Strange Winds Reveal Strongest Hints Yet of Magnetic Activity in Exoplanets

By measuring the strength of the invisible magnetic fields of seven ultra-hot Jupiters, astronomers have taken a major step toward understanding planets beyond our Solar System. A new study published today in Nature Astronomy reveals hints that the magnetic fields of some of the hottest known exoplanets are similar in strength to those of planets in our own Solar System.

“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,” says Julia Seidel, an astronomer at the Laboratoire Lagrange, Observatoire de la Côte d’Azur, France, and lead author of the study.

Earth’s magnetic field acts as a shield: it helps stop cosmic radiation from stripping away our atmosphere, keeping the planet habitable for life. Magnetic fields are also present on other Solar System planets, like Jupiter and Saturn. However, no one succeeded in directly measuring the strength of the magnetic fields of planets outside of our Solar System  until now.

The team, however, didn’t set out to measure magnetic fields but, rather, winds. They measured wind speeds on seven exoplanets orbiting different stars: gas giants like Jupiter, but each tidally locked to its host star and very close to it. Just as we always see only one side of the Moon from Earth, these planets always keep one side facing their host star, resulting in a scorching-hot day side and a freezing-cold night side. This temperature difference creates a climate completely different from the one on our planet, with extremely strong winds. The wind speeds in their sample ranged from around 7200 kilometers (4400 miles) per hour to over 25,000 kilometers (15,500 miles) per hour; in comparison, the fastest winds measured on Jupiter reach speeds of around 1500 kilometers (900 miles) per hour.

For their measurements, the team used data from the MAROON-X instrument on the Gemini North telescope in Hawaiʻi, one half of the International Gemini Observatory, partly funded by the U.S. National Science Foundation (NSF) and operated by NSF NOIRLab. They also used data from the ESPRESSO instrument on ESO’s VLT in the Chilean Atacama Desert. These powerful, high-resolution instruments allowed the team to measure wind speeds by detecting the light signature of specific chemicals and tracing their movements through the ultra-hot Jupiters’ atmospheres.

“The stability of MAROON-X makes it a powerful tool for detecting the subtle motion of Earth-sized planets around other stars, as well as tracing changes in the atmospheres of exoplanets depending on orbital phase,” says Andreas Seifahrt, Associate Director of Development for Gemini Observatory and study co-author. “The unexpected discovery that resulted from studying the winds of these seven ultra-hot Jupiters shows that there is even more that we can learn from the data. MAROON-X provides a world-class capability for these studies.”

When the researchers looked at how wind speeds varied with the planet’s temperature, they saw a very intriguing pattern emerge: the hotter the planet, the slower the wind. “This is totally counterintuitive because, all things being equal, hot planets have more energy to accelerate the winds! Something must happen that slows down the wind speeds for hotter objects,” says study co-author Vivien Parmentier, a professor at the Laboratoire Lagrange.

The team concluded that the most consistent explanation for this mystery is the presence of planet-wide magnetic fields, since these fields can work as a brake, slowing down the motion of charged particles in the atmosphere. The data, therefore, allowed the researchers to infer the strength of the magnetic field of each of the studied planets. They found them to be comparable in strength to those found in our Solar System: approximately four times as strong as Saturn’s magnetic field or about half the strength of Jupiter’s.

Such strong magnetic fields could affect more than just the wind on these distant planets. “Here on Earth, we know the beauty of the northern and southern lights, 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,” explains study co-author Bibiana Prinoth, a former PhD student at Lund University, Sweden, now an astronomer at ESO in Garching, Germany. On the studied exoplanets, the magnetically driven aurorae could be even more dramatic.