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SiO and a super-stellar C/O ratio in the atmosphere of the giant exoplanet WASP-121b

T0 review · 0 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read JWST finds SiO and a C/O ratio 1.96 times stellar on WASP-121b.

desk verdict Strong paper: first simultaneous refractory+volatile abundances from one JWST phase curve, with a credible new SiO detection; the stellar O non-LTE caveat shifts enrichment magnitudes but not the super-stellar conclusion. read the letter →

arxiv 2506.01771 v1 pith:HMFF4LQZ submitted 2025-06-02 astro-ph.EP

Thomas M. Evans-Soma , David K. Sing , Joanna K. Barstow , Anjali A. A. Piette , Jake Taylor , Joshua D. Lothringer , Henrique Reggiani , Jayesh M. Goyal
show 10 more authors
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
This is my paper · ORCID
classification astro-ph.EP
keywords exoplanetatmospheresultrahotJupiterWASP-121bJWSTNIRSpecphasecurvesiliconmonoxidecarbon-to-oxygenratiorefractoryelementsverticalmixing
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper claims that a single 37.8-hour JWST/NIRSpec phase curve of the ultrahot giant WASP-121b carries simultaneous detections of volatile molecules (H$_2$O and CO) and a refractory molecule (SiO) in the dayside atmosphere, plus CH$_4$ on the nightside. From those detections it derives super-stellar C/H, O/H, Si/H, and a C/O ratio 1.96 times the host star's value, arguing the planet built its envelope from volatile-enriched gas and later rocky additions. If correct, this is the first abundance set that jointly constrains refractory and volatile elements from one exoplanet observation, and it ties the presence of nightside CH$_4$ and the survival of dayside SiO to strong vertical mixing rather than horizontal quenching.

What carries the argument

The load-bearing object is the phase-resolved emission spectrum built from 349 wavelength channels of the JWST/NIRSpec G395H phase curve, fitted with a spherical-harmonic brightness map and a quadratic limb-darkening law. The retrieval machinery is a set of five independent atmospheric retrieval codes (ATMO, NEMESIS, CHIMERA, HyDRA, PETRA) that convert those spectra into abundances; the central identifications are the 4.0$-$4.3 $\mu$m SiO band on the dayside and the 3.3 $\mu$m CH$_4$ features on the nightside. The vertical-mixing mechanism is quantified with a C$-$H$-$N$-$O$-$S non-equilibrium chemical network, whose eddy diffusion coefficient $K_{zz}$ is matched to the observed nightside CH$_4$ abundance, and the stellar normalization uses non-LTE abundance corrections for the host star.

What would settle it

Recompute the WASP-121 stellar oxygen abundance with an independent non-LTE or 3D model grid: if the resulting stellar C/O rises to near the planetary 0.92, the central claim that WASP-121b has a super-stellar C/O loses its quantitative support. A second check is to observe the nightside at higher spectral resolution or longer wavelengths to confirm that the 3.3 micron feature is CH4 rather than a P-T profile artifact.

Watch

Extended reading notes

Core claim

Using the G395H grism of JWST's NIRSpec instrument to watch WASP-121b for 37.8 hours, the authors extract dayside and nightside emission spectra from the phase curve and fit them with five independent retrieval codes. They report decisive detections of dayside H$_2$O (5.5$-$13.5$\sigma$), CO (10.8$-$12.8$\sigma$), and SiO (5.7$-$6.2$\sigma$), and nightside CH$_4$ (3.1$-$5.1$\sigma$). Relative to updated stellar abundances, the dayside ratios are $(\mathrm{C/H})/(\mathrm{C/H})_\star=23.96$, $(\mathrm{O/H})/(\mathrm{O/H})_\star=12.19$, $(\mathrm{Si/H})/(\mathrm{Si/H})_\star=9.89$, and C/O $=0.92$, which is 1.96 times the stellar C/O of 0.47; all four ratios are super-stellar at $>99.99\%$ probability. The same data show a CH$_4$-rich nightside that cannot be explained by equilibrium chemistry alone, and the authors argue that vertical mixing ($K_{zz}\sim10^9$-$10^{11}\,\mathrm{cm^2\,s^{-1}}$) transports CH$_4$-rich gas from deep layers to the nightside photosphere while also keeping silicon from being cold-trapped, preserving the dayside SiO signal.

Load-bearing premise

The enrichment factors rest on the non-LTE correction to the host star's oxygen abundance; if that correction is wrong, the sizes of the super-stellar O/H and C/O ratios change, though their direction survives even with LTE stellar abundances.

