Presentation #503.02 in the session Atmospheres 4.
With thousands of confirmed detections of exoplanets, recent efforts are focusing on characterizing their atmospheres through transmission spectroscopy. However, opacity sources such as condensation clouds and photochemical hazes have been found to be common in exoplanet atmospheres, muting spectroscopic features of atmospheric species. In the atmospheres of temperate exoplanets (equilibrium temperature Teq < 1000 K), photochemical hazes are thought to be the dominant opacity source. To better understand atmospheric physics and chemistry in these exotic worlds, we use a combination of data analysis and laboratory experiments to constrain the formation and evolution of photochemical hazes in temperate exoplanet atmospheres.
We first analyzed statistical trends between various planetary parameters and exoplanet haziness using existing atmospheric characterization data of 23 temperate sub-Neptunes. We found that previously measured haze production rates alone cannot explain the haziness–temperature trend. Thus, we measured the surface energy for a matrix of laboratory-produced haze analogs (Teq < 800 K) and quantified the removal rates of hazes via sedimentation and cloud formation. We find a diverse range of surface energies and hence removal rates in exoplanet atmospheres. In particular, we find a transition temperature around 400-500 K, where photochemical hazes have relatively high production rates but the lowest removal rates, indicating we will likely encounter the haziest exoplanets in this temperature range. At lower or higher temperatures, haze removal rates are higher, leading to relatively less hazy atmospheres.
We also investigated the evolution of photochemical hazes through thermal decomposition experiments, mimicking haze transformation in the deep, hot part of the atmosphere or on hotter daysides through vertical or horizontal transport. We found none of the haze samples can be destroyed entirely even after being heated to extreme temperatures (~1500 K) and they are transformed into black-carbon-like materials around ~800-900 K. Thus, on exoplanets with large day-night temperature contrasts and/or strong horizontal/vertical mixing, we may expect to see aerosols behaving closer to black-carbon-like materials.