An extreme exoplanet has gotten even more extreme in the eyes of astronomers. In mapping the 3D structure of its atmosphere for the first time, scientists have discovered that a high-speed atmospheric jet whips around WASP-121b, an ultrahot gas giant planet. They have also confirmed the presence of titanium in the planet’s atmosphere, solving a yearslong mystery about the planet’s atmospheric chemistry.
“This planet’s atmosphere behaves in ways that challenge our understanding of how weather works—not just on Earth, but on all planets,” said Julia Seidel, an astrophysicist at the European Southern Observatory (ESO) in Santiago de Chile and colead researcher on the discoveries. “It feels like something out of science fiction.”
An ESPRESSO Boost
WASP-121b orbits a star bigger and hotter than the Sun in just 1.27 days, making its atmosphere a blistering 2,085°C (3,785°F). The planet is about 75% bigger but just 16% heavier than Jupiter, a so-called marshmallow planet. As WASP-121b zooms around its star, its low-density atmosphere distorts into the shape of an American or Australian football.
Astronomers have studied this planet extensively with ground- and space-based telescopes since it was discovered in 2016.
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The planet is big and bright and easy to see against its similarly big and bright star. That made it an easy pick when astronomers needed to test a new observing mode of ESO’s Very Large Telescope (VLT), a set of four 8-meter telescopes in Chile’s Atacama Desert.
“They wanted to go for a safe choice when trying out the mode. WASP-121b fell into their lap,” said Bibiana Prinoth, an astrophysicist at Lund University in Sweden and colead researcher on the discoveries.
In its new capacity, VLT’s four telescopes combine their observing power and achieve the resolution of a telescope twice the size. That light can then be fed into the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) instrument, which produces a high-resolution visible-light spectrum.
Testing that mode during the instrument’s commissioning phase in 2018, astronomers observed WASP-121b cross in front of its star and imprint its atmospheric spectrum on the star’s light.
“I got my data as a follow-up to the strange thing that they saw completely by accident.”
When Seidel looked at the ESPRESSO spectrum, she noticed something weird about the spectral lines from sodium. She proposed a second set of VLT observations, which took place in 2023.
“I got my data as a follow-up to the strange thing that they saw completely by accident,” Seidel said.
The researchers homed in on the spectral signatures of iron, sodium, and hydrogen, which were emitted from different depths within WASP-121b’s atmosphere. The planet’s puffiness helped turn its transmission spectrum into a 3D map of its atmosphere.
As a star’s light passes through a planet’s atmosphere, each wavelength of that light penetrates down to a different atmospheric depth, called the optical depth. An element’s strongest emission lines come from the physical depth that matches its optical depth. For planets of average density, those optical depths correspond to roughly the same physical depths, so transmission spectra map an atmosphere in one or two dimensions.
But for marshmallows like WASP-121b, those optical depths are more physically spread out, allowing astronomers to use the transmission spectrum to create a 3D map. Using atmospheric circulation models, the researchers traced the movement of material in three layers in the planet’s upper atmosphere—its “outer whimsical shell,” Seidel called it.
Superspeed Jet and Hidden Titanium
In the deepest observed layer, traced with iron’s spectral signature, the team found that heat flows from the planet’s permanent dayside to its permanent nightside both clockwise and counterclockwise. This behavior is typical for hot gas giant planets that, like WASP-121b, are tidally locked to their star, Seidel explained. In the shallowest observed layer, traced with hydrogen’s spectral lines, the team confirmed the puffy planet’s football shape, which is most pronounced in the outermost layer.
In the middle layer, traced with sodium, an atmospheric jet zips around the planet’s equator faster than the planet’s rotation. The oddity that first caught Seidel’s attention was the atmospheric jet distorting sodium’s well-known spectral lines.
The observation is a first, Seidel said. In solar system planets, atmospheric jets flow through deeper layers, and shallow layers dominate heat transport. For WASP-121b, “it’s flipped, and that’s weird, and we don’t know why.”
“We have given it to the theorists, and they have to fix it now.”
What’s more, the jet accelerates as it travels across the planet’s dayside, speeding up from 14 kilometers per second in the morning to about 27 kilometers per second in the evening.
“We have given it to the theorists, and they have to fix it now,” Prinoth joked.
That the upper and lower atmospheres flow so distinctly suggests that different mechanisms drive wind in each layer, said Elspeth Lee, an exoplanet climate modeler at the University of Bern in Switzerland who was not involved with this research. “This has been suggested in previous 3D atmospheric modelling efforts…but these observations provide much needed observational evidence of this phenomenon and provide guidance as to where 3D models require future improvement.”
Diagram of a planet’s atmosphere. The deepest layer is coded in green, the middle in yellow, and the upper in blue.
The upper atmosphere of WASP-121b contains three distinct layers, diagrammed here in a top-down view from the planet’s pole. In the deepest layer, traced with spectral features from iron, winds carry heat in both directions from the dayside to the nightside. In the middle layer, traced with sodium, an atmospheric jet speeds around the planet’s equatorial region faster than the planet’s rotation. In the upper layer, traced with hydrogen, the star’s intense radiation puffs up the atmosphere to a low density, distorts the planet’s shape, and causes some of the atmosphere to be lost to space. Credit: ESO/M. Kornmesser, CC BY 4.0
The observations also revealed that WASP-121b’s atmosphere contains titanium, which is known to shape the temperature and pressure structures of hot Jupiter atmospheres. Astronomers have debated for years whether WASP-121b has titanium—some telescopes could see it, whereas others could not. Prinoth explained that the signal from titanium was weaker than they expected it to be, which might explain conflicting past reports.
“There must be some mechanism that depletes it from the atmosphere or from the gas phase,” Prinoth said.
“This new [VLT] capability allowed a much deeper dive into the atmospheric composition and dynamical structure of WASP-121b, a canonical and well-studied ultrahot Jupiter, than ever before,” Lee said. This is also the strongest evidence yet that WASP-121b’s titanium exists and is just trapped deep within the atmosphere, “hiding it from being detected at expected levels,” she added.
These results were published in Nature and Astronomy and Astrophysics.
These observations of WASP-121b push the boundaries of what current telescopes and atmospheric models can map within exoplanetary atmospheres, Prinoth said. Prinoth, Seidel, and their colleagues plan to use the new VLT observing mode to study planets a bit smaller than WASP-121b and those on peculiar orbits.
Those observations will help astronomers prepare for the next generation of giant ground-based telescopes, like the Giant Magellan Telescope or ESO’s upcoming Extremely Large Telescope (ELT), which will provide even higher resolution spectra and enable this kind of 3D atmospheric study of planets that more closely resemble Earth.
“These two studies pave the way for a super promising ELT era of exoplanet atmosphere characterization,” Lee said.
—Kimberly M. S. Cartier (@astrokimcartier.bsky.social), Staff Writer
Citation: Cartier, K. M. S. (2025), First 3D map of exoplanet weather reveals superfast jet, Eos, 106,https://doi.org/10.1029/2025EO250103. Published on 17 March 2025.
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