Characterising the atmosphere of GJ1214 b with JWST observations.

GJ-1214b, discovered by Charbonneau et al. (2009), is a transiting super-Earth/mini-Neptune with an M4.5-type host star at 14.6pc from the Sun. GJ-1214b is notable for its featureless transmission spectrum (Gillon et al., 2014; Kreidberg et al., 2014), which has been interpreted as evidence for the presence of clouds (ZnS, KCL) or photochemical hazes (Morley et al. 2013, 2015; Charnay et al. 2015; Gao and Benneke 2018; Ohno & Okuzumi 2018, Adam et al. 2019; Kawasima et al. 2019; Lavvas et al. 2019; Ohno et al. 2020; Christie et al. 2022). Latest observations of GJ 1214 b with JWST reveal a high-albedo atmosphere (Kempton et al. 2023), possibly rich in photochemical hazes (Gao et al. 2023). 

We use a self-consistent model coupling among photochemistry, radiative transfer, thermal structure and haze & cloud microphysics to characterise the atmosphere of GJ1214b. We explore different metallicity cases to constrain the atmospheric composition and evaluate different scenarios for the photochemical haze mass fluxes and the eddy mixing efficient. We particularly focus on the interplay between photochemical hazes and clouds, considering the formation of ZnS, KCL and NaCL clouds on hazes.

We demonstrate that the novel JWST thermal emission observations provide much better constraints on the atmospheric metallicity compared to transit spectra alone, limiting the retrieved metallicity to values between ~1000x-3000x solar. Our results demonstrate that the atmospheric albedo is in the order of 10-20%, considering both the contributions of hazes and clouds. Transit spectra are mostly affected by hazes rather than clouds, but the implications of the clouds on the atmospheric thermal structure provide an indirect effect to affect the transit signature. We further discuss the role of sulfur chemistry in the formation of the haze and its potential signature in the transit spectra.