Free Astronomy Magazine May-June 2025

23 MAY-JUNE 2025 ASTRO PUBLISHING year there lasts only about 30 Earth hours. Moreover, one side of the planet is scorching, as it is always facing the star, while the other side is much cooler. The team has now probed deep in- side Tylos’s atmosphere and re- vealed distinct winds in separate layers, forming a map of the atmos- phere’s 3D structure. It’s the first time astronomers have been able to study the atmosphere of a planet outside our Solar System in such depth and detail. “What we found was surprising: a jet stream rotates material around the planet’s equator, while a sepa- rate flow at lower levels of the at- mosphere moves gas from the hot side to the cooler side. This kind of climate has never been seen before on any planet,” says Seidel, who is also a researcher at the Lagrange Laboratory, part of the Observatoire de la Côte d’Azur, in France. The ob- served jet stream spans half of the planet, gaining speed and violently churning the atmosphere high up in the sky as it crosses the hot side of Tylos. “Even the strongest hurri- canes in the Solar System seem calm in comparison,” she adds. To uncover the 3D structure of the exoplanet’s atmosphere, the team used the ESPRESSO instrument on ESO’s VLT to combine the light of its four large telescope units into a sin- gle signal. This combined mode of the VLT collects four times as much light as an individual telescope unit, revealing fainter details. By observing the planet for one full transit in front of its host star, ESPRESSO was able to detect signa- tures of multiple chemical elements, probing different layers of the at- mosphere as a result. “The VLT en- abled us to probe three different layers of the exoplanet’s atmos- phere in one fell swoop,” says study co-author Leonardo A. dos Santos, an assistant astronomer at the Space T ylos (or WASP-121b) is a gaseous, giant exoplanet located some 900 light-years away in the constellation Puppis. Using the ESPRESSO instrument on ESO’s Very Large Telescope (VLT), scientists have been able to prove into its atmosphere, reveal- ing its 3D structure. This is the first time that this has been pos- sible on a planet outside of the Solar System. The atmosphere of Tylos is divided into three layers, with iron winds at the bot- tom, followed by a very fast jet stream of sodium, and finally an upper layer of hydrogen winds. This kind of climate has never been seen before on any planet. [ESO/M. Kornmesser]