JWST Maps the Extreme Weather Differences Between Dawn and Dusk on an Alien World

For decades, astronomers have been forced to treat distant exoplanets as uniform, one-dimensional spheres, averaging their atmospheric data into a single global signature. That era has officially ended. Using the James Webb Space Telescope (JWST), researchers have successfully mapped the distinct weather patterns of an alien world's sunrise and sunset, revealing a planet where the atmosphere fundamentally transforms between dawn and dusk. The target of this breakthrough is WASP-121 b, an "ultra-hot Jupiter" located roughly 900 light-years away in the constellation Puppis.[1][2][7]

WASP-121 b is a world defined by extremes. Orbiting its host star at a punishingly close distance, a single year on the planet lasts just 30.5 hours. Because of this proximity, the gas giant is tidally locked—meaning one hemisphere permanently faces the star in unending daylight, while the other is trapped in perpetual darkness. The dayside is roasted to an average temperature of 2,770 Kelvin (2,500°C), while the nightside cools to a relatively frigid 1,000 Kelvin (725°C).[3][4][5][7]

Between these two extremes lie the terminators: the twilight zones separating day from night. On Earth, the terminator is a gentle transition of fading light. On WASP-121 b, the morning terminator (where night turns to day) and the evening terminator (where day turns to night) are violent atmospheric battlegrounds. Theoretical models have long predicted that these two zones should exhibit drastically different climates, but proving it required an instrument capable of dissecting the edge of a planet from trillions of miles away.[1][4][5]

The foundation of this discovery rests on a novel observational technique pioneered by a team led by Cyril Gapp at the Max Planck Institute for Astronomy (MPIA). When WASP-121 b passes in front of its host star—an event known as a transit—starlight filters through the planet's atmosphere before reaching Earth. Historically, astronomers combined the data from the entire transit to boost the signal, yielding a flat, averaged atmospheric profile.[1][3][4]

The MPIA team realized that during the course of a full transit, WASP-121 b rotates by approximately 30 degrees on its axis. By utilizing JWST's Near-Infrared Spectrograph (NIRSpec) and Near-Infrared Imager and Slitless Spectrograph (NIRISS), the researchers did not average the data. Instead, they measured how the absorption of starlight changed minute by minute as progressively hotter or colder atmospheric gas rotated into the telescope's view.[4][5][8]

While the precision of JWST makes this time-series analysis possible, the technique pushes the absolute limits of current technology. The signal variations between the start and end of the transit are minuscule, requiring rigorous calibration to ensure the observed asymmetries are not instrumental artifacts. However, the independent confirmation across multiple JWST instruments provides strong confidence in the resulting data.[8]

The primary finding of the Nature Astronomy study is a stark imbalance in how infrared light is absorbed on the two sides of the planet. The data clearly indicate that the evening zone intercepts a much larger fraction of starlight than the morning zone, confirming that the two terminators are physically distinct.[1][5][8]

This asymmetric absorption profile is the physical signature of atmospheric expansion. Gases expand when heated, and the JWST transmission spectrum proves that the atmosphere at the dusk terminator is puffed up considerably higher than at the dawn terminator.[1][7]

This extreme evening expansion is driven by the planet's supersonic wind system. Fierce equatorial winds, whipping at speeds up to 11,000 miles per hour, travel eastward across the planet. These winds act as a planetary conveyor belt, dragging the blistering heat from the dayside directly into the evening terminator. By the time the winds cross the nightside and reach the morning terminator, they have cooled significantly, resulting in a more contracted dawn atmosphere.[1][3][7]

The temperature gradient between dawn and dusk is so severe that it fundamentally alters the planet's chemistry. The JWST data revealed that the two terminators do not just have different temperatures; they are composed of different molecules.[1][4][8]

As the transit progressed and the evening terminator rotated into view, the researchers observed a distinct drop in the spectral signature of water (H2O). Conversely, the absorption signal for carbon monoxide (CO) remained robust and even appeared to increase, though researchers attribute the CO signal boost to temperature effects rather than a change in absolute abundance.[1][8]

The depletion of water on the dusk side is a textbook example of thermal dissociation. At temperatures exceeding 2,500°C, the thermal energy in the atmosphere is sufficient to physically rip water molecules apart into constituent hydrogen and oxygen atoms. Because carbon monoxide is held together by one of the strongest chemical bonds in nature, it survives the eastward journey through the dayside inferno intact, while water is systematically destroyed before it reaches the evening zone.[7][8]

If the evening is defined by the destruction of molecules, the morning is defined by their condensation. As the supersonic winds traverse the dark nightside, temperatures plummet to 725°C, allowing atoms to recombine and heavier elements to condense into liquid or solid states.[3][7]

The JWST transmission data from the dawn terminator shows a muted chemical signature, a classic indicator that clouds are blocking the starlight from penetrating deeper into the atmosphere. Given the temperatures involved, these are not clouds of water vapor. Planetary meteorologists theorize they are composed of silicate minerals, or even liquid metals and condensed gems like rubies and sapphires.[1][3][6]

The evidence for specific cloud compositions remains the weakest link in the current data pack. While the presence of a cloud deck on the morning side is strongly supported by the muted spectral lines, identifying the exact mineral makeup requires more sophisticated 3D atmospheric modeling and potentially broader wavelength coverage.[3][6]

The successful mapping of WASP-121 b's terminators represents a paradigm shift in exoplanet meteorology. "With its unprecedented observational quality, JWST gives us the most detailed glimpses into distant planets to date," noted Cyril Gapp, emphasizing that astronomers can now probe alien atmospheres "longitude by longitude."[1][6]

This methodology opens a new frontier for the James Webb Space Telescope. Researchers have already identified a catalog of other ultra-hot gas giants with suitable rotation rates and transit profiles. By applying this time-series technique to a larger sample size, astrophysicists hope to build a comprehensive taxonomy of 3D planetary climates, moving beyond single-pixel approximations of distant worlds.[2][6]

Ultimately, the WASP-121 b evidence pack proves that the universe is far more dynamic than our previous instruments could perceive. The boundary between day and night on this ultra-hot Jupiter is not just a line on a map, but a chaotic engine of supersonic winds, vaporized water, and mineral storms—a weather system we are only just beginning to understand.[3][7]