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-

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 which 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, since 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 K, 400 K, 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.3

Sub-Neptunes (R p ∼ 1.25–4 R Earth) remain the most commonly detected exoplanets to date. However, it remains difficult for observations to tell whether these intermediate-sized exoplanets have surfaces and where their surfaces are located. Here we propose that the abundances of trace species in the visible atmospheres of these sub-Neptunes can be used as proxies for determining the existence of surfaces and approximate surface conditions. As an example, we used a state-of-the-art photochemical model to simulate the atmospheric evolution of K2-18b and investigate its final steady-state composition with surfaces located at different pressures levels (P surf). We find that the surface location has a significant impact on the atmospheric abundances of trace species, making them deviate significantly from their thermochemical equilibrium and “no-surface” conditions. This result arises primarily because the pressure–temperature conditions at the surface determine whether photochemically produced species can be recycled back to their favored thermochemical equilibrium forms and transported back to the upper atmosphere. For an assumed H2-rich atmosphere for K2-18b, we identify seven chemical species that are most sensitive to the existence of surfaces: ammonia (NH3), methane (CH4), hydrogen cyanide (HCN), acetylene (C2H2), ethane (C2H6), carbon monoxide (CO), and carbon dioxide (CO2). The ratio between the observed and the no-surface abundances of these species can help distinguish the existence of a shallow surface (P surf < 10 bar), an intermediate surface (10 bar < P surf < 100 bar), and a deep surface (P surf > 100 bar). This framework can be applied together with future observations to other sub-Neptunes of interest.

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Sulfur gases significantly affect the photochemistry of planetary atmospheres in ourSolar System, and are expected to be important components in exoplanet atmospheres. However, sulfur photochemistry in the context of exoplanets is poorly understood due to a lack of chemical-kinetics information for sulfur species under relevant conditions.Here, we study the photochemical role of hydrogen sulfide (H2S) in warm CO2-rich exoplanet atmospheres (800 K) by carrying out laboratory simulations. We find that H2S plays a significant role in photochemistry, even when present in the atmosphere at relatively low concentrations (1.6%). It participates in both gas and solid phase chemistry, leading to the formation of other sulfur gas products (CH3SH/SO, C2H4S/OCS, SO2/S2, and CS2) and to an increase in solid haze particle production and compositional complexity. Our study shows that we may expect thicker haze with small particle sizes (20 to 140 nm) for warm CO2-rich exoplanet atmospheres that possess H2S.Observations 1-5 and laboratory simulations 6,7 have shown that clouds and/or hazes are likely ubiquitous in the atmospheres of exoplanets. These clouds and hazes play an important role in exoplanet atmospheres and affect the spectra of planets, therefore impacting our ability to observe their atmosphere and assess their habitability. Although a variety of atmospheric gases can condense at specific temperature and pressure conditions to form clouds, haze particles may be produced photochemically over a range of temperatures, pressures, and atmospheric compositions. 6−9 Among the various atmospheric components, sulfur gases significantly influence the photochemistry and haze formation in the atmospheres of Solar System bodies, such as Earth, Venus 10,11 , Jupiter 12,13 , and its moon, Io.