To the point:
- Ultrahot exoplanet, atmospheric differences: Researchers discovered clear differences in the atmosphere between the morning and evening sides of the ultrahot gas planet WASP-121 b using the James Webb Space Telescope (JWST).
- Temperature and chemical variations: The evening side absorbs more infrared light due to higher temperatures caused by strong winds moving heat eastward, while water molecules decrease in the evening terminator due to high temperatures breaking them apart.
- Planetary rotation and observation method: WASP-121 b’s synchronous rotation reveals different atmospheric regions during transit, allowing scientists to analyse changes in light absorption over time and longitude.
Astronomers find variations between the morning and the evening conditions of an ultra-hot exoplanet.
Astronomers have revealed distinct differences in atmospheric conditions between the morning and evening transition zones of the ultra-hot gas planet WASP-121 b, which separate day from night, commonly called terminators. This achievement was only possible due to the
unmatched sensitivity of the James Webb Space Telescope (JWST). Led by Cyril Gapp, a PhD student at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, a team of researchers detected this phenomenon, which had previously been predicted by theoretical
computations.
Confirmation of variations between dusk and dawn
The discovery corresponds to an asymmetry in the absorption of infrared light received from the host star, which is partially filtered through the planet’s atmosphere during its transit. The researchers interpret this as the result of non-uniform temperatures and chemical compositions in the exoplanet’s atmosphere.
With its unprecedented observational quality, JWST gives us the most detailed glimpses into distant planets to date: By measuring how star light absorption changes as WASP-121 b rotates, we probe its atmosphere longitude by longitude. Cyril Gapp, MPIA
The data indicate that the evening terminator absorbs more light than the morning side, consistent with the commonly accepted picture of powerful winds that transport intense heat from the day to the night side. Hot winds follow the planet’s rotation eastward, which heats the evening zone. With rising temperatures, this region is bound to expand, increasing the planet’s cross-section and allowing it to absorb stellar radiation more efficiently.
With its unprecedented observational quality, JWST gives us the most detailed glimpses into distant planets to date: By measuring how star light absorption changes as WASP-121 b rotates, we probe its atmosphere longitude by longitude. Cyril Gapp, MPIA
The data indicate that the evening terminator absorbs more light than the morning side, consistent with the commonly accepted picture of powerful winds that transport intense heat from the day to the night side. Hot winds follow the planet’s rotation eastward, which heats the evening zone. With rising temperatures, this region is bound to expand, increasing the planet’s cross-section and allowing it to absorb stellar radiation more efficiently.
Besides a general slight reduction in brightness towards the end of the transit, the data obtained by JWST’s NIRSpec (Near-infrared spectrograph) instrument also reveal an increase in the carbon monoxide (CO) signal. However, this appears to be a temperature effect, not related to an increase in carbon monoxide molecules.
In contrast, the amount of water (H2O) in the atmosphere appears to drop, which the astronomers interpret as a real decrease in water molecules. The temperatures in the upper atmosphere are high enough to break water molecules into their constituents. This result again corroborates the existence of hot winds heating the evening terminator region.
Two extreme sides of an ultra-hot planet
To detect these minute variations, the astronomers exploited a peculiar behaviour of hot gas planets. The proximity to their host stars slowly synchronizes their spin and orbital motion via tidal forces,such that eventually one rotation takes as long as one revolution. Finally, these planets exhibit two distinct hemispheres: a hot side constantly facing the star and an opposite, darker and cooler side.
“WASP-121 b is particularly extreme, with average temperatures on the dayside hemisphere being around 2770 Kelvin, while those on the nightside are closer to about 1000 Kelvin,” co-author Tom Evans-Soma from the University of Newcastle, Australia, explains. He previously determined the planet’s temperature range and is also affiliated with MPIA. These values translate to almost 2500 degrees Celsius, or about 4525 degrees Fahrenheit, on the dayside, and approximately 725 degrees Celsius, or 1340 degrees Fahrenheit, at night.
When astronomers observe such a planet transiting in front of a star, the planet rotates slightly between the points of ingress and egress, revealing different fractions of its atmosphere. While the planet mostly presents its night side, our point of view permits glimpses beyond the dusk and dawn towards the bright dayside, depending on the transit’s progress. The zone leading the planet’s orbit corresponds to the morning side, and the one trailing is the evening side.
Apart from recording the measured brightness variation over time, spectrographs break light into smaller components, which physicists call a spectrum, much as a prism produces a rainbow-like distribution of colours. Since atmospheric gases absorb light at distinct colours or wavelengths, a detailed analysis reveals their chemical composition.
