A World That Rains Glass

A star dims by a tiny, repeating fraction each time the planet passes in front of it. When that dimming is measured colour by colour rather than as a single number, the depth of the dip changes with wavelength. That wavelength dependence is the signal — the planet's atmosphere blocks some colours of starlight more than others.

HD 189733 b orbits extremely close to its star, so its atmosphere is hot enough that materials which are solid rock on Earth can exist as vapour and condense high in the air as fine particles.

Small particles scatter short wavelengths more efficiently than long ones — the same physics that makes Earth's sky blue. Models of a silicate-rich haze reproduce the scattering seen in the data, which is where the popular image of "glass rain blown sideways" comes from. It is a physically motivated picture, not a photograph.

Transit observations establish that a planet is there and how large it is relative to its star. Transmission spectroscopy — comparing transit depth across wavelengths — probes the thin ring of atmosphere lit from behind during each transit.

A planet orbits this star on a short period. Its atmosphere is detectable in transmission. The wavelength behaviour of the light is consistent with scattering by very small particles rather than a clear, cloud-free atmosphere.

We cannot state the exact composition, size or altitude distribution of the particles from scattering alone. Several haze compositions can produce similar slopes. Wind speeds and the idea of droplets "raining" horizontally are model-dependent interpretations, not direct measurements.