Even though subNeptunes likely represent the most common outcome of planet formation their natures remain poorly understood. In particular planets near 1.52.5Roplus often have bulk densities that can be explained equally well with widely different compositions and interior structures resulting in grossly divergent implications for their formation. Here we present the full 0.65.2mu mathrmm JWST NIRISSSOSSNIRSpecG395H transmission spectrum of the 2.2Roplus TOI270d 4.78Moplus Tmathrmeq350380 K delivering unprecedented sensitivity for atmospheric characterization in the subNeptune regime. We detect five vibrational bands of CH4 at 1.15 1.4 1.7 2.3 and 3.3mum 9.4sigma the signature of CO2 at 4.3mum 4.8sigma water vapor 2.5sigma and potential signatures of SO2 at 4.0mu mathrmm and CS2 at 4.6mumathrmm. Intriguingly we find an overall highly metalrich atmosphere with a mean molecular weight of 5.471.141.25. We infer an atmospheric metal mass fraction of 58128 and a CO of 0.470.190.16 indicating that approximately half the mass of the outer envelope is in highmolecularweight volatiles H2O CH4 CO CO2 rather than H2He. We introduce a subNeptune classification scheme and identify TOI270d as a miscibleenvelope subNeptune in which H2He is wellmixed with the highmolecularweight volatiles in a miscible supercritical metalrich envelope. For a fully miscible envelope we conclude that TOI270ds interior is 9043wt rockiron indicating that it formed as a rocky planet that accreted a few wt  of H2He with the overall envelope metal content explained by magmaoceanenvelope reactions without the need for significant ice accretion. TOI270d may well be an archetype of the overall population of subNeptunes.
['Astrophysics - Earth and Planetary Astrophysics']
JWST Reveals CH4 CO2 and H2O in a Metalrich Miscible Atmosphere on a TwoEarthRadius Exoplanet
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