Tylos (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, revealing its 3D structure. This is the first time that this has been possible on a planet outside of the Solar System. 

The atmosphere of Tylos is divided into three layers, with iron winds at the bottom, 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. Credit: ESO/ M Kornmesser.

In something seemingly straight out of science fiction astronomers have shown how an exoplanet’s atmosphere reveal a unique climate.

Astronomers have peered through the atmosphere of an exoplanet beyond the Solar System, mapping its 3D structure for the first time.

By combining all four telescope units of the European Southern Observatory’s Very Large Telescope (ESO’s VLT), they found powerful winds carrying chemical elements like iron and titanium, creating intricate weather patterns across the exoplanet’s atmosphere.

The discovery opens the door for detailed studies of the chemical makeup and weather of other alien worlds.

Lead author of the study, Julia Victoria Seidel, a researcher at the European Southern Observatory (ESO) in Chile, said: “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.”

This diagram shows the structure and motion of the atmosphere of the exoplanet Tylos (WASP-121b). The exoplanet is shown from above in this figure, looking at one of its poles. The planet rotates anti-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 transition 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 atmosphere 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 instrument 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 accelerates 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. Credit: ESO/ M Kornmesser.

Ultra-hot Jupiter

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 an exoplanet outside our Solar System in such depth and detail.

Seidel, who is also a researcher at the Lagrange Laboratory, part of the Observatoire de la Côte d’Azur, in France, said: “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.”

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.

Seidel added: “Even the strongest hurricanes in the Solar System seem calm in comparison.”

Fainter details

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.

Co-author Leonardo A dos Santos, an assistant astronomer at the Space Telescope Science Institute in Baltimore, United States, said: “The VLT enabled us to probe three different layers of the exoplanet’s atmosphere in one fell swoop.”

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.

Dos Santos added: “It’s the kind of observation that is very challenging to do with space telescopes, highlighting the importance of ground-based observations of exoplanets.”

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.

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, said: “It’s truly mind-blowing that we’re able to study details like the chemical make-up and weather patterns of a planet at such a vast distance.”

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.

Prinoth said: “The ELT will be a game-changer for studying exoplanet atmospheres.

“This experience makes me feel like we’re on the verge of uncovering incredible things we can only dream about now.”

The study is published in Nature.