First 3D mapping of an exoplanet atmosphere
Astronomers resolve the detailed atmospheric structure and composition of a planet 900 light years away.
Discoveries of planets beyond our solar system are coming in thick and fast, and in some cases we have some limited information on their atmospheric content and wind speeds. This is a tricky task when the planets are so far away, but observations are becoming ever more detailed, and the results challenge our understanding of the way in which weather works.
Using the European Southern Observatory’s Very Large Telescope (ESO VLT) in Chile, researchers have been looking1 at the planet WASP-121b, aka Tylos, in the constellation Puppis. This gas giant orbits its host star so closely that the Tylosian year lasts only around 30 Earth hours. The tight orbit also locks the rotation of Tylos so that one side of the planet forever faces the star, while the other is in permanent darkness and therefore much cooler.
What observations with the ground-based VLT show is that Tylos has three distinct layers in its atmosphere, with winds carrying chemical elements such as iron and titanium, creating intricate weather patterns.
“What we found was surprising,” says research leader Julia Seidel, who works in the Lagrange Laboratory at the Observatoire de la Côte d'Azur in France. “A jet stream rotates material around the planet’s equator, while a separate flow at lower levels of the atmosphere moves gas from the hot side to the cooler side. This kind of climate has never been seen before on any planet,”
The jet stream observed by Seidel and her colleagues spans half the planet, moving at great speed, and churning up the atmosphere as it crosses the hot, daylit side of Tylos. “Even the strongest hurricanes in the Solar System seem calm in comparison,” she notes.
In addition to tracking the movement of iron, sodium and hydrogen in the deep, mid and upper layers of Tylos’ atmosphere, the observations reveal the presence of titanium2 just below the jet stream. This was a surprise, as previous data had shown titanium element to be absent, possibly as it was hidden deep in the atmosphere.
Tylos is a massive, Jupiter-like planet. Studying the atmospheres of smaller, Earth-like planets will require larger telescopes than the VLT, and this is planned with ESO’s Extremely Large Telescope (ELT), currently under construction in Chile’s Atacama Desert. “The ELT will be a game-changer for studying exoplanet atmospheres,” says Seidel’s colleague Bibiana Prinoth. “This experience makes me feel like we’re on the verge of uncovering incredible things we can only dream about now.”
Seidel et al., “Vertical structure of an exoplanet’s atmospheric jet stream”, Nature (2025); doi:10.1038/s41586-025-08664-1.
Prinoth et al, “Titanium chemistry of WASP-121 b with ESPRESSO in 4-UT mode”, Astronomy & Astrophysics (2025); doi:10.1051/0004-6361/202452405.