Editorial extensions

If this is right

  • If the detections hold, WASP-121b becomes the first exoplanet whose atmosphere has been measured for both refractory (Si) and volatile (C, O) elements in a single observation, tying SiO to the dayside thermal inversion.
  • The super-stellar C/H and C/O support formation by gas enriched with inward-drifting, CH$_4$-rich pebbles evaporating beyond the H$_2$O ice line, while the super-stellar Si/H requires an additional roughly 21 Earth masses of rocky planetesimals.
  • The nightside CH$_4$ detection implies strong vertical mixing ($K_{zz}\sim10^9$-$10^{11}\,\mathrm{cm^2\,s^{-1}}$) rather than horizontal quenching, and predicts that the same mixing prevents refractory cold-trapping, explaining why SiO is seen on the dayside.
  • Future JWST observations should find SiO bands in other ultrahot giants with silicon- and oxygen-enriched atmospheres, complementing UV searches that suffer from dust extinction and low UV flux.
  • The measured C/O of 0.92 places the nightside opacity under CH$_4$ control, explaining the non-detection of nightside H$_2$O and CO.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Even if the non-LTE correction to the stellar oxygen abundance is wrong, the LTE stellar C/O of 0.203 still leaves the planetary C/O of 0.92 super-stellar, so the qualitative formation story survives; only the magnitude of the enrichment changes.
  • The paper reports a wavelength-dependent phase offset whose flat-line fit gives $\chi^2_\nu=1.74$, hinting at longitudinal or spectral variations in the brightness map that are not interpreted; a dedicated $\Delta\phi(\lambda)$ mapping could test the vertical-mixing picture.
  • The nightside CH$_4$ abundance predicted by vertical mixing could be verified by observing WASP-121b at longer wavelengths where CH$_4$ bands are stronger, or by comparing the inferred $K_{zz}$ with values from other tracers such as CO/CO$_2$ ratios.
  • If SiO is a strong shortwave absorber, the retrieved Si abundance should affect the altitude of the dayside thermal inversion; coupling that abundance into self-consistent GCMs would test whether the inversion pressure matches the data.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

0 major / 4 minor

Summary. The paper presents JWST NIRSpec G395H phase-curve observations of WASP-121b. From the dayside disc-integrated spectrum the authors report detections of H2O (5.5–13.5σ), CO (10.8–12.8σ), and SiO (5.7–6.2σ), and from the nightside spectrum a CH4 detection (3.1–5.1σ). Retrievals with five independent codes, including an equilibrium-chemistry retrieval (ATMO), yield super-stellar atmospheric C/H, O/H, and Si/H ratios relative to the host star, with C/O = 0.92, corresponding to a C/O ratio 1.96 times the adopted non-LTE stellar value. The authors interpret the super-stellar C/O and volatile/refractory ratios as evidence for pebble-driven gas enrichment and post-formation accretion of rocky material, and propose vertical mixing to explain the nightside CH4 abundance.

Significance. The result is significant because it demonstrates, from a single JWST observation, simultaneous constraints on a refractory species (SiO) and major volatiles (H2O, CO, CH4), opening a new window on giant-planet formation. The analysis is unusually thorough: five independent retrieval codes (ATMO, NEMESIS, CHIMERA, HyDRA, PETRA) give consistent elemental ratios; two independent data reductions (FIREFly and Eureka!) agree; detection significances are quantified via Bayesian evidence; and line-list differences are explicitly discussed. Data and code are public. The central caveat is the large non-LTE correction to the stellar oxygen abundance, which changes the magnitude of the quoted enrichment factors (e.g., (O/H)/(O/H)* from roughly 5 to 12 and (C/O)/(C/O)* from roughly 4.5 to 1.96) but does not reverse the qualitative super-stellar conclusion; the manuscript discloses this sensitivity in the Methods.

minor comments (4)
  1. [Results (first paragraph of 'Results')] The quoted enrichment factors, such as (O/H)/(O/H)star = 12.19 and (C/O)/(C/O)star = 1.96, assume non-LTE stellar oxygen abundances; because the O I non-LTE correction is large, please state explicitly in the main text that adopting the LTE stellar C/O of 0.203 instead of the non-LTE value of 0.470 would change (C/O)/(C/O)star from about 2 to about 4.5, and similarly for (O/H)/(O/H)star, while still leaving the ratios super-stellar.
  2. [Methods — Accretion of rocky material] The estimate of about 21 Earth masses of accreted rocky material depends directly on the assumed core mass (15 MEarth), the envelope hydrogen mass fraction (75%), and the assumption that the primordial envelope had a stellar Si/H ratio; the text gives one alternative (23.7 MEarth) for zero initial silicon, but it would be helpful to state how the result scales with a plausible range of core masses from the literature.
  3. [Methods — Atmospheric retrieval analyses] The nightside CH4 detection significance ranges from 3.1σ to 5.1σ across the retrieval codes, partly because of differing CH4 line lists; the paper should state explicitly that this spread is a systematic uncertainty from the opacity treatment, not just a statistical fluctuation, so that readers do not over-rely on the favored 4.7σ value.
  4. [Discussion (vertical mixing paragraph)] The inferred eddy diffusion coefficients Kzz of roughly 10^8–10^11 cm2/s are compared only with broad literature constraints; if possible, include a comparison with the Kzz profile expected from the retrieved nightside thermal structure or a simple mixing-length estimate, which would strengthen the physical plausibility argument.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the abundance ratios come from independent retrievals and stellar spectroscopy; the nightside CH4 consistency check is not the detection evidence, and the non-LTE oxygen normalization is a reported systematic, not a fitted input renamed as a prediction.

