Updated

Updated · ScienceBlog.com · Jun 10

JWST Detects 30-Degree Atmospheric Shift on WASP-121b, Exposing Hotter Dusk Than Dawn

Updated

Updated · ScienceBlog.com · Jun 10

JWST Detects 30-Degree Atmospheric Shift on WASP-121b, Exposing Hotter Dusk Than Dawn

Q: What exotic clouds on this searing exoplanet are causing our best atmospheric models to fail?

The clouds most likely tripping up the models are **silicate “rock clouds”**—condensed mineral particles, likely including magnesium silicate, forming on the cooler **morning side** of WASP-121 b. JWST observations showed the planet’s evening terminator is hotter and more expanded than the morning terminator, with a stronger mismatch than theory predicted. Researchers say the likely reason is that current atmospheric models do not fully capture how these rock clouds form, move and evaporate in such extreme conditions. On the cooler dawn side, silicate material can condense into clouds; on the hotter dusk side, those clouds are expected to thin or evaporate. That matters because the clouds can **mask heat and distort the atmospheric signal**. They can make the morning side appear cooler or more opaque than it otherwise would, exaggerating the contrast with the hotter evening side. This helps explain why the observed asymmetry was stronger than models forecast. The broader evidence from JWST studies of other ultra-hot Jupiters points the same way: mineral cloud cycles can be highly uneven across a planet, especially on tidally locked worlds with fierce winds and huge day-night temperature differences. In short, the “exotic clouds” are not water clouds but **vaporized-rock clouds made of silicate minerals**, and their complex behavior is exposing a major weakness in current exoplanet atmosphere models.

3 articles · Updated · ScienceBlog.com · Jun 10

Four and a half hours of JWST transit data showed WASP-121b rotating about 30 degrees, letting astronomers separate its morning and evening atmospheric edges for the first direct rotational-transit measurement.
The evening terminator absorbed more starlight than the morning side, indicating eastward winds carry heat toward dusk and puff up that side of the ultrahot Jupiter’s atmosphere.
Carbon monoxide strengthened as the evening side rotated into view while water weakened, a pattern consistent with higher temperatures there tearing H2O apart through thermal dissociation.
Models reproduced the asymmetry but underestimated its size, pointing to possible silicate-rock clouds on the cooler morning side that current simulations largely omit.
At 880 light years away, WASP-121b now serves as a test case for using transit rotation to compare dawn-and-dusk weather on other ultrahot gas giants.

Sources

ScienceBlog.com1d ago

WASP-121b's Atmospheric Rotation Reveals Distinct Dawn and Dusk Weather Patterns

The Daily Galaxy --Great Discoveries Channel1d ago

JWST Reveals Dramatic Day-Night Divide on Ultra-Hot Exoplanet WASP-121 b

Phys.org1d ago

JWST reveals dawn-dusk atmosphere split on ultra-hot exoplanet WASP-121 b

What exotic clouds on this searing exoplanet are causing our best atmospheric models to fail?

Now that we can map alien weather longitude-by-longitude, what other extreme worlds will be targeted next?

JWST Unveils Extreme Atmospheric Dynamics and Persistent Helium Escape on WASP-121b: New Insights and Modeling Challenges (2026)

Overview

The James Webb Space Telescope (JWST) is transforming our understanding of ultra-hot Jupiters like WASP-121b by enabling scientists to refine models and develop new techniques for studying these extreme exoplanets. Researchers are especially focused on probing the terminator regions—the boundaries between day and night—which helps reveal the complex longitudinal structure of these planets. By improving analytical models and observational methods, scientists aim to uncover important similarities and differences among ultra-hot gas planets. These advancements are crucial for maximizing the scientific value of JWST’s observations and deepening our knowledge of exoplanetary atmospheres and evolution.

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