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

TL;DR

This study uses JWST transmission spectroscopy to detect water and carbon dioxide in the atmosphere of the hot super-Neptune WASP-166b, providing insights into its composition and formation history.

Contribution

First detection of CO2 in WASP-166b's atmosphere using broad wavelength JWST data with free chemistry retrievals.

Findings

H2O detected at 15.2σ significance

CO2 detected at 14.7σ significance

Possible hint of NH3 at 2.3σ

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 ($0.85-5.17$ $\mu$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 ($15.2\sigma$ significance) and detect CO$_2$ ($14.7\sigma$) for the first time. We also find a possible hint of NH$_3$ ($2.3\sigma$) and an intermediate pressure cloud deck ($2.6\sigma$). 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.