full rationale

The central derivation chain is self-contained. Dayside C/H, O/H, and Si/H are free parameters in the ATMO equilibrium retrieval and are corroborated by independent free-chemistry codes (NEMESIS, HyDRA, CHIMERA, PETRA); molecule detection significances come from Bayesian model comparisons with and without each opacity source, not from the equilibrium chemistry itself. The planetary ratios relative to the host star divide these retrieved abundances by stellar abundances measured from ESPRESSO equivalent widths using standard MARCS/MOOG analysis and external non-LTE grids, so no fitted parameter is renamed as a prediction. The nightside CH4 signal is detected by free-chemistry retrievals, and the equilibrium-chemistry prediction of CH4 from the dayside C/O is explicitly used only as a consistency check ('this nonetheless demonstrates that the CH4 abundances we have inferred are physically plausible'), not as evidence for the detection. The rocky-material mass is an interpretive estimate from the measured Si/H with stated assumptions about envelope mass and Earth-like composition. The non-LTE oxygen correction alters the magnitude of the quoted stellar-normalized O/H and C/O ratios, but the paper reports both LTE and non-LTE stellar values, and the planetary C/O of 0.92 remains super-stellar relative to the LTE stellar C/O of 0.203; this is a transparent systematic, not a circular input. Self-citations are contextual (e.g., prior thermal-inversion detection) or methodological and are not load-bearing for the composition claim.

Assumptions & free parameters 7 free parameters · 6 assumptions · 0 invented entities

The paper's central claims depend on retrieval fits with several free abundance and PT parameters, plus assumptions about equilibrium chemistry and stellar reference abundances. No new physical entities are introduced.

free parameters (7)
  • [C/H] carbon abundance = C/H = 8036 ppm (dayside, ATMO)
    Free parameter in the retrieval, fitted to the dayside emission spectrum.
  • [O/H] oxygen abundance = O/H = 8741 ppm (dayside, ATMO)
    Free parameter in the retrieval, fitted to the dayside emission spectrum.
  • [Si/H] silicon abundance = Si/H = 491 ppm (dayside, ATMO)
    Free parameter in the retrieval, constrained primarily by the SiO band.
  • [M/H] metallicity of other heavy elements = Posterior distribution, median not stated in text
    Collective scaling of non-C/O/Si heavy elements relative to solar in the ATMO retrieval.
  • PT profile parameters = 3-, 5-, or 6-parameter analytic profiles depending on retrieval
    Parametric pressure-temperature profiles fitted to each hemisphere; the profile shape influences the retrieved abundances.
  • Molecular VMRs in free chemistry retrievals = e.g., log VMR CH4 nightside = -1.75 to -2.43 dex depending on code
    Free chemistry retrievals fit abundances of H2O, CO, CH4, SiO, etc. independently of equilibrium chemistry.
  • Assumed core mass M_core = 15 M_Earth (from Jupiter core estimate)
    Used in the rocky material accretion calculation; directly sets the envelope mass and therefore the inferred accreted silicon mass.
assumptions (6)
  • domain assumption Thermochemical equilibrium on the dayside
    The ATMO retrieval assumes Gibbs free energy minimization to convert elemental abundances to molecular abundances; free chemistry retrievals relax this.
  • domain assumption Dayside and nightside each described by a single parametric PT profile
    Retrievals use analytic PT profiles; inhomogeneities across the visible hemisphere are only partially addressed via dilution factors or weighted-sum modeling.
  • domain assumption Stellar abundances from LTE equivalent widths with non-LTE corrections
    Super-stellar ratios are computed relative to stellar C, O, Si abundances; if non-LTE corrections are inaccurate, the enrichment factors change, though the C/O direction is robust.
  • domain assumption Solar abundance pattern for heavy elements other than C, O, Si
    The retrieval scales all other heavy elements collectively with [M/H] relative to solar values; deviations from solar pattern for e.g. nitrogen, sulfur are not constrained.
  • domain assumption Spherical harmonic degree-1 brightness map for the planet
    The phase-curve fits adopt a dipole (Y1,0) brightness map, which can introduce biases in extracted spectra if the true map is higher-order.
  • domain assumption The accreted silicon is uniformly mixed throughout the atmospheric envelope
    The rocky material estimate assumes uniform enrichment of the whole envelope, which the authors explicitly acknowledge.

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Cite this review

Pith. "Pith review of SiO and a super-stellar C/O ratio in the atmosphere of the giant exoplanet WASP-121b." pith.science (2026). https://pith.science/paper/HMFF4LQZ

@misc{pith2026250601771,
  author       = {Pith},
  title        = {Pith review of: SiO and a super-stellar C/O ratio in the atmosphere of the giant exoplanet WASP-121b},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/HMFF4LQZ}},
  note         = {Machine review of arXiv:2506.01771}
}
abstract

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 H$_2$O (5.5-13.5$\sigma$), CO (10.8-12.8$\sigma$), and SiO (5.7-6.2$\sigma$) in the planet's dayside atmosphere, and CH$_4$ (3.1-5.1$\sigma$) 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 CH$_4$-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.

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Forward citations

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Reference graph

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