Strange winds reveal strongest hints yet of magnetic activity in exoplanets

On Earth, the magnetic field is vital as it prevents cosmic radiation from eroding our atmosphere. At the same time, it gives rise to impressive auroras. Magnetic fields also exist on other planets in our solar system, such as Jupiter and Saturn. But what about the planets outside our solar system, the so-called exoplanets?

An international research team with the participation of Dr. Elspeth Lee, Bernoulli Fellow at the Center for Space and Habitability (CSH) of the University of Bern, and Dr. Joost Wardenier from the Space Research and Planetary Sciences Division at the Physics Institute of the University of Bern, has now come a decisive step closer to answering this question. By determining the wind speeds on several extremely hot exoplanets similar to Jupiter, the researchers were able to show for the first time that the hotter the planet, the lower the wind speed. One explanation for this could be the presence of magnetic fields that slow down the winds of these planets. The study has just been published in Nature Astronomy.

Extreme exoplanet winds in the model

In the study, the researchers investigated the wind speeds on seven exoplanets orbiting different stars. These are gas giants similar to Jupiter, which are very close to their star. Elspeth Lee explains: "The rotation of the planets was synchronized with their orbits by the tidal forces of the parent star. Just as we only ever see one side of the moon, these planets always have one side facing the star. Thus, a glowing hot day side and a permanently dark night side have formed on the planets. The extreme temperature differences in turn lead to the generation of extremely strong winds."

For the measurements, the team used data from the ESPRESSO instrument at the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in the Chilean Atacama Desert and a similar instrument at the Gemini-North telescope in Hawaii, USA, which is partly funded by the US National Science Foundation (NSF) and operated by the NSF NOIRLab. Elspeth Lee and Joost Wardenier contributed to collaborated on the theoretical interpretation of the observational data, contributing their knowledge and experience of the atmospheres of "hot Jupiters". "We specialize in simulating the atmospheres of hot Jupiter planets with computer models. We investigate how their winds behave, how the chemical composition changes and how the climate develops on the entire planet," explains Elspeth Lee.

Raging winds on exoplanets

The research team was able to show that wind speeds of around 7,200 km/h to over 25,000 km/h prevail on the exoplanets. For comparison: the fastest winds measured on Jupiter reach speeds of around 1,500 km/h.

"Initially, we wanted to find out whether the atmospheric winds behave the same for all hot planets," explains Julia Seidel, astronomer at the Laboratoire Lagrange, Observatoire de la Côte d'Azur, France, and lead author of the study. However, when the researchers investigated the dependence of wind speeds on planetary temperature, a very fascinating pattern emerged: the hotter the planet, the slower the wind. "This completely contradicts intuition, because all other things being equal, hot planets have more energy to accelerate the winds! Something must be slowing down the wind on hotter objects," says Vivien Parmentier, co-author of the study and professor at the Laboratoire Lagrange.

Even more spectacular auroras than on Earth?

The team came to the conclusion that the most plausible explanation is the presence of planetary magnetic fields, as they act like a brake and can slow down the movement of charged particles in the atmosphere. Using the data, the researchers were therefore able to deduce the strength of the magnetic field on each of the planets studied. They found that these are comparable in strength to those in our solar system: about four times as strong as Saturn's or about half as strong as Jupiter's. " For the first time, we performed a consistent analysis of the wind speeds and magnetic field strengths of seven hot Jupiters, based on the very subtle Doppler shifts observed in their spectra. The data used in our work shed light on these shifts with impressive detail," explains Joost Wardenier.

Such strong magnetic fields could influence more than just the wind on these distant planets. "Here on Earth, we know the beauty of the auroras, where particles from the sun hit our magnetic field and are guided to the poles. There they collide with gases in the atmosphere and create colorful spectacles in green, pink and purple," explains Bibiana Prinoth, co-author of the study, former PhD student at Lund University in Sweden and now an astronomer at ESO in Garching, Germany. "On the exoplanets studied, the magnetically driven auroras could be even more spectacular," says Prinoth.

"This advance opens up completely new perspectives for exoplanet research. For the first time, we can compare the magnetic fields of other worlds – a crucial step towards ultimately understanding which planets can remain viable, retain their water and perhaps even one day harbor life as we know it," says Julia Seidel. "Overall, this is a big step towards understanding other worlds outside the solar system and a good omen for ESO's Extremely Large Telescope ELT, which will become operational in 2029 and will enable even more precise observations," concludes Elspeth Lee.