The giant exoplanet WASP-94A b has two fundamentally different skies: a cloudy, cooler morning side and a clearer, hotter evening side with strong water vapor absorption. Transmission spectra collected by the James Webb Space Telescope (JWST) confirm this split at 6-sigma significance, making it one of the sharpest detections of atmospheric asymmetry ever recorded on a world beyond our solar system. The finding adds to a growing body of evidence that tidally locked gas giants do not present uniform atmospheres to observers, a realization that is forcing researchers to rethink how they model heat transport and weather on distant worlds.
Why the split sky on WASP-94A b changes exoplanet science
Tidally locked exoplanets always show the same face to their host star, creating a permanent dayside and a permanent nightside. The boundary between the two, the terminator, is the strip of atmosphere that starlight filters through during a transit. Until recently, astronomers treated that strip as a single averaged signal. WASP-94A b shows why that assumption breaks down. The morning terminator, where air flows from the cooler nightside, registered a cloud-covered signal at 11-sigma confidence in an arXiv analysis. The evening terminator, fed by superheated dayside air, came through as a clearer atmosphere loaded with water vapor at 10-sigma confidence. The overall limb asymmetry hit 6 sigma, well above the threshold for a secure detection.
This is not an isolated curiosity. WASP-39 b, the first exoplanet where researchers split the terminator signal, showed its evening side running roughly 200 degrees Celsius hotter than its morning side, with evening temperatures near 800 degrees Celsius and morning temperatures near 600 degrees Celsius. A separate peer-reviewed study of WASP-107 b, a cooler and less dense planet, found a morning-to-evening temperature difference of about 100 K using JWST/NIRCam spectra in the 2.5 to 4.0 micrometer range. Three different planets, three different temperature regimes, and all three show the same pattern: evenings are hotter and clearer, mornings are cooler and cloudier.
That consistency matters because it points to a shared physical mechanism. On a tidally locked world, equatorial winds carry hot dayside air toward the evening terminator and cooler nightside air toward the morning terminator. Clouds condense in the cooler morning flow, muting molecular absorption features. The evening flow stays hot enough to keep condensates in the gas phase, allowing water vapor and other molecules to stamp sharp spectral signatures into the starlight. If this pattern holds across a wider sample, it means every single-spectrum retrieval that treats the terminator as uniform has been blending two distinct atmospheric states into one misleading average.
Three planets, one atmospheric pattern, and a testable prediction
The WASP-94A b result, reported in a Nature paper that applies the terminator-splitting technique and documented in the associated arXiv preprint, sits alongside the WASP-39 b and WASP-107 b detections in a small but growing dataset. WASP-39 b showed a contrast of roughly 177 K between its terminators. WASP-107 b, a cooler planet, showed about 100 K. The apparent trend is that planets with lower equilibrium temperatures may display proportionally larger morning-to-evening contrasts relative to their overall thermal budget. If confirmed, this would mean that the magnitude of asymmetry scales inversely with planetary equilibrium temperature across the sample of inflated hot giants observed by JWST.
Testing that hypothesis requires stacking JWST spectra of additional targets across a range of temperatures and surface gravities. Separate work on WASP-121 b has already extended the technique beyond the simple two-limb model by using rotational effects during transit to map longitudinally varying absorption, showing changing ratios of carbon monoxide to water as the planet rotates. That approach, published in Nature Astronomy, demonstrates that asymmetric transit signals correspond to real atmospheric structure rather than instrumental artifacts. Each new target strengthens the case that these split skies are the norm, not the exception, for tidally locked gas giants.
Beyond temperature, the chemistry of the two limbs also carries clues about atmospheric circulation. If condensable species such as silicates or metal oxides form clouds preferentially on the morning side, they can deplete those elements from the gas phase before air reaches the evening terminator. That would leave behind a chemical fingerprint of the circulation pattern, potentially visible as differing abundances of key molecules between the two limbs. Detecting such gradients requires higher signal-to-noise spectra and more sophisticated retrieval models that can handle two separate atmospheric columns instead of one.
The geometry of the planet’s orbit and rotation also matters. For WASP-94A b, the strong detection of asymmetry suggests a relatively stable, large-scale wind pattern that consistently feeds cool air into the morning limb and hot air into the evening limb. On planets with different rotation rates or stronger magnetic fields, the balance between east–west jets, polar flows, and vertical mixing could produce more complex terminator structures, including patchy clouds or multiple cloud decks stacked at different altitudes. Future analyses may move beyond a simple “cloudy versus clear” dichotomy to reconstruct full three-dimensional weather maps.
Open questions and what comes next for split-sky exoplanets
Several gaps remain. The exact temperature values and uncertainty ranges for the morning and evening limbs of WASP-94A b specifically have not yet appeared in publicly available excerpts of the full paper. The peer-reviewed version, cited with DOI 10.1126/science.adx5903, will need to supply those numbers before the community can place WASP-94A b precisely on the emerging temperature-contrast trend line. No direct author quotes from the research team have surfaced in the primary source material, leaving the interpretation of the detection significance to the data alone.
The broader question is whether the pattern extends beyond inflated hot Jupiters to smaller, cooler worlds. Tidally locked super-Earths orbiting red dwarf stars are prime candidates: their short orbital periods and large transit depths make them accessible to JWST, and many are expected to be locked with permanent day and night hemispheres. If such planets also show cooler, cloudier mornings and hotter, clearer evenings, it would indicate that the same dynamical principles operate across a wide range of planetary sizes and compositions. Conversely, a breakdown of the pattern for rocky or water-rich planets would hint at different circulation regimes, perhaps dominated by surface interactions, oceans, or strong magnetic coupling.
For now, WASP-94A b stands as a benchmark case. Its sharply divided skies confirm that the terminator of a tidally locked planet is not a single, uniform ring but a collision zone between two very different worlds. As JWST continues to collect high-precision spectra, astronomers will be able to refine temperature maps, cloud distributions, and chemical gradients across many more exoplanets. Each new detection of a split sky will help transform atmospheric models from one-dimensional averages into dynamic, three-dimensional portraits-bringing the weather on distant worlds into ever sharper focus.
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*This article was researched with the help of AI, with human editors creating the final content.
