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Water and an Escaping Helium Tail Detected in the Hazy and Methane-depleted Atmosphere of HAT-P-18b from JWST NIRISS/SOSS

Fu, Guangwei and Espinoza, Néstor and Sing, David K. and Lothringer, Joshua D. and Dos Santos, Leonardo A. and Rustamkulov, Zafar and Deming, Drake and Kempton, Eliza M.-R. and Komacek, Thaddeus D. and Knutson, Heather A. and Albert, Loïc and Pontoppidan, Klaus and Volk, Kevin and Filippazzo, Joseph (2022) Water and an Escaping Helium Tail Detected in the Hazy and Methane-depleted Atmosphere of HAT-P-18b from JWST NIRISS/SOSS. Astrophysical Journal Letters, 940 (2). Art. No. L35. ISSN 2041-8205. doi:10.3847/2041-8213/ac9977.

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JWST is here. The early release observation (ERO) program provides us with the first look at the scientific data and the spectral capabilities. One of the targets from the ERO is HAT-P-18b, an inflated Saturn-mass planet with an equilibrium temperature of ∼850 K. We present the NIRISS/SOSS transmission spectrum of HAT-P-18b from 0.6 to 2.8 μm and reveal the planet in the infrared beyond 1.6 μm for the first time. From the spectrum, we see clear water and escaping helium tail features in an otherwise very hazy atmosphere. Our free chemistry retrievals with ATMO show moderate Bayesian evidence (3.79) supporting the presence of methane, but the spectrum does not display any clearly identifiable methane absorption features. The retrieved methane abundance is ∼2 orders of magnitude lower than that of solar composition. The methane-depleted atmosphere strongly rejects simple equilibrium chemistry forward models with solar metallicity and a C/O ratio and disfavors high metallicity (100 times) and a low C/O ratio (0.3). This calls for additional physical processes such as vertical mixing and photochemistry, which can remove methane from the atmosphere.

Item Type:Article
Related URLs:
URLURL TypeDescription
Fu, Guangwei0000-0002-3263-2251
Espinoza, Néstor0000-0001-9513-1449
Sing, David K.0000-0001-6050-7645
Lothringer, Joshua D.0000-0003-3667-8633
Dos Santos, Leonardo A.0000-0002-2248-3838
Rustamkulov, Zafar0000-0003-4408-0463
Deming, Drake0000-0001-5727-4094
Kempton, Eliza M.-R.0000-0002-1337-9051
Komacek, Thaddeus D.0000-0002-9258-5311
Knutson, Heather A.0000-0002-5375-4725
Albert, Loïc0000-0003-0475-9375
Pontoppidan, Klaus0000-0001-7552-1562
Volk, Kevin0000-0002-3824-8832
Filippazzo, Joseph0000-0002-0201-8306
Group:Division of Geological and Planetary Sciences
Issue or Number:2
Record Number:CaltechAUTHORS:20221213-185576500.3
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:118333
Deposited By: Research Services Depository
Deposited On:18 Jan 2023 18:19
Last Modified:18 Jan 2023 18:19

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