A Possible Metal-dominated Atmosphere below the Thick Aerosols of GJ 1214 b Suggested by Its JWST Panchromatic Transmission Spectrum

Abstract

GJ 1214b is the archetype sub-Neptune for which thick aerosols have prevented us from constraining its atmospheric properties for over a decade. In this study, we leverage the panchromatic transmission spectrum of GJ 1214b established by the Hubble Space Telescope (HST) and JWST to investigate its atmospheric properties using a suite of atmospheric radiative transfer, photochemistry, and aerosol microphysical models. We find that the combined HST, JWST/NIRSpec, and JWST/MIRI spectrum can be well explained by atmospheric models with an extremely high metallicity of [M/H] ∼ 3.5 and an extremely high haze production rate of Fhaze ∼ 10−8 to 10−7 g cm−2 s−1. Such high atmospheric metallicity is suggested by the relatively strong CO2 feature compared to the haze absorption feature or the CH4 feature in the NIRSpec-G395H bandpass of 2.5–5 μm. The flat 5–12 μm MIRI spectrum also suggests a small scale height with a high atmospheric metallicity that is needed to suppress a prominent ∼6 μm haze feature. We tested the sensitivity of our interpretation to various assumptions for uncertain haze properties, such as optical constants and production rate, and all models tested here consistently suggest extremely high metallicity. Thus, we conclude that GJ 1214b likely has a metal-dominated atmosphere where hydrogen is no longer the main atmospheric constituent. We also find that different assumptions for the haze production rate lead to distinct inferences for the atmospheric C/O ratio. We stress the importance of high-precision follow-up observations to confirm the metal-dominated atmosphere, as it challenges the conventional understanding of interior structure and evolution of sub-Neptunes.