Webb telescope captures first daily cloud cycle on the exoplanet WASP-94A b
A Cambridge and ESA team reports in Science that Webb telescope observations show magnesium-silicate clouds on the hot Jupiter WASP-94A b forming in the morning and evaporating by evening, the first direct evidence of a diurnal weather cycle beyond the Solar System.
A team of astronomers at the University of Cambridge and the European Space Agency, led by Nikku Madhusudhan, reported on May 21, 2026 in Science the first direct evidence of a daily weather cycle on an exoplanet.
Webb telescope data, collected with the NIRSpec and MIRI instruments, show that on the hot Jupiter WASP-94A b, a planet about 700 light-years from the Sun, clouds made of magnesium silicate systematically form in the morning hours and evaporate by evening. It is the first time a complete daily cycle of cloud condensation and evaporation has been observed beyond the Solar System.
What WASP-94A b is
WASP-94A b is a hot Jupiter, a gas planet comparable in mass to Jupiter but with a much higher temperature because of its proximity to its host star. It was discovered in 2014 by the transit method, which detects the periodic dip in the star’s brightness as the planet passes in front of it.
Only with the arrival of the Webb telescope has it become possible to analyse its atmosphere in detail with spectroscopy. The planet is tidally locked, which means:
- one side permanently faces the star
- the other is always in shadow
- the temperature difference between day side and night side exceeds 1,000 degrees Celsius
The authors use this difference as a natural laboratory for studying atmospheric dynamics.
Methodology
The observations were carried out over several consecutive transits of the planet. That allowed comparison of spectra as starlight passed through different parts of the atmosphere.
At the morning terminator, the boundary between day and night, the spectrum shows a distinct absorption line consistent with magnesium silicates. The composition is similar to rocky cloud particles found in the Solar System.
At the evening terminator the same line is absent. The particles have either evaporated, or sunk to lower altitudes where they no longer absorb light. Atmospheric circulation models suggest that strong winds carry material from the cooler night side to the hot day side, where clouds condense in the morning and then dissipate.
Madhusudhan put it this way:
For the first time we can observe weather changes in the atmosphere of a planet orbiting another star in near real time, which opens the path to similar measurements on rocky planets in habitable zones.
What this does NOT prove about life
The result describes a weather cycle that resembles Earth’s atmospheric processes, but it does not suggest conditions suitable for life. WASP-94A b is a gas giant with no solid surface, with daytime temperatures of about 1,500 degrees Celsius, which makes any biological process impossible.
The value of the finding lies in demonstrating that the Webb telescope is sensitive enough to measure atmospheric changes on exoplanets. The atmosphere also shows up as roughly five times richer in carbon than Jupiter’s.
A second important limitation is that the measured signals are close to the instrument’s noise threshold. Madhusudhan’s team had to combine multiple independent observations to raise statistical significance. Further confirmation will require additional transits, as well as observations of other similar exoplanets to establish whether the cloud cycle is a general phenomenon or specific to WASP-94A b. The next target is WASP-39 b, a well-studied exoplanet whose composition and atmosphere are known but whose daily cycle has not yet been demonstrated.
Relevance for Serbian astronomy
The result matters because it shows that many planets we have so far considered static worlds are in fact dynamic systems with weather patterns similar to Earth’s. The Belgrade Astronomical Observatory has been collaborating with European exoplanet networks for several years, particularly through the CHEOPS programme. That gives Serbian researchers access to raw Webb telescope data for their own analysis.
Will we be able to observe clouds on smaller, rocky planets by 2030?
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