Detection of H2O and CO2 in the Atmosphere of the Hot Super-Neptune WASP-166b with JWST

We characterize the atmosphere of the hot super-Neptune WASP-166b (P = 5.44 days, Rp = 6.9 ± 0.3 R⊕, Mp = 32.1 ± 1.6 M⊕, Teq = 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 ( log(Z)=1.57−0.18+0.17 , Z=37−13+18 ) and planetary C/O ratio ( C/O=0.282−0.053+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.