WASP-121 b belongs to a class of exoplanets known as hot Jupiters, gas giants orbiting perilously close to their stars. Its proximity results in tidal locking—the synchronous rotation where one hemisphere permanently faces the star while the opposite side remains shrouded in near-freezing darkness. This unique dynamic creates sharply defined day/night hemispheres with temperature differentials reaching thousands of degrees Celsius. Daytime skies on WASP-121 b blaze at nearly 2770 Kelvin (approx. 2500°C), while the nightside cools dramatically to about 1000 Kelvin (approx. 725°C), a difference capable of fundamentally shaping atmospheric dynamics and chemistry.

The research team, led by Cyril Gapp of the Max Planck Institute for Astronomy (MPIA), leveraged JWST’s Near Infrared Spectrograph (NIRSpec) to analyze the planet as it passed in front of its star. By measuring variations in the starlight filtered through the planet’s atmosphere along a transit and correlating them with the planet’s rotation, the scientists discerned more than mere average brightness dips. Instead, they detected distinct asymmetries in infrared absorption between the morning and evening terminators—boundaries marking the transitions between day and night sides—as the planet rotated about 30 degrees during the transit.

Detailed spectral analyses revealed that the evening terminator’s atmosphere absorbs significantly more stellar infrared radiation than the morning side. This bespoke absorption pattern aligns precisely with expectations from robust eastward winds that transport intense daytime heat toward the night hemisphere. These winds elevate atmospheric temperatures on the evening side, causing its gaseous layers to expand and increase its effective cross-sectional area occulting the star. This expanded atmosphere thus filters more infrared light, resulting in the observed enhanced absorption signature.

Interestingly, while the carbon monoxide (CO) absorption feature intensifies towards the evening terminator, the researchers attribute this to temperature-related shifts in molecular excitation rather than an increase in CO molecule abundance. Conversely, the water vapor (H₂O) signature diminishes markedly on the evening side, indicating genuine molecular depletion. Scientists interpret this as photodissociation driven by extreme temperatures breaking water molecules into hydrogen and oxygen, a hallmark of ultra-hot planetary atmospheres subjected to relentless stellar irradiation.

The innovative approach exploited the planet’s tidally locked nature to parse atmospheric composition with longitudinal specificity—a refined spectroscopic “longitude scan” rarely attainable for exoplanets. Traditionally, transit observations amalgamate data over ingress to egress, masking subtle spatial differences. Here, accounting for the planet’s rotation mid-transit provided richer insights and improved model fits, affirming genuine asymmetries due to localized physical phenomena rather than observational noise.

However, when researchers compared these observations to advanced atmospheric circulation models simulating heat distribution, some discrepancies arose. Specifically, the observed amplitude of variation surpassed theoretical predictions, hinting at missing or underestimated mechanisms modulating the atmospheric properties. One plausible explanation relates to the presence of mineral clouds—composed of silicates and other condensates—that preferentially form on the cooler morning terminator. Clouds can efficiently absorb or scatter infrared radiation, complicating measurements by masking hotter, deeper layers and suppressing apparent emission. Incorporating these cloud effects into models brought simulations into closer harmony with JWST’s unprecedented data.

The study exemplifies the extraordinary capabilities of the JWST in unveiling detailed atmospheric physics in distant exoplanets, heralding a new era of precision exoplanetology. By characterizing longitudinal structure across terminator zones, scientists can now probe global circulation patterns, chemistry, and cloud formation in worlds vastly different from those in our solar system. These insights shed light not only on atmospheric dynamics under extreme irradiation but also on planetary formation and evolution processes.

Moreover, this approach provides a blueprint for future investigations targeting similar ultra-hot gas giants within optimal temperature and rotation regimes. Expanding such longitudinal studies to a broader exoplanet sample will enable comparative atmospheric climatology, revealing whether WASP-121 b’s asymmetries are unique or reflect widespread characteristics among tidally locked hot Jupiters. Unlocking this diversity will advance understanding of atmospheric escape, chemistry, and heat transport mechanisms under conditions alien to our own planetary neighborhood.

WASP-121 b’s case also spotlights challenges in exoplanet atmospheric modeling, emphasizing the importance of incorporating cloud microphysics and non-equilibrium chemistry alongside thermal and dynamical factors. Robust, multi-dimensional models capturing these intricacies are necessary to interpret forthcoming JWST data accurately and to unravel the interplay of radiative transfer, chemical kinetics, and fluid dynamics shaping these extreme atmospheres. Future observational campaigns supported by enhanced modeling will ultimately refine our ability to reconstruct exoplanet atmospheric compositions with confidence.

In summary, this compelling research not only confirms predicted asymmetrical atmospheric structures on a tidally locked ultra-hot Jupiter but also highlights the nuanced complexity of exoplanet atmospheres revealed through state-of-the-art infrared transit spectroscopy. The successful detection and characterization of the dawn-dusk differences on WASP-121 b showcase cutting-edge exoplanet science propelled by JWST’s capabilities, affirming its vital role in decoding the secrets of distant worlds and their climates.

