Astronomers using the James Webb Space Telescope (JWST) have found a clear difference between the morning and evening sides of the atmosphere on the ultra-hot exoplanet WASP-121 b. The planet does not just show a simple day and night contrast. Even the narrow boundary between them behaves differently depending on whether it is dawn or dusk.
The result comes from transit observations, where scientists analyze starlight that passes through a planet’s atmosphere as it moves across its host star. According to the study published in Nature Astronomy, this approach now allows astronomers to examine exoplanet atmospheres with a level of detail that separates different regions along the planet’s edge instead of blending everything together.
WASP-121 b is a gas giant orbiting extremely close to its star. One hemisphere is permanently exposed to intense heat, while the opposite side remains in constant darkness. This creates extreme temperature differences and strong atmospheric winds that shape how heat and chemicals move through the planet.
A Clear Difference Between Dawn And Dusk
Using JWST’s NIRSpec instrument, researchers tracked how infrared light changed during the transit. Based on the study, published in Nature Astronomy, the evening side of the terminator absorbs more starlight than the morning side. This indicates that the two regions are not physically identical.
The planet rotates by about 30 degrees during the transit. That movement lets scientists separate signals from the leading edge, which corresponds to morning, and the trailing edge, which corresponds to evening. The evening region consistently shows stronger absorption, suggesting higher temperatures or an expanded atmospheric layer. Lead author Cyril Gapp from the Max Planck Institute for Astronomy explained:
“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.”
Researchers link this pattern to strong winds that transport heat from the extremely hot dayside toward the nightside. This shifts the warmest region toward the evening terminator and helps explain the asymmetry in the data.
Chemical Signals Do Not Behave The Same Way
The observations also show that different molecules respond in different ways across the planet.Carbon monoxide (CO) appears stronger toward the end of the transit. As explained by the researchers, this does not necessarily mean there is more CO in that region. Instead, temperature changes likely affect how the gas interacts with infrared light, which alters the strength of the signal.
Water shows a different pattern. The data suggest a real decrease in water molecules in parts of the upper atmosphere. At the extreme temperatures on WASP-121 b, water can break apart into hydrogen and oxygen, which changes the chemical composition of the atmosphere.
The temperature contrast on the planet is extreme. The dayside reaches about 2770 K (around 2500°C), while the nightside is closer to 1000 K (around 725°C). Co-author Tom Evans-Soma from the University of Newcastle said:
“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.” This difference drives strong global winds that redistribute heat across the planet.
Models, Clouds, And What Is Still Missing
When scientists compared the JWST data with atmospheric models, they found that the general pattern could be reproduced, but not the full strength of the difference between morning and evening. The real atmosphere shows a stronger contrast than simulations predict.
One possible explanation involves mineral clouds, likely made of silicates. These clouds may form more easily on the cooler morning side and block infrared radiation coming from deeper layers. That would make the morning region appear cooler than it actually is in the observations.
When simplified cloud effects are included in the models, the results move closer to what JWST observes. Still, cloud formation and behavior in such extreme environments remains difficult to model with precision, so the picture is not fully settled.
