James Webb Telescope Maps Distinct Dawn and Dusk Weather on an Ultra-Hot Exoplanet

For decades, exoplanets have been treated as uniform spheres of data—distant points of light yielding only average temperatures and blended chemical signatures. But the James Webb Space Telescope (JWST) has fundamentally changed the resolution of planetary science. In a landmark study published in Nature Astronomy, researchers have successfully mapped the distinct morning and evening weather systems of WASP-121 b, an ultra-hot gas giant located 880 light-years from Earth. By dissecting the planet's atmosphere longitude by longitude, astronomers have provided the first direct observational evidence that an exoplanet's dawn and dusk can feature wildly different climates and chemistry.[1][2][4][5]

WASP-121 b belongs to a class of extreme worlds known as "hot Jupiters." It orbits its host star at a punishingly close distance, completing a full "year" in just 30.5 hours. Because of this extreme proximity, the host star's immense gravitational pull has tidally locked the planet, forcing it to complete one rotation in the exact time it takes to finish one orbit. As a result, one hemisphere is permanently baked in unending daylight, while the other is perpetually shrouded in the freezing darkness of space.[3][6][7]

The temperature gradient between these two halves is staggering. The dayside hemisphere reaches a searing 2,500 degrees Celsius (2,770 Kelvin)—hot enough to vaporize iron and other heavy metals. Meanwhile, the nightside cools to a relatively frigid 725 degrees Celsius (1,000 Kelvin). Between this blistering day and sweltering night lie two narrow transition zones known as terminators: the morning terminator, where night turns to day, and the evening terminator, where day fades into night.[1][2][3][4][6]

Theoretical models have long predicted that the morning and evening terminators of hot Jupiters should exhibit different atmospheric conditions. However, previous observatories, including the Hubble Space Telescope, lacked the sensitivity to separate these regions, forcing scientists to rely on an "average spectrum" that blended the entire planet's atmospheric profile into a single data point. JWST's Near-Infrared Spectrograph (NIRSpec) has now shattered that limitation, allowing researchers to peel apart the planet's atmospheric layers.[1][2][4][5][7]

The breakthrough relies on a technique called transmission spectroscopy, combined with precise timing. When WASP-121 b transits—or crosses in front of—its host star, a fraction of the starlight filters through the planet's gaseous envelope before reaching Earth. Different chemical elements and molecules absorb specific wavelengths of this infrared light, leaving a distinct fingerprint in the spectrum.[4][5]

The critical innovation in this study was leveraging the planet's rotation during the transit itself. Because WASP-121 b is so close to its star, it rotates by approximately 30 degrees during the four-and-a-half-hour transit event. As the planet creeps across the stellar disk, fresh longitudes of its scorched atmosphere rotate into view. The leading edge of the planetary disk reveals the morning side, while the trailing edge exposes the evening side.[1][2][7]

Instead of averaging the data across the entire transit, the research team, led by Cyril Gapp at the Max Planck Institute for Astronomy, allowed the signal to vary minute by minute. By tracking these subtle changes, they effectively translated elapsed time into planetary longitude, creating a dynamic map of the terminators. "By measuring how starlight absorption changes as WASP-121 b rotates, we probe its atmosphere longitude by longitude," Gapp explained.[1][2][4][6][7]

The primary claim validated by the JWST data is that the evening terminator is significantly hotter and more expanded than the morning terminator. The observations revealed a pronounced asymmetry: the evening side absorbs substantially more infrared starlight than the dawn side. This absorption pattern perfectly matches the physical behavior of a gas expanding under extreme heat, presenting a larger cross-section to the incoming starlight.[1][4][5][6]

This thermal expansion is driven by some of the most violent winds in the known universe. To redistribute the immense energy accumulating on the dayside, WASP-121 b generates powerful eastward jet streams that whip around the equator at thousands of miles per hour. These winds carry superheated gas from the dayside directly into the evening terminator, baking the dusk region before the gas eventually cools and sinks into the nightside.[3][4][5][6]

The extreme heat at the evening terminator triggers a secondary, dramatic chemical claim: the temperatures are so severe that they literally tear water molecules apart. While water vapor is a common feature in the atmospheres of hot Jupiters, the JWST data showed a sharp, localized drop in the water signal specifically at the dusk terminator.[4][5]

Planetary chemists interpret this not as an absence of oxygen or hydrogen, but as thermal dissociation. The upper layers of the evening atmosphere become so intensely heated by the eastward winds that the thermal energy overcomes the chemical bonds holding the water molecules together, breaking them down into their constituent atoms. As the gas circulates to the cooler nightside, the atoms recombine back into water vapor.[3][4][5]

Conversely, the morning terminator presents a very different meteorological puzzle. The JWST observations indicate that the dawn region is cooler than the evening side, but the data also revealed a discrepancy: the morning terminator appeared even cooler than the 3D atmospheric models had predicted. This anomaly has led researchers to propose a third major claim regarding the planet's weather.[4][6]

The leading hypothesis is that the morning terminator is shielded by exotic mineral clouds. On the cooler nightside, temperatures drop low enough for vaporized metals like iron, magnesium, and aluminum to condense into liquid droplets or solid particles. Aluminum condensing with oxygen forms corundum—the exact mineral that, on Earth, constitutes rubies and sapphires.[3][4][6]

As the eastward winds carry these metallic clouds out of the night and into the morning terminator, they create a thick, reflective cloud deck. These silicate and corundum clouds would block infrared radiation from escaping the deeper, hotter layers of the atmosphere, making the dawn region appear artificially cool to JWST's sensors. Once these clouds cross fully into the searing dayside, they rapidly vaporize back into gas, leaving the afternoon skies clear.[3][4][6]

While the temperature and chemical asymmetries between dawn and dusk are now observationally confirmed, the exact composition of the morning clouds remains an area of transparent uncertainty. The researchers note that more sophisticated, high-resolution 3D models will be required to definitively prove whether these clouds are made of silicates, corundum, or other exotic compounds.[3][6]

The implications of this study extend far beyond WASP-121 b. By proving that JWST can resolve the longitudinal differences of an exoplanet's atmosphere, astronomers have unlocked a new tool for studying the cosmos. The era of treating exoplanets as flat, uniform data points is ending. Scientists can now begin to map the complex, three-dimensional meteorology of alien worlds, comparing the sunrises and sunsets of distant planets to understand the universal laws of atmospheric physics.[1][4][7]