Astronomers measure powerful magnetic fields around hot Jupiter exoplanets

By Brooks Hays
Because hot Jupiters orbit so close to their host stars, their magnetic fields interact with their sun's emissions, as pictured in this illustration. Photo by NASA/ESA/A. Schaller for STScI
Because hot Jupiters orbit so close to their host stars, their magnetic fields interact with their sun's emissions, as pictured in this illustration. Photo by NASA/ESA/A. Schaller for STScI

July 22 (UPI) -- Astronomers have for the first time directly measured the powerful magnetic fields surrounding hot Jupiters, gas-giant exoplanets orbiting close to their host stars.

Unlike Jupiter, which is positioned in the outer solar system, hot Jupiters trace more intimate orbits. Their proximity to their host stars makes them "hot."


Of the thousands of exoplanets discovered by astronomers, a larger percentage are hot Jupiters. Astronomers theorized the gas giants boasted intense magnetic fields, but until now, scientists were unable to directly measure electromagnetic forces around hot Jupiters.

"Our study is the first to use observed signals to derive exoplanet magnetic field strengths," Evgenya Shkolnik, professor of space sciences at Arizona State University, said in a news release. "These signals appear to come from interactions between the magnetic fields of the star and the tightly orbiting planet."

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Scientists have previously used radio observations to establish upper limits for the magnetic fields around hot Jupiters. In gas giants, magnetic fields are generated by the swirling movements of conducting materials in the planet's liquid-metalic core.

"We combined measurements of increased stellar emission from the magnetic star-planet interactions together with physics theory to calculate the magnetic field strengths for four hot Jupiters," said Wilson Cauley of the University of Colorado.


Because magnetic fields interact with stellar emissions, researchers were able to calculate the strength of the distant planets' magnetic fields by capturing high-resolution images of the stars' magnetically heated chromospheres. The images allowed researchers to measure emissions from ionized calcium.

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"We used the power estimates to calculate magnetic field strengths for the planets using a theory for how the planets' magnetic fields interact with the stellar magnetic fields," Shkolnik said.

Researchers detailed their calculations this week in the journal Nature Astronomy.

"Magnetic fields like to be in a state of low energy. If you twist or stretch the field like a rubber band, this increases the energy stored in the magnetic field." said Cauley. "Hot Jupiters orbit very close to their parent stars and so the planet's magnetic field can twist and stretch the star's magnetic field."

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"When this happens, energy can be released as the two fields reconnect, and this heats the star's atmosphere, increasing the calcium emission," Cauley said.

When scientists compared the results of their calculations with the predictions of magnetic field models, the figures were in agreement.

"We are pleased to see how well the magnitude of the field values corresponded to those predicted by the internal heat flux theory," said Shkolnik. "This may also help us work toward a clearer understanding of magnetic fields around temperate rocky planets."


The analysis showed the hot Jupiters' magnetic fields are surprisingly strong, several times greater than the magnetic field produced by Jupiter or Earth.

Cauley, Shkolnik and their colleagues suggest the new observations will help scientists better understand what's happening deep inside faraway gas giants.

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