don’t have an example of a hot Jupiter in our Solar System, studying them might tell us a lot about how planets form.  

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