Image an alien planet where savage winds rip through the atmosphere carrying molten iron and titanium particles. Such a planet actually exists. Known as WASP-121b, this exoplanet is located 900 light-years away in the constellation Puppis. Using the European Southern Observatory’s Very Large Telescope (VLT) in Chile, astronomers have mapped the 3D structure of the atmosphere of WASP-121b, marking the first study of the atmosphere of an exoplanet in such depth and detail.
It’s one of the wildest exoplanets astronomers have ever encountered.
“This planet’s atmosphere behaves in ways that challenge our understanding of how weather works — not just on Earth, but on all planets,” says Julia Victoria Seidel, a researcher at the European Southern Observatory (ESO) and lead author of the study. “It feels like something out of science fiction.”
A World of Extremes
WASP-121b, also known as Tylos, is an ultra-hot Jupiter, a gas giant that orbits its host star so closely that a year there lasts just 30 Earth hours. The planet is tidally locked, meaning one side perpetually faces the star, reaching scorching temperatures of up to 2,500°C — hot enough to vaporize metals like iron. The other side, shrouded in eternal night, is significantly cooler. This extreme temperature difference drives a chaotic and violent atmosphere.
By combining the light from all four telescope units of the VLT, the team used the ESPRESSO instrument to observe Tylos during a full transit across its star. This allowed them to probe the planet’s atmosphere in unprecedented detail, detecting the signatures of iron, sodium, hydrogen, and even titanium. These elements acted as tracers, enabling the researchers to map winds across three distinct layers of the atmosphere, with iron winds at the bottom, followed by a very fast jet stream of sodium, and finally an upper layer of hydrogen winds.
“What we found was surprising: 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,” says Seidel. “This kind of climate has never been seen before on any planet.”
The jet stream, which spans half the planet, reaches speeds of up to 70,000 kilometers per hour — nearly double the speed of the fastest winds previously recorded on an exoplanet earlier this year. “Even the strongest hurricanes in the solar system seem calm in comparison,” Seidel adds.
“It’s truly mind-blowing that we’re able to study details like the chemical makeup and weather patterns of a planet at such a vast distance,” says Bibiana Prinoth, a PhD student at Lund University and ESO, who led a companion study published in Astronomy & Astrophysics. “This experience makes me feel like we’re on the verge of uncovering incredible things we can only dream about now.”
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Tylos is obviously not your typical exoplanet. But it’s so bizarre and strange that it’s defying what astronomers thought they knew about planetary formation. Unlike the atmospheres of planets in our solar system, where jet streams are typically driven by internal temperature differences, Tylos’s jet stream appears to be influenced by its star’s intense heat and possibly its magnetic field. “What we see now is actually exactly the inverse of what comes out of theory,” says Seidel.
It’s a strange world, indeed — and many more like it likely lie beyond our solar system. While the VLT has proven capable of studying the atmospheres of hot Jupiters like Tylos, smaller, Earth-like planets remain out of reach — for now. The construction of next-generation telescopes, such as ESO’s Extremely Large Telescope (ELT) in Chile’s Atacama Desert, promises to bridge this gap. Best of all, with its advanced instruments, the ELT will be able to probe the atmospheres of rocky exoplanets, potentially uncovering signs of habitability or even life.
“The ELT will be a game-changer for studying exoplanet atmospheres,” says Prinoth. “This is just the beginning of what we can achieve.”
The findings appeared in the journal Nature.