An international team measured wind speeds on seven ultra-hot Jupiters — gas giants orbiting extremely close to their stars, tidally locked with one side permanently facing the star. The day side is scorching, the night side freezing, and this contrast drives ferocious winds: from 7,200 to 25,000 km/h. For comparison, Jupiter's fastest winds reach only about 1,500 km/h.
But the most intriguing finding wasn't the speeds themselves — it was the pattern: the hotter the planet, the slower the wind. This is completely counterintuitive — more energy should mean stronger winds. Something is acting as a brake.
The most consistent explanation is planet-wide magnetic fields. In the atmospheres of such hot planets, gas is partially ionized, and a magnetic field slows down charged particles. This allowed the team to estimate exoplanet magnetic field strengths for the first time — and they turned out to be comparable to those of Solar System planets: roughly four times stronger than Saturn's field and about half as strong as Jupiter's.
Why does this matter? Earth's magnetic field is a shield that protects the atmosphere from cosmic radiation and keeps the planet habitable. Understanding magnetic fields of other worlds is a step toward answering which planets can retain water and atmosphere.
Observations were made with MAROON-X on the Gemini North telescope (Hawai'i) and ESPRESSO on ESO's Very Large Telescope (Chile). The results are published in Nature Astronomy.