2025AJ....170...50M

We characterize the atmosphere of the hot super-Neptune WASP-166b (P = 5.44 days, R_p = 6.9 ± 0.3 R_⊕, M_p = 32.1 ± 1.6 M_⊕, T_eq = 1270 ± 30 K) orbiting an F9V star using JWST transmission spectroscopy with NIRISS and NIRSpec (0.85-5.17 μm). With this broad wavelength range, NIRISS provides strong constraints on H₂O and clouds (where NIRSpec performs poorly) while NIRSpec captures CO₂ and NH₃ (where NIRISS performs poorly). Our POSEIDON free chemistry retrievals confirm the detection of H₂O (15.2σ significance) and detect CO₂ (14.7σ) for the first time. We also find a possible hint of NH₃ (2.3σ) and an intermediate pressure cloud deck (2.6σ). Finally, we report inconclusive support for the presence of SO₂, CO, and Na, as well as nondetections of CH₄, C₂H₂, HCN, H₂S, and K. We verify our results using a TauREx free chemistry retrieval. We also measure with POSEIDON equilibrium chemistry retrievals a superstellar planetary atmospheric metallicity ($\mathrm{log}\,(Z)=1.5{7}_{-0.18}^{+0.17}$, $Z=3{7}_{-13}^{+18}$) and planetary C/O ratio (${\rm{C}}/{\rm{O}}=0.28{2}_{-0.053}^{+0.078}$) consistent with the stellar C/O ratio (C/O_* = 0.41 ± 0.08). These results are compatible with various planetary formation pathways, especially those that include planetesimal accretion followed by core erosion or photoevaporation. WASP-166b also resides near the edge of the Hot Neptune Desert, a sparse population of intermediate-sized planets at high insolation fluxes; thus, these results and further atmospheric observations of Hot Neptunes will help determine the driving processes in the formation of the Hot Neptune Desert.