Determine the presence and properties of clouds and hazes in rocky exoplanet atmospheres

Characterize the presence, altitude, and optical properties of clouds and hazes in the atmospheres of rocky exoplanets, and quantify their impact on spectral observables and atmospheric interpretation.

Background

Clouds and hazes can flatten or mute transmission spectra, confounding efforts to diagnose atmospheric composition and even the existence of an atmosphere. The working group underscores the uncertainty in the cloud/haze properties of rocky worlds and notes that stellar contamination further complicates interpretation of transmission data.

Understanding aerosol properties is necessary to design effective observation strategies and to interpret spectra obtained across wavelengths, including with JWST and supporting HST UV data.

References

Clouds and hazes are an additional unknown, though extrapolating from the Solar System and gaseous exoplanets, their presence would not come as a surprise.

— Report of the Working Group on Strategic Exoplanet Initiatives with HST and JWST  (2404.02932 - Redfield et al., 2024) in Section 7, Frequently Asked Questions: Why not transits?

We therefore adopt the resulting P--T profiles as fixed background atmospheres (with respect to contact angle and vertical mixing) and leave a self-consistent treatment of aerosol radiative feedback (including feedback due to clouds and hazes) to future work.

— Wet Removal and Cloud Enhancement: The Microphysics of Cloud-Haze Interactions on Sub-Neptunes  (2608.19100 - Nagpal et al., 19 Aug 2026) in Section 2.2, “Modeling haze formation”

Finally, whether cloud-haze interactions produce a unique observational signature remains an open and interesting question.

— Wet Removal and Cloud Enhancement: The Microphysics of Cloud-Haze Interactions on Sub-Neptunes  (2608.19100 - Nagpal et al., 19 Aug 2026) in Section 7.5, “Observational Strategies and Implications”

The presence of small mass mixing ratio of haze is not ruled out, though.

— Asymmetric Aerosol Distribution on the Terminators of the Warm Saturn WASP-69 b Revealed by JWST NIRISS/SOSS  (2608.19756 - Wang et al., 20 Aug 2026) in Section 7.2, Discussion, subsection “Aerosol Properties”

An uncertainty in our study is the presence of the SiO cloud within the atmosphere of GJ 499 C.

— Benchmark Brown Dwarfs as Chemical Laboratories: Linking System Bulk Properties to Atmospheric Retrievals  (2608.26029 - Kecskeméthy et al., 26 Aug 2026) in Section “Is there an SiO cloud in the atmosphere of GJ 499 C?” (Section 5.4)