Exoplanets, -moons, -comets

SiO and a Super-stellar C/O Ratio in the Atmosphere of the Giant Exoplanet WASP-121b

By Keith Cowing
Status Report
astro-ph.EP
June 8, 2025
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SiO and a Super-stellar C/O Ratio in the Atmosphere of the Giant Exoplanet WASP-121b
Properties of the atmosphere of WASP-121b. a, PT profiles inferred from the retrieval analysis for the dayside (red) and nightside (blue) atmospheres assuming thermochemical equilibrium. Condensation curves for refractory species are shown as labelled dash-dot lines. b, Normalized contribution functions for the dayside (red) and nightside (blue) atmospheres integrated across the G395H passband, illustrating the relative amount of radiation originating from each pressure level. c, Volume mixing ratios (VMRs) inferred for key molecules in the nightside atmosphere. d, The same as c for the dayside atmosphere. Solid lines in a, c, and d are the median values obtained at each pressure level and the shaded regions indicate the associated 1 sigma uncertainties defined by the 16th and 84th percentiles (68% credible interval) based on n = 1,000 posterior samples. — astro-ph.EP

Refractory elements such as iron, magnesium, and silicon can be detected in the atmospheres of ultrahot giant planets.

This provides an opportunity to quantify the amount of refractory material accreted during formation, along with volatile gases and ices. However, simultaneous detections of refractories and volatiles have proved challenging, as the most prominent spectral features of associated atoms and molecules span a broad wavelength range.

Here, using a single JWST observation of the ultrahot giant planet WASP-121b, we report detections of H2O (5.5-13.5σ), CO (10.8-12.8σ), and SiO (5.7-6.2σ) in the planet’s dayside atmosphere, and CH4 (3.1-5.1σ) in the nightside atmosphere. We measure super-stellar values for the atmospheric C/H, O/H, Si/H, and C/O ratios, which point to the joint importance of pebbles and planetesimals in giant planet formation.

The CH4-rich nightside composition is also indicative of dynamical processes, such as strong vertical mixing, having a profound influence on the chemistry of ultrahot giant planets.

Thomas M. Evans-Soma, David K. Sing, Joanna K. Barstow, Anjali A. A. Piette, Jake Taylor, Joshua D. Lothringer, Henrique Reggiani, Jayesh M. Goyal, Eva-Maria Ahrer, Nathan J. Mayne, Zafar Rustamkulov, Tiffany Kataria, Duncan A. Christie, Cyril Gapp, Jiayin Dong, Daniel Foreman-Mackey, Soichiro Hattori, Mark S. Marley

Comments: Published in Nature Astronomy (publisher version is Open Access)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2506.01771 [astro-ph.EP] (or arXiv:2506.01771v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2506.01771
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Submission history
From: Thomas Evans-Soma
[v1] Mon, 2 Jun 2025 15:17:47 UTC (9,960 KB)
https://arxiv.org/abs/2506.01771
Astrobiology, Astrochemistry,

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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