Water And An Escaping Helium Tail Detected In The Hazy And Methane-depleted Atmosphere Of HAT-P-18b From JWST NIRISS/SOSS

JWST is here. The early release observation program (ERO) provides us with the first look at the scientific data and the spectral capabilities.

One of the targets from ERO is HAT-P-18b, an inflated Saturn-mass planet with an equilibrium temperature of ∼850K. We present the NIRISS/SOSS transmission spectrum of HAT-P-18b from 0.6 to 2.8μm and reveal the planet in the infrared beyond 1.6μm 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 C/O ratio and disfavors high metallicity (100 times) and 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.

Guangwei Fu, Néstor Espinoza, David K. Sing, Joshua D. Lothringer, Leonardo A. Dos Santos, Zafar Rustamkulov, Drake Deming, Eliza M.-R. Kempton, Thaddeus D. Komacek, Heather A. Knutson, Loïc Albert, Klaus Pontoppidan, Kevin Volk, Joseph Filippazzo

Comments: Accepted to ApJL, JWST is awesome!
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2211.13761 [astro-ph.EP] (or arXiv:2211.13761v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2211.13761
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Submission history
From: Guangwei Fu
[v1] Thu, 24 Nov 2022 19:00:01 UTC (4,217 KB)
https://arxiv.org/abs/2211.13761
Astrobiology, Astrochemistry