UV radiation can induce photochemical processes in exoplanet atmospheres and produce haze particles. Recent observations suggest that haze and/or cloud layers could be present in the upper atmospheres of exoplanets. Haze particles play an important role in planetary atmospheres and may provide a source of organic material to the surface that may impact the origin or evolution of life. However, very little information is known about photochemical processes in cool, high-metallicity exoplanetary atmospheres. Previously, we investigated haze formation and particle size distribution in laboratory atmosphere simulation experiments using AC plasma as the energy source. Here, we use UV photons to initiate the chemistry rather than the AC plasma, as photochemistry driven by UV radiation is important for understanding exoplanet atmospheres. We present photochemical haze formation in current UV experiments; we investigated a range of atmospheric metallicities (100×, 1000×, and 10000× solar metallicity) at three temperatures (300, 400, and 600 K). We find that photochemical hazes are generated in all simulated atmospheres with temperature-dependent production rates: the particles produced in each metallicity group decrease as the temperature increases. The images taken with atomic force microscopy show the particle size (15–190 nm) varies with temperature and metallicity. Our laboratory experimental results provide new insight into the formation and properties of photochemical haze, which could guide exoplanet atmosphere modeling and help to analyze and interpret current and future observations of exoplanets.

CO is an important component in many atmospheres, including Titan, Triton, and Pluto, and has also been detected in the atmosphere of a number of exoplanets. Numerous experimental simulations have been carried out in the laboratory to understand the chemistry in atmospheres, but very few simulations have included CO in the initial gas mixtures. The effect of CO on the chemistry occurring in these atmospheres is still poorly understood. We have investigated the effect of CO on both gas and solid phase chemistry in a series of planetary atmosphere simulation experiments using gas mixtures of CO, , and with a range of CO mixing ratios from 0.05% to 5% at low temperature (∼100 K). We find that CO affects the gas phase chemistry, the density, and the composition of the solids. Specifically, with the increase of CO in the initial gases, there is less but more , HCN, , and produced in the gas phase, while the density, oxygen content, and degree of unsaturation of the solids increase. The results indicate that CO has an important impact on the chemistry occurring in our experiments and accordingly in planetary atmospheres.

Super-Earths and mini-Neptunes are the most abundant types of planets among the ∼3500 confirmed exoplanets, and are expected to exhibit a wide variety of atmospheric compositions. Recent transmission spectra of super-Earths and mini-Neptunes have demonstrated the possibility that exoplanets have haze/cloud layers at high altitudes in their atmospheres. However, the compositions, size distributions, and optical properties of these particles in exoplanet atmospheres are poorly understood. Here, we present the results of experimental laboratory investigations of photochemical haze formation within a range of planetary atmospheric conditions, as well as observations of the color and size of produced haze particles. We find that atmospheric temperature and metallicity strongly affect particle color and size, thus altering the particles’ optical properties (e.g., absorptivity, scattering, etc.); on a larger scale, this affects the atmospheric and surface temperature of the exoplanets, and their potential habitability. Our results provide constraints on haze formation and particle properties that can serve as critical inputs for exoplanet atmosphere modeling, and guide future observations of super-Earths and mini-Neptunes with the Transiting Exoplanet Survey Satellite, the James Webb Space Telescope, and the Wide-Field Infrared Survey Telescope.