Free Astronomy Magazine May-June 2025

24 MAY-JUNE 2025 ASTRO PUBLISHING “It’s truly mind-blowing that we’re able to study details like the chem- ical makeup and weather patterns of a planet at such a vast distance,” says Bibiana Prinoth, a PhD student at Lund University, Sweden, and ESO, who led the companion study and is a co-author of the Nature paper. To uncover the atmosphere of smaller, Earth-like planets, though, larger telescopes will be needed. They will include ESO’s Extremely Large Telescope (ELT), which is cur- rently under construction in Chile’s Atacama Desert, and its ANDES in- strument. “The ELT will be a game- changer for studying exoplanet atmospheres,” says Prinoth. “This experience makes me feel like we are on the verge of uncovering in- credible things we can only dream about now.” T his diagram shows the structure and motion of the atmosphere of the exo- planet Tylos (WASP-121b). The exoplanet is shown from above in this figure, looking at one of its poles. The planet rotates counter-clockwise, in such a way that it always shows the same side to its parent star, so it’s always day on one half of the planet and night on the other. The tran- sition between night and day is the “morning side” while the “evening side” represents the transition between day and night; its morning side is to the right and its evening side to the left. As the planet crosses in front of its host star, atoms in the planet’s at- mosphere absorb specific colours or wavelengths of the star’s light, which can be measured with a spectrograph. From this data –– obtained in this case with the ESPRESSO instru- ment on ESO’s Very Large Telescope –– astronomers can reconstruct the composition and velocity of different layers in the atmosphere. The deepest layer is a wind of iron that blows away from the point of the planet where the star is directly overhead. Above this layer there is a very fast jet of sodium that moves faster than the planet rotates. This jet actually ac- celerates as it moves from the morning side to the evening side of the planet. Finally, there is an upper layer of hydrogen wind blowing outwards. This hydrogen layer overlaps with the sodium jet below it. [ESO/M. Kornmesser] Telescope Science Institute in Balti- more, United States. The team tracked the movements of iron, sodium and hydrogen, which al- lowed them to trace winds in the deep, mid and shallow layers of the planet’s atmosphere, respectively. “It’s the kind of observation that is very challenging to do with space telescopes, highlighting the impor- tance of ground-based observations of exoplanets,” he adds. Interest- ingly, the observa- tions also revealed the presence of tita- nium just below the jet stream, as high- lighted in a compan- ion study published in Astronomy and Astrophysics . This was another surprise since previous obser- vations of the planet had shown this ele- ment to be absent, possibly because it’s hidden deep in the atmosphere. T his video illustrates how astronomers have revealed the 3D structure of an exoplanet’s atmosphere for the first time. [ESO] !