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Arxiv: JWST Reveals CH_4, CO_2, and H_2O in a Metal-rich Miscible Atmosphere on a Two-Earth-Radius Exoplanet Published: 3/5/2024 11:07:43 PM Updated: 3/5/2024 11:07:43 PM

Paper abstract: Even though sub-Neptunes likely represent the most common outcome of planetformation, their natures remain poorly understood. In particular, planets near1.5-2.5\,R_\oplus often have bulk densities that can be explained equallywell with widely different compositions and interior structures, resulting ingrossly divergent implications for their formation. Here, we present the full0.6-5.2\,\mu \mathrm{m} JWST NIRISS/SOSS+NIRSpec/G395H transmission spectrumof the 2.2\,R_\oplus TOI-270d (4.78\,M_\oplus, T_\mathrm{eq}=350-380 K),delivering unprecedented sensitivity for atmospheric characterization in thesub-Neptune regime. We detect five vibrational bands of CH_4 at 1.15, 1.4,1.7, 2.3, and 3.3\,\mum (9.4\sigma), the signature of CO_2 at 4.3\,\mum(4.8\sigma), water vapor (2.5\sigma), and potential signatures of SO_2 at4.0\,\mu \mathrm{m} and CS_2 at 4.6\,\mu\mathrm{m}. Intriguingly, we findan overall highly metal-rich atmosphere, with a mean molecular weight of5.47_{-1.14}^{+1.25}. We infer an atmospheric metal mass fraction of58_{-12}^{+8}\% and a C/O of 0.47_{-0.19}^{+0.16}, indicating thatapproximately half the mass of the outer envelope is in high-molecular-weightvolatiles (H_2O, CH_4, CO, CO_2) rather than H_2/He. We introduce asub-Neptune classification scheme and identify TOI-270d as a "miscible-envelopesub-Neptune" in which H_2/He is well-mixed with the high-molecular-weightvolatiles in a miscible supercritical metal-rich envelope. For a fully miscibleenvelope, we conclude that TOI-270d's interior is 90_{-4}^{+3}\,wt\,\%rock/iron, indicating that it formed as a rocky planet that accreted a few wt %of H_2/He, with the overall envelope metal content explained bymagma-ocean/envelope reactions without the need for significant ice accretion.TOI-270d may well be an archetype of the overall population of sub-Neptunes.