Artist’s impression of an exoplanet with a magnetic field. Credit: ESO
Picture a planet with no sunrise and no sunset.
One hemisphere is scorched by a star it can never turn away from. The other is locked in permanent night. Between them, colossal winds rip through the atmosphere at thousands of miles per hour, fast enough to make Jupiter’s worst storms look almost tame.
These are ultra-hot Jupiters, some of the most extreme planets we know of. And now, they may have revealed something astronomers have been chasing for years: magnetic fields around worlds beyond our solar system.
Hot wind
Astronomers have been searching for magnetic fields on other planets for years. A strong magnetic field can act like a planetary shield, deflecting charged particles from the host star before they can erode the upper atmosphere. That matters especially for planets orbiting close to active stars, where stellar winds and radiation can be brutal.
But finding magnetic fields on planets beyond our solar system is extremely hard. Astronomers can’t simply “see” these fields directly, especially around planets drowned out by the light of their host stars. Instead, they have to look for indirect clues, such as radio emissions, atmospheric escape, auroras, or unusual behavior in the planet’s winds.
Now, a team of astronomers has found the strongest evidence yet for such a magnetic field.
The team didn’t even set out to find something like this. Instead, the clue appeared as a strange pattern in wind speeds. Researchers measure wind speeds by looking at tiny Doppler shifts in the light absorbed by atoms, such as iron, in the planet’s atmosphere as it passes in front of its star. If that atmospheric gas is moving toward or away from us, the absorption lines shift slightly, letting astronomers calculate how fast the winds are blowing.
A team led by Julia Seidel, an astronomer at the Laboratoire Lagrange, Observatoire de la Côte d’Azur, France, found that the hotter the planet, the slower the winds were.
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But that doesn’t seem to make sense.
“This is totally counterintuitive because, all things being equal, hot planets have more energy to accelerate the winds,” said co-author Vivien Parmentier, a professor at the Laboratoire Lagrange. “Something must happen that slows down the wind speeds for hotter objects.”
So the team looked for an explanation.
Magnetic brakes, hidden in plain sight
Seidel and her team trained two of the world’s most precise spectrographs on seven ultra-hot Jupiters. Using the ESPRESSO instrument on ESO’s Very Large Telescope in Chile and MAROON-X on the Gemini North telescope in Hawaiʻi, they tracked Doppler shifts in iron lines in each planet’s atmosphere to clock their day-to-night-side wind speeds.
The winds were extraordinary — ranging from roughly 4,475 mph to over 15,500 mph (7,200 km/h to over 25,000 km/h). For context, the fastest winds on Jupiter top out at around 930 mph (1,500 km/h).
You’d expect hotter planets to have stronger winds because larger temperature differences create stronger pressure gradients. This gives the atmosphere more energy to accelerate air from hot regions toward cooler ones.
However, that’s not what was observed/ Instead, the data showed the opposite: the hotter the planet, the slower the wind.
The team worked through every alternative explanation. Atmospheric shocks? Shear instabilities? Both predict wind speeds that still rise with temperature. Only one mechanism fits the data: Ohmic drag — the braking force a magnetic field exerts on electrically charged particles.
Above about 2,420 Fahrenheit (1,327 Celsius), these atmospheres become so hot that alkali metals ionize, turning the air itself into a conductor. When a conductive gas moves through a magnetic field, electromagnetic induction slows it down. This is the same principle behind maglev trains. The hotter the planet, the more conductive its atmosphere, and the stronger the braking. That naturally explains the cooling-wind paradox.
“This breakthrough opens a completely new window on exoplanet research. It’s the first time we can compare the magnetic environments of other worlds — a key step toward ultimately understanding which planets can stay alive, keep their water, and perhaps even, one day, host life as we know it,” said Seidel, lead author of the study.
Why magnetic fields matter
Magnetic fields can shape a planet’s atmosphere, influence its long-term evolution, and help determine how much punishment it can take from its star.
Earth’s magnetic field is a shield, deflecting solar wind particles that would otherwise strip away the atmosphere. Mars lost its global magnetic field billions of years ago and, with it, most of its atmosphere and any liquid water. The connection between magnetism and habitability is written into the geological record of our nearest neighbor.
That is why exoplanet magnetism matters for the search for life.
The finding helps calibrate theoretical scaling laws used to predict magnetic fields across all planet types, including Earth-sized rocky worlds. By anchoring those models to real data, this study paves the way toward estimating whether smaller, cooler exoplanets in habitable zones carry protective fields of their own.
Of course, these ultra-hot Jupiters aren’t even remotely habitable. They’re giant, searing planets orbiting close to their stars. But they give astronomers a way to test how magnetic fields behave on alien worlds. That can help calibrate models used to predict magnetic fields on smaller planets, including rocky worlds in habitable zones.
Stunning lights
The next step is to push this method further. ESO’s Extremely Large Telescope, now under construction in Chile’s Atacama Desert, should give astronomers far more power to probe smaller and cooler worlds. Eventually, researchers hope to search for magnetic signatures on rocky planets.
That dream is still distant. But it no longer feels like pure speculation.
Researchers also suspect that these magnetic fields do something else: they trigger some mind-blowing aurorae.
“Here on Earth, we know the beauty of the northern and southern lights,” Prinoth said, “where particles from the Sun hit our magnetic field and are guided toward the poles, colliding with gases in the atmosphere to produce colourful displays of green, pink, and purple,” said co-author Bibiana Prinoth, a former PhD student at Lund University now at ESO in Garching, Germany. “I like to imagine that some of these worlds have a sky filled not only with stars, but with vast curtains of colourful light dancing across a planet that’s half in perpetual day and half in endless night.”