Subject of Research: Not applicable

Article Title: Atmospheric asymmetries in WASP-121 b revealed by rotational transits detected with JWST

News Publication Date: 10-Jun-2026

Web References: http://dx.doi.org/10.1038/s41550-026-02887-6

References: Nature Astronomy journal, DOI: 10.1038/s41550-026-02887-6

Image Credits: Patricia Klein and MPIA

Keywords: WASP-121 b, ultra-hot Jupiter, exoplanet atmosphere, atmospheric asymmetry, JWST, NIRSpec, tidally locked, infrared transit spectroscopy, atmospheric dynamics, hot Jupiters

LHS 3844 b orbits its host star — a cool red dwarf — in an extraordinarily close orbit, completing a revolution every 11 hours. This tight gravitational embrace results in tidal locking, where one hemisphere perpetually faces the star, basking in relentless daylight with surface temperatures soaring around 1000 Kelvin, approximately 725 degrees Celsius. Situated just 48.5 light-years away, this proximity offers an unparalleled glimpse into the characteristics of rocky planets outside our Solar System.

Employing JWST’s unmatched sensitivity, the researchers were able to detect the infrared emission directly emanating from the planet’s searing hot surface. Intriguingly, rather than a vibrant, atmosphere-rich world, LHS 3844 b resembles an inert, barren rock — dark and devoid of an atmosphere. This crucial insight was gleaned not by direct imaging, but through precision measurements of the combined brightness fluctuations of the star and planet system, as the planet’s position changes relative to Earth.

By dissecting the planet’s infrared radiation between 5 and 12 micrometers into finely resolved spectral bins, the team created a detailed emission spectrum. This spectrum, augmented with previous data from the Spitzer Space Telescope, enabled the astronomers to compare the observed wavelengths against laboratory models of minerals found on Earth, the Moon, and Mars. These comparisons decisively excluded compositions similar to Earth’s silicate-rich crust, such as granite, pointing towards a fundamentally different geological makeup.

Insights from terrestrial geological science suggest that Earth’s silicate crust forms through protracted tectonic and hydrological processes, requiring plate tectonics and significant water presence to recycle mantle material and generate lighter minerals at the surface. The absence of such a crust on LHS 3844 b implies a tectonically inactive world with minimal water content, indicating a geologic environment starkly contrasting with Earth’s.

The spectral data strongly favor a surface dominated by basaltic or magmatic rocks, mineralogically abundant in magnesium and iron, potentially enriched with olivine. Interestingly, the data suggest the presence of solid, extended rock formations or crushed rubble rather than fine powdery grains. The implications are profound: without an atmosphere to shield it, the surface endures relentless space weathering from intense stellar radiation and bombardment by micrometeorites, which gradually darkens and modifies surface materials.

A nuanced statistical fit to the infrared spectrum paints two plausible geological scenarios. One posits freshly exposed basaltic rock, resurfaced recently by geological activity such as widespread volcanism. The alternative scenario envisages an older, more heavily weathered regolith — a layer of dark, fine sedimentary rock fragments akin to the lunar surface — reflecting a long period of geological dormancy. Both scenarios maintain the dark and airless nature of the planet but differ in their interpretations of geological dynamism.

Further differentiating between these two models, the research team searched for signs of volcanic outgassing, particularly sulphur dioxide (SO₂), a hallmark byproduct of active volcanism. The absence of SO₂ absorption features in the MIRI data heavily favors the latter scenario — a geologically quiescent planet with a heavily weathered surface. This suggests LHS 3844 b may closely resemble Mercury, marked by an ancient crust and a surface sculpted over eons by space weathering rather than active geophysical processes.

Moving forward, the astronomers are poised to leverage additional JWST observations equipped to distinguish subtle variations in surface texture by analyzing how light is emitted or reflected from solid slabs versus powdered material. Surface roughness influences emission angles, enabling the team to refine their models and decisively identify the geological state of LHS 3844 b. This technique, adapted from asteroid studies within our Solar System, heralds a new era in exoplanetary geology with the potential to unlock the mysteries of countless rocky worlds.

This research, integrating observational astronomy with planetary geology, not only enriches our understanding of exoplanet surfaces but challenges existing paradigms about tectonic activity and planetary evolution beyond the Solar System. By revealing a barren, basalt-like world subjected to intense space weathering, the study underscores the diversity of rocky exoplanets and the myriad evolutionary paths they may follow.

The success of this study also showcased the collaborative effort behind JWST and MIRI, involving numerous institutions and international partners including NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA), alongside scientific bodies like the Max Planck Society and several universities. The groundbreaking data harnessed by JWST’s MIRI instrument reinforces its status as the premier observatory for illuminating the cosmos in unprecedented detail.

As the astronomical community anticipates forthcoming JWST observation cycles, the methodologies demonstrated in this study set a robust precedent for characterizing the geological properties of rocky exoplanets. With each successive insight, humanity edges closer to unraveling the complex histories and potential habitability of Earth-like worlds scattered across our galaxy.

Subject of Research: Not applicable

Article Title: The dark and featureless surface of rocky exoplanet LHS 3844 b from JWST mid-infrared spectroscopy

News Publication Date: 4-May-2026

Web References: https://doi.org/10.1038/s41550-026-02860-3

References: Zieba, S., Kreidberg, L., et al. (2026). The dark and featureless surface of rocky exoplanet LHS 3844 b from JWST mid-infrared spectroscopy. Nature Astronomy.

Image Credits: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington (cropped)

Keywords

Exoplanet, LHS 3844 b, James Webb Space Telescope, MIRI, mid-infrared spectroscopy, rocky super-Earth, space weathering, basaltic surface, planetary geology, tidal locking, volcanism, sulphur dioxide