Source: Ars Technica

Cloudy with a chance of excessive heat

The differences seen here could be throwing off how we study planetary atmospheres.

WASP‑94A b is a hot, tidally locked gas giant orbiting close to one of the stars in a binary system roughly 690 light‑years away from Earth. In a new Science study, scientists led by Sagnick Mukherjee, an astrophysicist at Johns Hopkins University, used the James Webb Space Telescope to learn what the weather looks like out there.

“Tidal locking means that you no longer have day‑ and night‑side temperature differences sweeping across the planet. We wanted to understand the atmospheres of such planets. Are they static or dynamic? Do they have winds? Do they have clouds?” – Sagnick Mukherjee

His team found that, on WASP‑94A b, it’s cloudy in the morning but the skies are clear in the evening. The fact that we didn’t know this already means we might have gotten the chemistry of this and many other exoplanets surprisingly wrong.

Averaged atmospheres

WASP‑94A b has a mass slightly below half of Jupiter but a diameter that’s over 70 % larger.

“This means the planet has low density, and its atmosphere extends further out into space, which makes it easier to observe,” Mukherjee explains.

When astronomers study atmospheres like this, they usually rely on transmission spectroscopy. By analyzing the spectrum of starlight that filters through the planet’s atmosphere as it transits its star, they can determine the atmosphere’s chemical composition.

The problem with this approach is that the light passing through the entire circumference of the planet’s silhouette is averaged, as though the atmosphere were a homogeneous ball of gas. For tidally locked planets, this is a massive oversimplification.

On tidally locked worlds, there are extreme temperature swings between the day and night sides, which usually lead to differences in atmospheric density between the two hemispheres. These differences, combined with the Coriolis effect that stems from the planet’s slow rotation, give rise to a phenomenon called equatorial super‑rotation—winds on the equator blow eastward faster than the planet itself rotates. Circulation models predict this is exactly what’s happening on WASP‑94B a.

• Morning limb – the leading edge of the planet’s disk where the atmosphere rotates out of the colder night side and into the hot day side.
• Evening limb – the trailing edge where heated daytime gases cross over into the dark side.

To capture this process in motion, Mukherjee and his colleagues employed a technique called limb‑resolved spectroscopy.

Slicing Transits

Because it takes a little bit of time for the planet to fully cross the star’s edge during the beginning and end of the transit, the telescope sees the leading morning limb block the starlight slightly before the trailing evening limb does. Using JWST’s Near‑Infrared Imager and Slitless Spectrograph (NIRISS), the team measured the light curves as WASP‑94A b transited and split the signal. This allowed them to extract two separate chemical transmission spectra for the exoplanet: one for its morning limb and one for its evening limb. The two spectra were markedly different.

Morning Limb

• The spectrum appears as a sloped line that rises toward shorter wavelengths.
• This slope indicates high‑altitude aerosols (dust and cloud particles) that block light from deeper layers of the atmosphere.

“You would see a lot of dust and cloud particles at very high altitudes,” Mukherjee says. “Going deeper, the clouds likely clear up, and you would probably find water vapor and these kinds of gases.”

Evening Limb

• The spectrum shows no substantial evidence of aerosols.
• Distinct spikes of gaseous water vapor are present, suggesting a clearer view of the atmospheric gases.

“This would be a different view where you do not encounter many clouds through your journey, but what you see is just gas—water vapor mostly and other gases, maybe like carbon dioxide,” Mukherjee suggests.

Atmospheric Modeling

By feeding the JWST data into computer models, the team could also predict the weather engine on WASP‑94 b and visualize how atmospheric conditions change as the planet rotates from its morning to evening limb.

Equatorial Winds

The average temperature on WASP‑94A b exceeds 1,500 K, and Mukherjee’s team confirmed that the evening limb is about 450 K hotter than the morning limb—hot enough to evaporate potential aerosol materials such as iron or magnesium silicate. This temperature contrast drives the planet’s weather dynamics.

• Night‑side condensation:
  On the permanent night side, gases in the atmosphere condense into droplets, forming clouds.

• Equatorial transport:
  “These cloud particles are then dragged by the equatorial wind towards the morning side,” Mukherjee explains. As the clouds are pushed into the heated day side, most droplets evaporate. By the time the winds reach the evening limb again, the clouds are almost completely gone, leaving clear skies.

Clouds vs. Hazes

Based on the observed day‑side/night‑side aerosol distribution, the team concluded that WASP‑94 b possesses actual clouds rather than hazes.

• Hazes are photochemical smog produced when intense ultraviolet radiation breaks down molecules.
• Because hazes form under UV illumination, they should appear preferentially on the permanent day side. Global jet streams would then carry them toward the evening limb, making sunsets hazy and mornings relatively clear—the exact opposite of what the data show.

Cloud‑Support Mechanism

The researchers also calculated how the atmosphere keeps the clouds aloft. The equatorial wind is strong enough to push the heavy mineral droplets across the night side faster than gravity can pull them down.

Impact of Data Treatment

Finally, the team performed an experiment in which they re‑analyzed their precise JWST data without splitting it into two limbs.

“This had a huge effect on our understanding of the composition of this planet,” Mukherjee says.

When the atmosphere was averaged in a traditional, single‑limb model, the resulting composition differed markedly—an outcome that could have broader implications for exoplanet science.

Biased Composition

Because the thick morning clouds diluted the clear water‑vapor signals from the evening, the single‑sphere model concluded that the planet’s metallicity—the abundance of elements heavier than hydrogen and helium—was suspiciously high.

“With the limbs resolved, we’ve got an oxygen enrichment of this planet that was three to five times higher than our Sun,” Mukherjee explains. When the team averaged the spectrum, the oxygen enrichment came out about 100 times higher.

This bias in the composition estimates, he argues, probably affects other tidally locked exoplanets, including sub‑Neptunes and super‑Earths that are smaller than WASP‑94A b. For now, however, we have not been able to resolve the morning‑and‑evening asymmetries in these smaller planets, even with JWST. The team believes there is still a lot we can do before concluding that an even larger telescope is required.

“We need to think harder about how to mitigate this bias,” Mukherjee says. The answer, he suggests, might be figuring out how to disentangle morning and evening limbs in smaller planets based on the data we get from the instruments we have. “And even if we don’t have this kind of measurement, we can develop our theoretical models to mitigate this even if we have an averaged spectrum of the planet,” he adds.

Science, 2026. DOI: 10.1126/science.adx5903

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About the Author

Jacek Krywko is a freelance science and technology writer who covers space exploration, artificial‑intelligence research, computer science, and all sorts of engineering wizardry.

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