Formation of organic hazes in CO-rich sub-Neptune atmospheres within the graphite-stability regime
arXiv:2508.05974 · doi:10.3847/1538-4357/adfa0a
Abstract
Super-Earths and sub-Neptunes are the most common exoplanets, with a "radius valley" suggesting that super-Earths may form by shedding sub-Neptunes' gaseous envelopes. Exoplanets that lie closer to the super-Earth side of the valley are more likely to have lost a significant fraction of their original H/He envelopes and become enriched in heavier elements with CO gaining in abundance. It remains unclear which types of haze would form in such atmospheres, potentially significantly affecting spectroscopic observations. To investigate this, we performed laboratory simulations of two CO-rich gas mixtures (with 2000 times solar metallicity at 300 K and 500 K). We found that under plasma irradiation, organic hazes were produced at both temperatures with higher haze production rate at 300 K probably because condensation occurs more readily at lower temperature. Gas-phase analysis demonstrates the formation of various hydrocarbons, oxygen- and nitrogen-containing species, including reactive gas precursors like CH, CHO, and HCN, for haze formation. The compositional analysis of the haze particles reveals various functional groups and molecular formulas in both samples. The 500 K haze sample has larger average molecular sizes, higher degree of unsaturation with more double or triple bonds presence, and higher nitrogen content incorporated as N-H, C=N bonds, indicating different haze formation pathways. These findings not only improve the haze formation theories in CO-rich exoplanet atmospheres but also offer important implications for the interpretation of future observational data.
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