The James Webb Space Telescope (JWST) has captured unprecedented details of the ultra-hot gas giant WASP-121 b, revealing stark differences between its morning and evening atmospheric regions. Published in Nature Astronomy, this discovery offers the first observational confirmation that some exoplanets possess a pronounced dawn-dusk split in temperature and chemical composition, reshaping our understanding of extreme planetary environments.
Probing The Atmosphere Longitude By Longitude
Astronomers led byCyril Gapp at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, analyzed infrared light absorption from WASP-121 b as it transited its host star.
“With its unprecedented observational quality, JWST gives us the most detailed glimpses into distant planets to date: By measuring how starlight absorption changes as WASP-121 b rotates, we probe its atmosphere longitude by longitude,” said Gapp.
By observing subtle differences in light filtering through the planet’s atmosphere, the team detected variations between the morning (dawn) and evening (dusk) terminators. The evening terminator absorbs more starlight than the morning side, a result consistent with powerful eastward winds that carry intense heat from the dayside to the nightside. This uneven heating expands the atmosphere in the evening zone, making it a more effective absorber of stellar radiation.
Extreme Temperatures Across The Planet
WASP-121 b is a dramatic example of a tidally locked hot Jupiter, with one side permanently facing its star while the other remains in near-perpetual night.
“WASP-121b is particularly extreme, with average temperatures on the dayside hemisphere being around 2,770 Kelvin, while those on the nightside are closer to about 1,000 Kelvin,” explained co-author Tom Evans-Soma from the University of Newcastle, Australia.
These temperatures translate to nearly 2,500°C (4,525°F) on the dayside, and roughly 725°C (1,340°F) at night, creating a vast thermal gradient. JWST’s Near-Infrared Spectrograph (NIRSpec) allowed astronomers to not only detect these extremes but also analyze the chemical composition of the atmosphere, revealing that water molecules break down at high temperatures, while carbon monoxide signals rise mainly due to heating effects.
Observing The Transit And Atmospheric Dynamics
During a transit, WASP-121 b rotates slightly, allowing scientists to sample different slices of its atmosphere. The morning terminator leads the planet’s orbit, while the evening side trails. This dynamic view, combined with precise spectrographic measurements, captures the temperature and chemical variations across the planet’s edge.
The team found that brightness changes over time aligned with heat redistribution by planetary winds, confirming long-standing theoretical predictions. Minor differences between observed and modeled signals suggest that additional factors, such as mineral clouds, may modulate infrared absorption, especially on the morning side. Clouds composed of silicates could shield deeper, hotter layers, masking heat and creating the asymmetry observed.
Gaps In Atmospheric Models And Future Research
Existing models of hot gas planet atmospheres struggle to fully reproduce the observed differences between dawn and dusk. By incorporating cloud effects and updated simulations, the researchers achieved better alignment with JWST data, but more sophisticated modeling will be required to definitively characterize these extreme exoplanets.
The study also paves the way for future investigations into ultra-hot Jupiters, with additional targets identified for similar transit observations. Comparing multiple planets with these techniques may reveal universal patterns, or striking diversity, in longitudinal atmospheric structures, shedding light on the most extreme climates in the galaxy.
The Dawn Of High-Resolution Exoplanet Meteorology
With JWST’s extraordinary sensitivity, astronomers can now study exoplanet atmospheres with unprecedented spatial resolution, moving beyond average temperature measurements to reveal longitudinal variations and dynamic weather patterns. The results for WASP-121 b, as published in NatureAstronomy, highlight the intricate and violent climate of ultra-hot gas giants and offer a blueprint for the next generation of atmospheric studies.