Astronomy & Telescopes

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

By Keith Cowing

Status Report

astro-ph.EP

January 3, 2025

Filed under astro-ph.EP, astrochemistry, astrogeology, atmosphere, biosignature, exoplanet, hot Neptune desert, hot super-Neptune, JWST, NIRISS SOSS, NIRSpec, POSEIDON, super-Neptunes, Transmission spectroscopy, WASP-166 b

Detection of H2O and CO2 in the Atmosphere of the Hot Super-Neptune WASP-166b with JWST — The spectral decomposition of the Reference case retrieval of WASP-166b with the absorption contributions of the key atmospheric constituents shown. Spectra are plotted with resolution R = 100 for clarity. The combined forward model, shown as a heavy black line, is the median Reference retrieval result (see Table 5). The colored lines correspond to the individual opacity/absorption contributions (including Rayleigh scattering and collision absorption) of the various constituent molecules and clouds at specific wavelengths. (H2S is plotted but is not distinguishable from the bulk species line.) The transmission spectrum is dominated by contributions from H2O (15.2σ), and CO2 (14.7σ). The more subtle effects of NH3 (2.3σ) may be apparent at ∼ 2.3 µm, and possibly at ∼ 3.9 µm. The effects of the cloud deck (flat purple line at transit depth of ∼ 0.274 x10−2 ) are apparent in clipping the troughs of the water features from 0.85 to 1.7 µm — astro-ph.EP

We characterize the atmosphere of the hot super-Neptune WASP-166b (P=5.44 d, Rp=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 observations obtained with NIRISS SOSS Order-1 and NIRSpec BOTS G395M/F290LP.

Our combined spectrum spans wavelengths 0.85 to 5.17 μm (GO ID 2062, PI: Mayo). WASP-166b resides near the edge of the Hot Neptune Desert, a scarcity of intermediate-sized planets at high insolation fluxes; thus, exploring the atmospheric composition and formation processes of WASP-166b can provide insights into the mechanisms sculpting this parameter space.

Our POSEIDON free chemistry retrievals confirm the detection of H₂O (15.2σ significance) and detect CO₂ (14.7σ) for the first time in the planet atmosphere. We also find a hint of NH₃ (2.3σ) and an intermediate pressure cloud deck (2.6σ). Finally, we report non-detections of CH₄, CO, C₂H₂, HCN, SO₂, and H₂S. We verify our results using a TauREx free chemistry retrieval.

We also measure with POSEIDON a high planetary atmospheric metallicity (log(Z)=1.57+0.17−0.18, Z=37+18−13) and a potentially substellar C/O ratio for the planet (C/O=0.282+0.078−0.053) compared to the star (C/O∗=0.41±0.08), suggesting a formation pathway for WASP-166b that includes planetesimal accretion followed by core erosion or photoevaporation, which may indicate these to be plausible driving processes in the formation of the Hot Neptune Desert.

Andrew W. Mayo, Charles D. Fortenbach, Dana R. Louie, Courtney D. Dressing, Steven Giacalone, Caleb K. Harada, Emma V. Turtelboom

Comments: Submitted to AAS Journals, 42 pages, 21 figures, 9 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2501.00609 [astro-ph.EP] (or arXiv:2501.00609v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2501.00609
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Submission history
From: Andrew Mayo
[v1] Tue, 31 Dec 2024 19:12:19 UTC (19,071 KB
https://arxiv.org/abs/2501.00609
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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