The James Webb Space Telescope (JWST) has mapped distinct atmospheric differences between the morning and evening “twilight zones” of WASP-121 b, an ultra-hot gas giant. According to research led by Cyril Gapp of the Max Planck Institute for Astronomy, the evening terminator shows higher carbon monoxide levels and lower water concentrations than the morning side due to extreme heat and fierce winds.
Mapping the Twilight Zones of WASP-121 b
WASP-121 b exists in a state of tidal locking, meaning one hemisphere permanently faces its host star while the other remains in eternal darkness. This creates a brutal temperature gradient. The dayside reaches roughly 2,500 degrees Celsius, while the night side sits at 725 degrees, a gap of nearly 1,800 degrees across a single world.
To understand this environment, Cyril Gapp and his team tracked the planet as it passed in front of its star. Because the planet rotates about 30 degrees during its transit, the JWST could observe slivers of the dayside beyond the dawn and dusk boundaries. Rather than averaging the data—a common practice in previous studies—the team analyzed starlight changes second by second.
Did you know? A single year on WASP-121 b lasts barely 30 hours because of its extreme proximity to its star.
Chemical Divergence: Water vs. Carbon Monoxide
The data revealed that the morning and evening terminators are chemically distinct. The evening terminator is characterized by fierce winds that sweep heat eastward from the dayside. This heat surge increases the signal of carbon monoxide.
Water behaves differently. According to the Max Planck Institute for Astronomy, temperatures at the evening terminator are high enough to tear water molecules apart. This results in significantly less water remaining on the evening side compared to the cooler morning boundary.
Comparison of Planetary Hemispheres
| Feature | Morning Terminator | Evening Terminator |
|---|---|---|
| Temperature | Relatively Cooler | Searing Heat |
| Water Vapor | Higher Concentration | Molecules Torn Apart |
| Carbon Monoxide | Lower Signal | Elevated Signal |
The Mineral Cloud Puzzle
When the research team compared their findings to existing computer models, the real-world signal was stronger than predicted. This discrepancy suggests the presence of clouds made of vaporized minerals, such as silicates.
These mineral clouds likely cool the morning terminator by blocking infrared light from the hotter layers beneath. According to the researchers, this gap between the model and the observation provides a critical clue for improving how scientists model exoplanet atmospheres, which remains notoriously difficult.
A New Era of Longitude-Based Astronomy
This study marks a shift in how astronomers view distant worlds. Instead of treating an exoplanet as a “single averaged blob” of atmosphere, the JWST can now trace conditions longitude by longitude.
The team has already identified other ultra-hot planets that fit this observation method. This approach allows for the creation of an “atlas of alien weather,” moving beyond general snapshots to detailed, regional climate maps of planets hundreds of light-years away.
Pro Tip: To follow these developments, monitor updates from the NASA James Webb Space Telescope mission and the Max Planck Institute for Astronomy.
Frequently Asked Questions
What is a tidally locked planet?
It is a planet where the rotation period matches its orbital period, causing one side to always face the star and the other to always face away.
Why is WASP-121 b called an “ultra-hot gas giant”?
Due to its close orbit, its dayside reaches temperatures of approximately 2,500 degrees Celsius.
What are the “terminators” of a planet?
The terminators are the boundary lines between the day side and the night side, essentially the zones of permanent dawn and dusk.
Do you think we will find a planet with a stable twilight zone capable of supporting life? Tell us your thoughts in the comments or subscribe to our newsletter for more deep-space discoveries.
Worth a look