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Abstract Photochemical hazes are suspected to obscure molecular features, such as water, from detection in the transmission spectra of exoplanets with atmospheric temperatures <800 K. The opacities of laboratory produced organic compounds (tholins) from Khare et al. have become a standard for modeling haze in exoplanet atmospheres. However, these tholins were grown in an oxygen-free, Titan-like environment that is very different from typical assumptions for exoplanets, where C/O ∼ 0.5. This work presents the 0.13–10 μ m complex refractive indices derived from laboratory transmission measurements of tholins grown in environments with different oxygen abundances. With the increasing uptake of oxygen, absorption increases across the entire wavelength range, and a scattering feature around 6 μ m shifts toward shorter wavelengths and becomes more peaked around 5.8 μ m, due to a C = O stretch resonance. Using GJ 1214 b as a test case, we examine the transmission spectra of a sub-Neptune planet with C/O ratios of solar, 1, and 1000 to evaluate the effective differences between our opacities and those of Khare. For an atmosphere with solar hydrogen and helium abundances, we find a difference of 200–1500 ppm, but for high-metallicity ( Z = 1000) environments, the difference may only be 20 ppm. The 1–2 μ m transmission data for GJ 1214 b rule out the Titan-like haze model, and are more consistent with C/O = 1 and C/O = solar haze models. This work demonstrates that using haze opacities that are more consistent with underlying assumptions about bulk atmospheric composition are important for building self-consistent models that appropriately constrain the atmospheric C/O ratio, even when molecular features are obscured.
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This content will become publicly available on September 10, 2026
The Impact of Organic Hazes and Graphite on the Observation of CO 2 -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, CO2becomes 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 analogs and graphite samples and computed their optical constants across the measured wavelength range from 0.4 to 25μm. The organic haze exhibits strong vibrational absorption bands, notably at 3.0, 4.5, and 6.0μm, while graphite shows featureless broadband absorption. The derived optical constants of haze and graphite provide the first data set for organic haze analogs 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 near-infrared 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 superpuff exoplanets. Our work supplies essential optical to infrared data for interpreting observations of CO2-rich exoplanet atmospheres.
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- Award ID(s):
- 2206245
- PAR ID:
- 10656898
- Publisher / Repository:
- IOP
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 990
- Issue:
- 2
- ISSN:
- 2041-8205
- Page Range / eLocation ID:
- L66
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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