Astronomers have captured the first direct evidence of a daily weather cycle on a planet outside our solar system. The hot Jupiter WASP-94A b, roughly 700 light-years from Earth, shows mineral clouds blanketing its morning horizon that dissipate into clear skies by evening, with a temperature difference of about 450 K between the two sides. The finding, drawn from limb-resolved transmission spectroscopy performed by the James Webb Space Telescope (JWST), represents the sharpest look yet at how clouds form and vanish on a gas giant in real time.
How JWST split morning from evening on a distant gas giant
When a transiting exoplanet crosses in front of its host star, starlight filters through a thin ring of atmosphere along the planet’s edge. That ring has two halves: a leading limb, which corresponds to the planet’s morning terminator, and a trailing limb, its evening terminator. On tidally locked hot Jupiters like WASP-94A b, the morning side faces away from the substellar point and stays cooler, while the evening side receives heat carried by powerful equatorial winds from the dayside. The JWST team exploited this geometry by using the telescope’s NIRISS instrument in its slitless spectroscopy mode to record separate near-infrared spectra for each limb during transit.
The result was stark. The cooler morning limb appeared far more opaque than the warmer evening limb, a difference measured at greater than 5 sigma statistical significance according to a comparative study of hot Jupiters observed with NIRISS. That level of confidence places the asymmetry well beyond the threshold for a chance fluctuation. The peer-reviewed Science paper reporting the primary result interprets the excess opacity as mineral condensate clouds that form on the cooler morning side and evaporate as atmospheric circulation carries material toward the hotter evening side. Because the two limbs are probed almost simultaneously during a single transit, instrumental systematics and stellar variability are strongly constrained, strengthening the case that the difference is truly atmospheric.
Mineral clouds, 450 K contrasts, and what ground telescopes missed
According to a Johns Hopkins release, Sagnick Mukherjee served as lead investigator on the study, with David Sing as principal investigator. The same release describes the morning-side clouds as consisting of magnesium silicate minerals, essentially vaporized rock that condenses into aerosol particles when temperatures drop below a critical threshold on the cooler limb. The roughly 450 K temperature contrast between the two limbs, confirmed in the AAAS press material for the Science paper, provides the thermal gradient that drives this cycle: hot enough on the evening side to keep silicates in the gas phase, cool enough on the morning side for them to condense into clouds.
Ground-based telescopes had already detected atmospheric signatures on WASP-94A b. Observations with the NTT/EFOSC2 instrument identified sodium absorption and Rayleigh scattering in the planet’s atmosphere. Those measurements confirmed that the atmosphere contained detectable aerosols and alkali metals, but the ground-based data lacked the precision, continuous time coverage, and wavelength range to separate morning from evening. JWST’s infrared sensitivity and the NIRISS time-series capability closed that gap, turning what used to be a single averaged spectrum into two distinct atmospheric portraits, each tied to a different local time of day on the planet.
A separate JWST program using the NIRSpec/G395H grating also performed spectroscopy of WASP-94A b, providing independent molecular abundance constraints. That work, described in a detailed analysis of the planet’s composition and thermal structure, focuses on formation and migration history rather than cloud asymmetry. Even so, it adds a second instrument’s perspective on the same atmosphere and demonstrates that WASP-94A b is becoming one of the most thoroughly characterized hot Jupiters in the JWST era, with multiple datasets that can be cross-checked for consistency.
What the cloud cycle reveals about atmospheric physics
The morning-to-evening cloud clearing on WASP-94A b is not just a curiosity. It offers a direct test of how fast mineral clouds can form and be destroyed on a tidally locked world. Two competing processes set the timescale. Zonal winds carry cloud particles from the cooler nightside toward the hotter dayside, a process called advection. Radiative heating on the dayside and evening terminator works to evaporate those particles. The ratio between advection speed and radiative heating rate determines where, along the terminator, clouds survive and where they vanish.
If advection dominates, clouds should extend farther onto the evening limb before burning off. If radiative heating wins, the cloud deck should retreat closer to the morning side. The WASP-94A b data, showing a clean split with thick clouds on the morning limb and largely clear skies on the evening limb, suggests that radiative heating is efficient enough to destroy mineral condensates before winds carry them far past the terminator. A prediction follows from this: observing the planet at slightly different orbital phases should reveal measurable shifts in the evening-limb cloud base altitude, because the balance between wind transport and heating changes with viewing geometry.
In practice, that means future JWST phase-curve observations could track how the cloud boundary migrates as different longitudes rotate into view. Models of hot Jupiter circulation already predict strong eastward jets and large day–night temperature contrasts. The new limb-resolved spectra provide an anchor point for those models, showing exactly where a key condensate species transitions from solid particles to vapor. By matching the observed cloud asymmetry and temperature contrast, theorists can refine estimates of wind speeds, vertical mixing, and radiative timescales in the upper atmosphere.
The mineral clouds themselves also matter for the planet’s energy budget. Silicate aerosols are highly reflective at some wavelengths and strongly absorbing at others, altering how starlight and thermal radiation move through the atmosphere. On WASP-94A b, a persistent morning cloud deck could brighten the planet’s appearance in reflected light and mute spectral features from deeper layers, while the clearer evening side would allow more thermal emission to escape. Together, those effects could shape the infrared phase curves and eclipse depths that observers measure, tying the microphysics of cloud particles to the macroscopic brightness variations seen over an orbit.
Implications for exoplanet climate and future surveys
Beyond WASP-94A b itself, the new result points toward a broader goal: building a comparative climatology of exoplanets. The NIRISS hot Jupiter program that captured this dataset includes multiple targets, enabling astronomers to ask whether strong morning–evening asymmetries are common or exceptional. If mineral cloud cycles like this one turn out to be widespread, they will need to be incorporated into retrieval frameworks that currently assume a single, uniform terminator profile for most transiting planets.
For upcoming surveys, the work underscores the value of limb-resolved spectroscopy as a standard observing strategy. Splitting a transit into morning and evening components effectively doubles the information content without requiring more telescope time, as long as the instrument is stable enough to disentangle the two halves. JWST has now demonstrated that capability in practice. Future facilities, from the Extremely Large Telescope on the ground to proposed space missions focused on exoplanet atmospheres, can build on that template to probe weather patterns on smaller and cooler worlds.
Ultimately, the first direct glimpse of a daily weather cycle on WASP-94A b is a proof of concept: even on a world where a year lasts just a few days and temperatures soar above 1,000 K, familiar atmospheric processes-cloud formation, transport, and evaporation-still shape the climate. With more targets and more precise measurements, astronomers hope to extend that insight to a diverse population of exoplanets, tracing how alien skies evolve from one limb to the other and, eventually, from one world to the next.
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*This article was researched with the help of AI, with human editors creating the final content.
