Silicate clouds on hot exoplanets should settle. Gravity pulls the particles downward; only turbulent lofting and condensation at altitude keep them aloft. Microphysical models balance these forces and predict a characteristic altitude where cloud opacity peaks. JWST can now measure that altitude directly through transit spectroscopy — the wavelength-dependent absorption as the planet crosses its star's disk.
The clouds are too high.
Arfaux and Min (arXiv:2603.13167, March 2026) compare JWST observations of silicate cloud features on four exoplanets with the Nimbus microphysical model. The observed cloud features sit at altitudes significantly above where the models place them. The discrepancy is not small — matching the observations requires either an extremely low settling efficiency (fsed below 0.1, meaning particles barely fall) or a sticking efficiency so low that particles barely grow.
The sticking physics is the key. Cloud particles grow by colliding and sticking — just like water droplets in terrestrial clouds. The rate of growth determines the particle size, the particle size determines the settling speed, and the settling speed determines the altitude. If particles stick less efficiently on these worlds — because the silicate grains have different surface energies, different charging states, or different collision velocities — they stay small, fall slowly, and remain at high altitudes.
The structural implication is that the sticking physics governing cloud formation on hot exoplanets is fundamentally different from the assumptions imported from solar system cloud models. Earth-based intuition about how particles aggregate fails at 1500 kelvin in hydrogen-dominated atmospheres with silicate condensates. The clouds are not defying gravity — they are made of particles that grow too slowly to fall.
The measurement is the first empirical constraint on exoplanet cloud microphysics from direct spectral comparison. Before JWST, cloud models were tuned to reproduce broadband photometry — a blunt tool that constrains total cloud opacity but not altitude. Spectral features pin the altitude, and the altitude constrains the particle physics. The telescope is finally sharp enough to tell us what the clouds are made of by telling us where they float.