“WASP-121 b is particularly extreme, with average temperatures on the dayside hemisphere being around 2770 Kelvin, while those on the nightside are closer to about 1000 Kelvin,” co-author Tom Evans-Soma from the University of Newcastle, Australia, explains. He previously determined the planet’s temperature range and is also affiliated with MPIA. These values translate to almost 2500 degrees Celsius, or about 4525 degrees Fahrenheit, on the dayside, and approximately 725 degrees Celsius, or 1340 degrees Fahrenheit, at night.
When astronomers observe such a planet transiting in front of a star, the planet rotates slightly between the points of ingress and egress, revealing different fractions of its atmosphere. While the planet mostly presents its night side, our point of view permits glimpses beyond the dusk and dawn towards the bright dayside, depending on the transit’s progress. The zone leading the planet’s orbit corresponds to the morning side, and the one trailing is the evening side.
Apart from recording the measured brightness variation over time, spectrographs break light into smaller components, which physicists call a spectrum, much as a prism produces a rainbow-like distribution of colours. Since atmospheric gases absorb light at distinct colours or wavelengths, a detailed analysis reveals their chemical composition.
Elapsed time converts to longitude
Hence, the variation along the direction of rotation translates into a time-dependent change of the filtered signal. In the case of WASP-121 b the rotation angle during a full transit amounts to about 30 degrees, which is sufficient to probe the morning (dawn) and evening (dusk) terminators with high precision in longitude.
Astronomers usually average the measurements over the entire transit to achieve a clearer signal. However, to determine how the signal changes during the planet’s trajectory across the star, Gapp and his colleagues allowed for a temporal variation while the planet rotates. By applying statistical methods, they found that their procedure provides a significantly better fit to the data, indicating that they indeed detected a significant variation.
Astronomers usually average the measurements over the entire transit to achieve a clearer signal. However, to determine how the signal changes during the planet’s trajectory across the star, Gapp and his colleagues allowed for a temporal variation while the planet rotates. By applying statistical methods, they found that their procedure provides a significantly better fit to the data, indicating that they indeed detected a significant variation.
Notable gaps in atmospheric models
To verify the measured temperatures that would cause local expansion, the astronomers ran models simulating heat distribution in the upper layers of a gas planet, depending on the planet's properties and the constellation of the planet and its host star. While these atmospheric models confirmed the asymmetric effect caused by spatial temperature variations, the data revealed a larger signal amplitude than the models predicted.
The astronomers suspected that cooling mechanisms at the morning terminator might be at work that the models didn’t account for. Previous studies have indicated that clouds may be present, albeit composed not of water droplets but of minerals such as silicates. Clouds can efficiently shield infrared light emitted from hot gaseous layers below, mimicking lower temperatures. Infamously, simulating the physics of clouds, condensation, and evaporation in a dynamic environment is hard. Therefore, physical models commonly applied to exoplanet atmospheres, such as the one used in this study, do not account for clouds, which can yield unrealistic results.
After tweaking the simulation to better approximate the effect of clouds on infrared radiation from deeper layers, the results were more consistent with observations. However, only more sophisticated models will be able to confidently confirm the presence of clouds.
The astronomers suspected that cooling mechanisms at the morning terminator might be at work that the models didn’t account for. Previous studies have indicated that clouds may be present, albeit composed not of water droplets but of minerals such as silicates. Clouds can efficiently shield infrared light emitted from hot gaseous layers below, mimicking lower temperatures. Infamously, simulating the physics of clouds, condensation, and evaporation in a dynamic environment is hard. Therefore, physical models commonly applied to exoplanet atmospheres, such as the one used in this study, do not account for clouds, which can yield unrealistic results.
After tweaking the simulation to better approximate the effect of clouds on infrared radiation from deeper layers, the results were more consistent with observations. However, only more sophisticated models will be able to confidently confirm the presence of clouds.
A blueprint for future studies
Model updates will also improve future investigations using this method. The astronomers have already identified additional suitable targets within the required temperature range and rotation speed to successfully probe the terminator regions. This will help them establish a sample of ultrahot gas planets, revealing their longitudinal structure, and potentially discover similarities and differences among these extreme worlds.
Additional information
MPIA astronomers involved in this study were Cyril Gapp (also Heidelberg University), Thomas M. Evans-Soma (also University of Newcastle, Australia), and Eva-Maria Ahrer.
