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Arxiv: A Possible Metal-Dominated Atmosphere Below the Thick Aerosols of GJ 1214 b Suggested by its JWST Panchromatic Transmission Spectrum Published: 10/14/2024 9:09:28 AM Updated: 10/14/2024 9:09:28 AM

Paper abstract: GJ1214b is the archetype sub-Neptune for which thick aerosols have preventedus from constraining its atmospheric properties for over a decade. In thisstudy, we leverage the panchromatic transmission spectrum of GJ1214bestablished by HST and JWST to investigate its atmospheric properties using asuite of atmospheric radiative transfer, photochemistry, and aerosolmicrophysical models. We find that the combined HST, JWST/NIRSpec and JWST/MIRIspectrum can be well-explained by atmospheric models with an extremely highmetallicity of [M/H]~3.5 and an extremely high haze production rate ofF_{\rm haze}{~}10^{-8}--10^{-7} g cm^{-2} s^{-1}. Such highatmospheric metallicity is suggested by the relatively strong CO2 featurecompared to the haze absorption feature or the CH4 feature in the NIRSpec-G395Hbandpass of 2.5--5 \mum. The flat 5--12 \mum MIRI spectrum also suggests asmall scale height with a high atmospheric metallicity that is needed tosuppress a prominent 6 \mum haze feature. We tested the sensitivity of ourinterpretation to various assumptions for uncertain haze properties, such asoptical constants and production rate, and all models tested here consistentlysuggest extremely high metallicity. Thus, we conclude that GJ1214b likely has ametal-dominated atmosphere where hydrogen is no longer the main atmosphericconstituent. We also find that different assumptions for the haze productionrate lead to distinct inferences for the atmospheric C/O ratio. We stress theimportance of high precision follow-up observations to confirm themetal-dominated atmosphere and to constrain the C/O ratio, which providesfurther insights on the planet formation process. The confirmation of themetal-dominated atmosphere is particularly crucial, as it challenges theconventional understanding of interior structure and evolution of sub-Neptunes.