For the first time in 15 years of trying, astronomers have detected the magnetic fields wrapped around planets orbiting other stars — opening what one researcher called 'a completely new window' on the search for habitable worlds.
The discovery, published last week in Nature Astronomy, came from observations of seven scorching Jupiter-like exoplanets using the European Southern Observatory's Very Large Telescope (VLT) in Chile and the Gemini North telescope in Hawaii. A magnetosphere — the bubble of magnetic field that surrounds a planet — is widely considered a prerequisite for keeping an atmosphere intact, which in turn is the prerequisite for liquid water and life.
Earth has one. Jupiter and Saturn have powerful ones. Mars lost its long ago, and with it most of its atmosphere and water. Until now, no one had ever measured the magnetic field of a planet beyond our solar system.
'This breakthrough opens a completely new window on exoplanet research,' said lead author Julia Seidel, an astronomer at the Laboratoire Lagrange at the Observatoire de la Côte d'Azur in France. '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.'
The team stumbled onto the result while trying to do something else entirely. They had set out to measure wind speeds on the seven gas giants, each of them tidally locked to its star with one face in scorching permanent daylight and the other in eternal frozen night. The resulting climate produced winds ranging from 7,200 kilometers per hour to more than 25,000 kilometers per hour — far faster than anything in our solar system, where Jupiter's fiercest winds top out around 1,500 km/h.
But the data revealed a puzzle. 'In the beginning we set out to check if the atmospheric winds behaved the same way for all hot planets,' Seidel explained. Instead, the team found something counterintuitive: the hotter the planet, the slower the wind.
'This is totally counter intuitive 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.'
The best explanation, the team concluded, was magnetism. A planet-wide magnetic field acts as a brake on charged particles in the upper atmosphere, dragging on the wind. By working backward from the slowdown, the researchers could estimate the strength of each magnetic field. The result: roughly four times the strength of Saturn's, or about half of Jupiter's — squarely within the range of magnetospheres in our own solar system.
For Bibiana Prinoth, an astronomer at the ESO station in Garching, Germany, the implication is almost cinematic.
'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 colorful displays of green, pink, and purple,' Prinoth said. On the seven exoplanets, the auroras could be 'even more dramatic.'
The result is a meaningful step. Magnetic fields are notoriously hard to detect across interstellar distances — they don't glow, and they don't emit a clean signal. Inferring them from atmospheric wind drag is a clever workaround that turns every future spectroscopic survey into a potential magnetism survey too.
The next step is bigger eyes. ESO's Extremely Large Telescope, currently under construction in Chile, should be able to extend the technique from hot Jupiters down to smaller, more Earth-like worlds — and possibly catch the chemical fingerprints of auroras themselves. After 15 years of guessing, the field finally has a way to ask one of the most basic questions about another planet: does it have a shield?
