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 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 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 (15.2 σ significance) and detect CO 2 (14.7 σ ) for the first time. We also find a possible hint of NH 3 (2.3 σ ) and an intermediate pressure cloud deck (2.6 σ ). Finally, we report inconclusive support for the presence of SO 2 , CO, and Na, as well as nondetections 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>Z</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>1.5</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.18</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.17</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>Z</mml:mi> <mml:mo>=</mml:mo> <mml:mn>3</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>13</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>18</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ) and planetary C/O ratio ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">C</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">O</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.28</mml:mn> <mml:msubsup> <mml:mn>2</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.053</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.078</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ) 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.