JWST Reveals CH₄, CO₂, and H₂O in a Metal-rich Miscible Atmosphere on a Two-Earth-Radius Exoplanet

Even though sub-Neptunes likely represent the most common outcome of planet formation, their natures remain poorly understood. In particular, planets near 1. 5-2. 5\, R_ 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. 6-5. 2\, m JWST NIRISS/SOSS+NIRSpec/G395H transmission spectrum of the 2. 2\, R_ TOI-270d (4. 78\, M_, Tₑq=350-380 K), delivering unprecedented sensitivity for atmospheric characterization in the sub-Neptune regime. We detect five vibrational bands of CH₄ at 1. 15, 1. 4, 1. 7, 2. 3, and 3. 3\, m (9. 4), the signature of CO₂ at 4. 3\, m (4. 8), water vapor (2. 5), and potential signatures of SO₂ at 4. 0\, m and CS₂ at 4. 6\,. Intriguingly, we find an overall highly metal-rich atmosphere, with a mean molecular weight of 5. 47-₁. ₁₄^+1. 25. We infer an atmospheric metal mass fraction of 58-₁₂^+8\% and a C/O of 0. 47-₀. ₁₉^+0. 16, indicating that approximately half the mass of the outer envelope is in high-molecular-weight volatiles (H₂O, CH₄, CO, CO₂) rather than H₂/He. We introduce a sub-Neptune classification scheme and identify TOI-270d as a "miscible-envelope sub-Neptune" in which H₂/He is well-mixed with the high-molecular-weight volatiles in a miscible supercritical metal-rich envelope. For a fully miscible envelope, 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 H₂/He, with the overall envelope metal content explained by magma-ocean/envelope reactions without the need for significant ice accretion. TOI-270d may well be an archetype of the overall population of sub-Neptunes.