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

Abstract

We characterize the atmosphere of the hot super-Neptune WASP-166b ($P = 5.44$ d, $R_p = 6.9 \pm 0.3$ R$_\oplus$, $M_p = 32.1 \pm 1.6$ M$_\oplus$, $T_\mathrm{eq} = 1270 \pm 30$ K) orbiting an F9V star using JWST transmission spectroscopy with NIRISS and NIRSpec ($0.85-5.17$ $μ$m). With this broad wavelength range, NIRISS provides strong constraints on H$_2$O and clouds (where NIRSpec performs poorly) while NIRSpec captures CO$_2$ and NH$_3$ (where NIRISS performs poorly). Our POSEIDON free chemistry retrievals confirm the detection of H$_2$O ($15.2σ$ significance) and detect CO$_2$ ($14.7σ$) for the first time. We also find a possible hint of NH$_3$ ($2.3σ$) and an intermediate pressure cloud deck ($2.6σ$). Finally, we report inconclusive support for the presence of SO$_2$, CO, and Na, as well as non-detections of CH$_4$, C$_2$H$_2$, HCN, H$_2$S, 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 ($\log(Z) = 1.57^{+0.17}_{-0.18}$, $Z = 37^{+18}_{-13}$) and planetary C/O ratio ($C/O = 0.282^{+0.078}_{-0.053}$) consistent with the stellar C/O ratio ($C/O_* = 0.41 \pm 0.08$). 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 scarcity 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.