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,” Mukherjee says. “Are they static or dynamic? Do they have winds? Do they have clouds?” 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 has a diameter that’s over 70 percent wider. “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 light filtering through the planet’s atmosphere as it crosses in front of its star, they can figure out its chemical composition.
The problem with this approach is that the light filtering through the entire circumference of the planet’s silhouette was averaged out, as though its atmosphere was one homogenous ball of gas. For tidally locked planets, this was a massive oversimplification.
On tidally locked worlds, there are massive temperature swings between day and night sides, which usually lead to differences in atmospheric density between the day side and the night side. These differences, combined with the Coriolis effect that stems from the planet’s slow rotation, cause a phenomenon called equatorial super-rotation. This is where winds on the equator blow eastward faster than the planet is spinning. Circulation models predicted this is exactly what’s happening on WASP-94B a.
The leading edge of the planet’s disk, called the morning limb, is the region where the local atmosphere is rotating out of the colder night side and into the hot day side. The trailing edge at the evening limb is where the heated daytime gases are crossing over into the dark side. To catch 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 way, they managed to extract two separate chemical transmission spectra for the exoplanet: one for its morning, and one for its evening limb. And there was quite a difference between the two.
The morning limb’s spectrum was just a sloped line, rising at shorter wavelengths, which indicated high-altitude aerosols blocking the light from deeper in 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.”
On the evening limb, the spectrum showed no substantial evidence of aerosols and revealed spikes of gaseous water vapor. “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.
By feeding the JWST data into computer models, the team could also predict what the weather engine on WASP-94 b looks like in motion.
Equatorial winds
The average temperature on WASP-94A b exceeds 1,500 Kelvin, and Mukherjee’s team confirmed the evening limb is around 450 Kelvin hotter than the morning limb—hot enough to evaporate potential aerosol materials like iron or magnesium silicate. This temperature difference dictates the weather dynamics on the planet.
On the permanent night side, gases in the atmosphere condense into droplets due to lower temperature, forming clouds. “These cloud particles are then dragged by the equatorial wind towards the morning side,” Mukherjee says. As the clouds are pushed into the heat of the day side, most of these droplets evaporate. By the time the winds reach the evening limb again, the clouds are almost completely gone, leaving the skies clear.
Based on this day-side/night-side aerosol distribution, the team determined WASP-94 b has actual clouds rather than hazes. The latter are basically photochemical smog created when intense radiation breaks the molecules down. Because hazes are produced by ultraviolet light, they should preferentially appear on the planet’s permanent day side. Global jet streams would then blow them into the evening limb, making the sunset hazy and the morning relatively clear—the exact reverse of what showed up in the data.
The team even managed to calculate how the atmosphere keeps the clouds aloft. The equatorial wind is apparently strong enough to push the heavy mineral droplets through the night side faster than gravity can pull them down.
Finally, the researchers ran an experiment where they took their precise JWST data and reanalyzed it without splitting it into two to resolve the limbs. “This had a huge effect on our understanding of the composition of this planet,” Mukherjee says. The results the researchers got when they averaged the atmosphere in a traditional model turned out a bit alarming for exoplanet science in general.
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, though, we have not been able to resolve the morning and evening asymmetries in these smaller planets, even using the JWST. But the team thinks there is still a lot we can do before concluding we need an even bigger telescope.
“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 measurements, we can think about how to develop our theoretical models to mitigate this even if we have an averaged spectrum of the planet,” Mukherjee claims.
Science, 2026. DOI: 10.1126/science.adx5903
Jacek Krywko is a freelance science and technology writer who covers space exploration, artificial intelligence research, computer science, and all sorts of engineering wizardry.
