The impact of organic hazes and graphite on the observation of CO2-rich sub-Neptune atmospheres

Abstract: Many sub-Neptune and super-Earth exoplanets are expected to develop metal-enriched atmospheres due to atmospheric loss processes such as photoevaporation or core-powered mass loss. Thermochemical equilibrium calculations predict that at high metallicity and a temperature range of 300-700 K, CO2 becomes the dominant carbon species, and graphite may be the thermodynamically favored condensate under low-pressure conditions. Building on prior laboratory findings that such environments yield organic haze rather than graphite, we measured the transmittance spectra of organic haze analogues and graphite samples, and computed their optical constants across the measured wavelength range from 0.4 to 25 {\mu}m. The organic haze exhibits strong vibrational absorption bands, notably at 3.0, 4.5, and 6.0 {\mu}m, while graphite shows featureless broadband absorption. The derived optical constants of haze and graphite provide the first dataset for organic haze analogues formed in CO2-rich atmospheres and offer improved applicability over prior graphite data derived from bulk reflectance or ellipsometry. We implemented these optical constants into the Virga and PICASO cloud and radiative transfer models to simulate transit spectra for GJ 1214b. The synthetic spectra with organic hazes reproduce the muted spectral features in the NIR observed by Hubble and general trends observed by JWST for GJ 1214b, while graphite models yield flat spectra across the observed wavelengths. This suggests haze features may serve as observational markers of carbon-rich atmospheres, whereas graphite's opacity could lead to radius overestimation, offering a possible explanation for super-puff exoplanets. Our work supplies essential optical to infrared data for interpreting observations of CO2-rich exoplanet atmospheres.