In theory, the hottest giant planets in our galaxy ought to boast the swiftest winds. The hotter a planet, the more vigorous its atmospheric currents should be, and the category of exoplanets known as hot Jupiters encompasses the hottest worlds we’ve ever found.
They orbit their stars at such incredibly close distances that some are literally being vaporized by the heat…
However, a new analysis of seven hot Jupiters reveals that the stellar wind speeds there are surprisingly low compared to what astronomers anticipated.
According to a team of astronomers led by Laetitia Delrez from the University of Liège in Belgium, the best explanation for this unexpected phenomenon is that something is actively reining the winds in. And the mechanism that best accounts for this powerful braking effect is a magnetic field. The findings from this study have been published in the journal Nature Astronomy.
If the team’s results hold true, these sluggish winds could offer the best evidence yet of magnetic activity on a planet beyond our solar system.
“This breakthrough opens up entirely new avenues for exoplanet research,” says Delrez. “It’s the first time we can compare the magnetic conditions of other worlds—a crucial step towards understanding which planets might remain habitable, retain water, and perhaps even one day become homes for life as we know it.”
Hot Jupiters are already considered among the most remarkable exoplanets in the Milky Way. These worlds orbit so close to their stars that, in the most extreme cases, their orbital periods are less than a single day.
This means two things are typically true for hot Jupiters. First, they are tidally locked, with one side perpetually bathed in daylight and facing the star, while the other is in constant darkness, facing away.
This creates a temperature differential that should lead to wildly extreme weather.
Second, these worlds are generally heated to equilibrium temperatures of several thousand degrees, which should drive even more vigorous atmospheric circulation.
While we cannot directly measure magnetic fields on exoplanets at this time, previous studies of individual hot Jupiters have suggested that by tracking evaporated iron in their atmospheres, wind speeds can be determined.
Since magnetic fields are known to act as a brake on electrically charged gases, researchers hypothesized that the wind speed on a hot Jupiter could serve as an indicator of magnetic field activity.
They utilized the MAROON-X instrument on the Gemini North telescope and the ESPRESSO instrument on the European Southern Observatory’s Very Large Telescope to measure wind speeds above seven hot Jupiters.
The observed wind speeds on these planets are still far greater than anything we see in the Solar System. Researchers recorded howling winds ranging from 2 to 7 kilometers per second. Jupiter, the fastest in our Solar System, reaches only about 0.4 kilometers per second.
However, what makes hot Jupiters intriguing is the apparent correlation between wind speed and temperature.
The researchers discovered that the hotter an exoplanet is, the slower its winds blow.
Other explanations for slower-than-expected winds on hot Jupiters exist; however, the researchers argue that other possibilities would still show the opposite trend, with wind speeds increasing as temperatures rise.
“This is completely counterintuitive, because all else being equal, hotter planets have more energy to accelerate stellar winds!” says astronomer Vivien Parmentier from the University of Bern in Switzerland. “Something must be happening to slow down stellar winds on hotter objects.”
Researchers propose that this “something” is most likely magnetic fields… and based on the trends from their observations, they were even able to estimate the strength of the field responsible for this effect.
The researchers found that hot Jupiters would need magnetic fields of only a few Gauss, which is roughly comparable to Jupiter’s own magnetic field.
As this is an indirect measurement method, further observations may be required to confirm the team’s conclusions.
Nevertheless, it is still a wonderful result—demonstrating how far we’ve come in understanding alien worlds, moving from characterizing individual planets to performing statistical analyses that are beginning to reveal patterns.
“Here on Earth, we know the beauty of the Aurora Borealis and Australis, where particles from the Sun collide with our magnetic field and are channeled towards the poles, where they strike gases in the atmosphere and create dazzling displays of green, pink, and purple light,” says astronomer Bibiana Prinoth of the European Southern Observatory (ESO). “I like to imagine that on some of these worlds, the sky is filled not just with stars, but with vast curtains of multi-colored light dancing across a planet that is half in perpetual day and half in endless night.”
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