This illustration shows magnetic activity in an exoplanet. The planet is a gas giant like Jupiter, but it’s very close to its host star and tidally locked: one side always faces the star and is scorching hot, whereas the other side is extremely cold. This steep temperature difference creates fast winds that blow from the day side to the night side. The planet’s magnetic field, shown here with blue lines, can slow these winds down. Credit: ESO/ M Kornmesser, L Calçada.

A team of astronomers has found the strongest evidence yet that some planets outside our Solar System may be magnetic.

Using the European Southern Observatory’s Very Large Telescope (ESO’s VLT) and the Gemini North telescope, the researchers measured wind speeds on seven very hot, Jupiter-like exoplanets.

The observations revealed that the winds on these planets are most likely governed by magnetic fields, providing the first robust measurement of magnetism on planets outside the Solar System.

Lead author of the study, Julia Seidel, an astronomer at the Laboratoire Lagrange, Observatoire de la Côte d’Azur, France, said: “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.”

This diagram illustrates how astronomers can infer the strength of magnetic fields in exoplanets from their effect on the winds on these planets. Using spectrographs astronomers can measure the temperature and wind speed on exoplanets. A trend of decreasing wind speed with increasing temperature can betray the presence of magnetic fields on these planets. Credit: ESO/ M Kornmesser, L Calçada.

Habitable planets

Earth’s magnetic field influences our atmosphere in complex ways, and is therefore a key factor in understanding what keeps the planet habitable for life.

Magnetic fields are also present in other Solar System planets, like Jupiter and Saturn.

However, for the past 15 years, no one succeeded in directly measuring the strength of the magnetic fields of exoplanets — 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, these planets always keep one face towards the 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 7,200 km/h to over 25,000 km/h; in comparison, the fastest winds measured on Jupiter reach speeds of around 1,500 km/h.

Seidel, who was previously an astronomer at ESO in Chile, said: “In the beginning we set out to check if the atmospheric winds behaved the same way for all hot planets.”

For their measurements, the team used data from the ESPRESSO instrument on ESO’s VLT, in the Chilean Atacama Desert, and from a similar instrument on the Gemini North telescope in Hawaiʻi, USA.

But when they looked at how the wind speeds varied with planet temperature, they saw a very intriguing pattern emerge: the hotter the planet, the slower the wind.

Co-author Vivien Parmentier, a professor at the Laboratoire Lagrange, said: “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.”

Planet-wide magnetic fields

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 in 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 or about half the strength of Jupiter’s.

Such strong magnetic fields could affect more than just the wind on these distant planets.

Co-author Bibiana Prinoth, a former PhD student at Lund University, Sweden, now an astronomer at ESO in Garching, Germany, said: “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.”

On the studied exoplanets, the magnetically driven aurorae could be even more dramatic.

The team eagerly anticipates the arrival of ESO’s Extremely Large Telescope, which will help to characterise not only large, Jupiter-like exoplanets but also smaller ones like Earth, possibly even detecting gases that could produce aurorae on these distant worlds.

Prinoth said: “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.”

The study is published in Nature Astronomy.