‘Out of science fiction’: First 3D observations of an exoplanet’s atmosphere reveal a unique climate

“This planet’s atmosphere behaves in ways that challenge our understanding of how weather works — not just on Earth, but on all planets. It feels like something out of science fiction,” says Julia Victoria Seidel, a researcher at the European Southern Observatory (ESO) in Chile and lead author of the study, published today in Nature.

The planet, WASP-121b (also known as Tylos), is some 900 light-years away in the constellation Puppis. It’s an ultra-hot Jupiter, a gas giant orbiting its host star so closely that a 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 inside Tylos’s atmosphere and revealed distinct winds in separate layers, forming a map of the atmosphere’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 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,” says Seidel, who is also a researcher at the Lagrange Laboratory, part of the Observatoire de la Côte d’Azur, in France. The observed 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 hurricanes 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 single 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 signatures of multiple chemical elements, probing different layers of the atmosphere as a result.

“The VLT enabled us to probe three different layers of the exoplanet’s atmosphere in one fell swoop,” says study co-author Leonardo A. dos Santos, an assistant astronomer at the Space Telescope Science Institute in Baltimore, United States. The team tracked the movements of iron, sodium and hydrogen, which allowed 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 importance of ground-based observations of exoplanets,” he adds.

Interestingly, the observations also revealed the presence of titanium just below the jet stream, as highlighted in a companion study published in Astronomy and Astrophysics. This was another surprise since previous observations of the planet had shown this element to be absent, possibly because it’s hidden deep in the atmosphere.

“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, 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 currently under construction in Chile’s Atacama Desert, and its ANDES instrument. “The ELT will be a game-changer for studying exoplanet atmospheres,” says Prinoth. “This experience makes me feel like we’re on the verge of uncovering incredible things we can only dream about now.”

The companion research, uncovering the presence of titanium, was published in the journal Astronomy & Astrophysics in a paper titled “Titanium chemistry of WASP-121 b with ESPRESSO in 4-UT mode” (doi: 10.1051/0004-6361/202452405)