Three dimensional simulation of photochemical haze formation in the atmosphere of HD 189733b

Star system: HD 189733 A | Distance from the Sun: 64.5 light years

Photochemical haze—tiny solid particles formed by chemical reactions triggered by starlight—plays a critical role in shaping the appearance and environment of exoplanetary atmospheres. A new 3D model was applied to the well-studied exoplanet HD 189733b to understand how these particles form, move, and affect the planet's overall temperature and light-blocking properties over time.

The simulation used a microphysical scheme coupled with a general circulation model to track haze particles in three dimensions. The model shows that particles do not grow larger than approximately 30 nanometers. Their distribution is heavily influenced by the atmosphere's vertical air currents, leading to a build-up of haze along the eastern and western limbs of the planet.

These haze particles create an enhanced opacity, meaning they effectively block more light at the planet's edges. The simulation also reveals that the timing of haze production significantly influences the planet's thermal structure. If haze forms slowly, it increases the light-blocking effect, while formation on the nightside provides stronger thermal feedback that increases the heat emitted from the dayside.

Key findings
- Haze particles in the atmosphere of HD 189733b do not exceed 30 nanometers in size.
- Distribution of haze follows atmospheric vertical velocity patterns, concentrating at the east and west limbs.
- Equatorial convergence of material is observed deeper in the atmosphere at approximately 0.01 bar pressure.
- Radiative feedback from haze significantly alters the temperature and pressure structure in the upper atmosphere.
- Longer haze-production timescales result in stronger haze opacity effects on transmission spectra.

Scientific study

Three dimensional temporal evolution of photochemical haze in exoplanet atmospheres I. Description and test application to HD 189733b (external link)