Optical properties of organic haze analogues in water-rich exoplanet atmospheres observable with JWST

The James Webb Space Telescope (JWST) has begun its scientific mission, which includes the atmospheric characterization of transiting exoplanets. Some of the first exoplanets to be observed by JWST have equilibrium temperatures below 1,000 K, which is a regime where photochemical hazes are expected to form. The optical properties of these hazes, which control how they interact with light, are critical for interpreting exoplanet observations, but relevant experimental data are not available. Here we measure the density and optical properties of organic haze analogues generated in water-rich exoplanet atmosphere experiments. We report optical constants (0.4 to 28.6 μm) of organic haze analogues for current and future observational and modelling efforts covering the entire wavelength range of JWST instrumentation and a large part of Hubble. We use these optical constants to generate hazy model atmospheric spectra. The synthetic spectra show that differences in haze optical constants have a detectable effect on the spectra, impacting our interpretation of exoplanet observations. This study emphasizes the need to investigate the optical properties of hazes formed in different exoplanet atmospheres and establishes a practical procedure for determining such properties. The reported optical properties of organic hazes produced in water-rich exoplanet atmospheres differ from those in nitrogen-rich atmospheres. Such differences have a detectable effect on the spectra, impacting interpretation of JWST observations.