The discovery comes from measurements of infrared starlight passing through the planet's atmosphere as WASP-121 b crossed in front of its host star. By examining how the atmosphere filtered that light during the transit, researchers found an uneven pattern of absorption.

According to the team, the asymmetry is best explained by differences in temperature and chemistry between the morning and evening sides of the planet.

"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," said Cyril Gapp, MPIA

The observations show that the evening terminator absorbs more light than the morning terminator. This matches current ideas about powerful atmospheric winds that transport heat from the intensely hot day side toward the cooler night side. Because those winds move eastward in the direction of the planet's rotation, they heat the evening region more strongly.

As temperatures rise, the atmosphere expands. A larger atmosphere presents a bigger cross-section to incoming starlight, allowing it to absorb more radiation.

Data collected with JWST's NIRSpec (Near-infrared spectrograph) instrument also revealed a stronger carbon monoxide (CO) signal toward the end of the transit. Researchers believe this change is caused by temperature effects rather than an actual increase in carbon monoxide abundance.

Water (H₂O) told a different story. The observations suggest that water molecules become less abundant in the hotter atmospheric regions. Scientists interpret this as a genuine decrease in water because temperatures in the upper atmosphere are high enough to break water molecules apart into their constituent elements. This finding provides additional evidence that hot winds are warming the evening terminator.

A Planet of Permanent Day and Night

Detecting such subtle atmospheric differences required taking advantage of a common characteristic of close-orbiting gas giants.

Over time, tidal forces synchronize the planet's rotation with its orbit, causing one rotation to take the same amount of time as one trip around its star. As a result, one hemisphere constantly faces the star while the other remains in perpetual darkness.

"WASP-121b 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.

Those temperatures correspond to nearly 2,500 degrees Celsius (4,525 degrees Fahrenheit) on the day side and about 725 degrees Celsius (1,340 degrees Fahrenheit) on the night side.

As the planet transits its star, it rotates slightly between the beginning and end of the event. This small rotation allows astronomers to observe different sections of the atmosphere. Although the night side remains mostly in view, scientists can also glimpse portions of the dawn and dusk regions and, depending on the stage of the transit, even small areas closer to the blazing day side.

The leading side of the orbit corresponds to the morning terminator, while the trailing side corresponds to the evening terminator.

Turning Transit Time Into Atmospheric Maps

To study the atmosphere, researchers analyzed how the planet's brightness changed throughout the transit. They also examined spectra, which are created when light is separated into its component wavelengths, much like a prism creating a rainbow.

Different gases absorb specific wavelengths of light, allowing scientists to identify the chemicals present in the atmosphere.

Because the planet rotates as it moves across the face of its star, changes in the signal over time correspond to different longitudes on the planet. During a complete transit, WASP-121 b rotates by roughly 30 degrees, enough to distinguish the morning (dawn) and evening (dusk) terminators with remarkable precision.

Astronomers often combine all transit measurements into a single average signal to improve clarity. In this study, however, Gapp and his colleagues allowed the signal to vary over time as the planet rotated. Statistical analysis showed that this approach matched the observations significantly better, providing strong evidence that the atmospheric differences are real.

Possible Clouds Missing From Current Models

To understand the observations, the researchers ran computer simulations of heat transport in the upper atmosphere of a gas giant. The models successfully reproduced the general asymmetry caused by temperature differences, but the observed effect was stronger than predicted.

That discrepancy suggests that additional processes may be influencing the atmosphere.

One possibility is that the morning terminator experiences extra cooling that current models fail to capture. Previous research has suggested that clouds may exist in these regions. Unlike Earth's clouds, they would likely consist of minerals such as silicates rather than water droplets.

Such clouds could block infrared radiation coming from hotter layers below, making the atmosphere appear cooler than it actually is.

Modeling cloud formation, condensation, and evaporation in a rapidly changing environment remains extremely difficult. As a result, many exoplanet atmosphere models, including those used in this study, do not fully incorporate cloud physics.

When the team modified their simulations to approximate the effects of clouds, the results aligned more closely with the observations. Even so, more advanced modeling will be needed before researchers can confidently confirm the presence of clouds on WASP-121 b.

A New Way To Study Extreme Exoplanets

Future improvements to atmospheric models could make this technique even more powerful.

The researchers have already identified other ultra-hot gas giants with suitable temperatures and rotation rates for similar studies. By applying the same method to a larger sample of planets, astronomers hope to compare how atmospheric conditions change across different worlds and gain a deeper understanding of their three-dimensional structures.

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 program led by NASA and its partners ESA and CSA (Canadian Space Agency).