Other researchers were: Aurélien Falco (Sorbonne Université, Paris, France), David K. Sing (Johns Hopkins University, Baltimore, USA), Shashank Dholakia (University of Queensland, St. Lucia, Australia), Vivien Parmentier (Université de la Côte d’Azur, Nice, France), Jérémy Leconte (Université Bordeaux, France), and Guangwei Fu (Johns Hopkins University).
The JWST observations used in this study were conducted as part of GO program #1729 (PI: Thomas Evans-Soma, Co-PI: Tiffany Kataria) titled “A NIRSpec Phase Curve for the ultrahot Jupiter WASP-121b” and GTO program #1201 (PI: David Lafreniere) labelled “NIRISS Exploration of the Atmospheric diversity of Transiting exoplanets (NEAT).”
NIRSpec (Near Infrared Spectrograph) was built by European industry to the European Space Agency’s (ESA) specifications and managed by the ESA JWST Project at ESTEC (European Space Research and Technology Centre), the Netherlands. The prime contractor was Airbus Defence and Space in Ottobrunn, Germany. MPIA contributed to the development and manufacture of NIRSpec’s filter and grating wheels. The NIRSpec detector and micro-shutter array subsystems were provided by NASA’s Goddard Space Flight Center (GSFC).
The James Webb Space Telescope is the world’s leading observatory for space research. It is an international programme led by NASA and its partners ESA and CSA (Canadian Space Agency).
Other researchers were: Aurélien Falco (Sorbonne Université, Paris, France), David K. Sing (Johns Hopkins University, Baltimore, USA), Shashank Dholakia (University of Queensland, St. Lucia, Australia), Vivien Parmentier (Université de la Côte d’Azur, Nice, France), Jérémy Leconte (Université Bordeaux, France), and Guangwei Fu (Johns Hopkins University).
The JWST observations used in this study were conducted as part of GO program #1729 (PI: Thomas Evans-Soma, Co-PI: Tiffany Kataria) titled “A NIRSpec Phase Curve for the ultrahot Jupiter WASP-121b” and GTO program #1201 (PI: David Lafreniere) labelled “NIRISS Exploration of the Atmospheric diversity of Transiting exoplanets (NEAT).”
NIRSpec (Near Infrared Spectrograph) was built by European industry to the European Space Agency’s (ESA) specifications and managed by the ESA JWST Project at ESTEC (European Space Research and Technology Centre), the Netherlands. The prime contractor was Airbus Defence and Space in Ottobrunn, Germany. MPIA contributed to the development and manufacture of NIRSpec’s filter and grating wheels. The NIRSpec detector and micro-shutter array subsystems were provided by NASA’s Goddard Space Flight Center (GSFC).
The James Webb Space Telescope is the world’s leading observatory for space research. It is an international programme led by NASA and its partners ESA and CSA (Canadian Space Agency).
Contacs:
Dr. Markus Nielbock
Press and outreach officer
Tel: +49 6221 528-134
Email: pr@mpia.de
MPIA press team
Max Planck Institute for Astronomy, Heidelberg, Germany
Cyril Gapp
Tel: +49 6221 528-328
Email: gapp@mpia.de
Cyril Gapp / MPIA
Max Planck Institute for Astronomy, Heidelberg, Germany
Dr. Thomas M. Evans-Soma
Tel: +61 2 4055-3229
Email: tom.evans-soma@newcastle.edu.au
Homepage Thomas Evans-Soma
School of Information and Physical Sciences, The University of Newcastle, Callaghan, Australia
Max-Planck-Institut für Astronomie, Heidelberg, Deutschland
Original publication
Cyril Gapp, Aurélien Falco, Thomas M. Evans-Soma, et al. (incl. Eva-Maria Ahrer)
Atmospheric asymmetries in WASP-121 b revealed by rotational transits detected with JWST
Nature Astronomy (2026). DOI: 10.1038/s41550-026-02887-6
Source
Orbit of WASP-121 b around its host star (Video)
This animation illustrates the orbit of the exoplanet WASP-121 b around its host star, as well as its tidal locking. The perspective shifts from a top-down view of the orbit to the alignment during observation. During the transit, it becomes apparent that at the beginning and the end of the passage, a portion of the planet's illuminated dayside appears as a narrow crescent. T. Müller (MPIA/HdA)
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mpia-pm_wasp121b_2026_animation1080p (8.3 MB)
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mpia-pm_wasp121b_2026_fig1_de 455.47 kB
mpia-pm_wasp121b_2026_fig1_en 455.92 kB
mpia-pr_wasp-121b_mikal-evans_2022_teaser 1.6 MB
