Unraveling the non-equilibrium chemistry of the temperate sub-Neptune K2-18 b

A. Y. Jaziri; N. Carrasco; O. Sohier; O. Venot

REVIEW 3 major objections 5 minor 46 references

Unraveling the non-equilibrium chemistry of the temperate sub-Neptune K2-18 b

T0 review · 3 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash

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Pith's one-line read A non-equilibrium chemistry grid fit to JWST data makes K2-18 b metal-rich and carbon-rich, with methane as the only secure absorber.

desk verdict Wide non-equilibrium grid for K2-18 b, honest about its limits, but the non-equilibrium versus equilibrium claim is not supported by its own fits. read the letter →

arxiv 2507.14983 v1 pith:NQ4L6XPD submitted 2025-07-20 astro-ph.EP

A. Y. Jaziri , O. Sohier , O. Venot , N. Carrasco This is my paper

classification astro-ph.EP

keywords K2-18bsub-Neptuneatmospheresnon-equilibriumchemistrytransmissionspectroscopyJWSTmetallicityC/Oratioverticalmixing

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

K2-18 b, a temperate sub-Neptune in an M-dwarf habitable zone, is argued on the basis of a non-equilibrium chemistry grid to have a metal-rich, carbon-rich atmosphere. The best-fit models favor a metallicity of $266^{+291}_{-104}$ times solar and a C/O ratio above 2.1 at 2$\sigma$, with methane securely detected at $\log_{10}[\mathrm{CH_4}] \approx -0.3$ at 1 mbar. Carbon dioxide, water, ammonia, and other species remain unconfirmed, and vertical mixing ($K_{zz}$) is unconstrained. A constant-abundance retrieval cannot capture this chemistry, but the current data also cannot prove disequilibrium, since the equilibrium fit is statistically indistinguishable. The result matters because it changes what K2-18 b is made of and makes any simple habitable-ocean interpretation premature.

What carries the argument

The machinery is FRECKLL, a one-dimensional steady-state non-equilibrium chemical model that solves production-loss-transport continuity equations for each species over a network of roughly 2000 reactions among more than 100 neutral species containing H, C, N, and O, with vertical transport set by an eddy diffusion coefficient $K_{zz}$ and photolysis driven by a stellar UV spectrum. TauREx 3's forward model converts the resulting abundance profiles into transmission spectra that are compared to the JWST data by $\chi^2$. The load-bearing step is the three-dimensional grid in metallicity, C/O, and $K_{zz}$; the grid is collapsed along $K_{zz}$ by taking the lowest $\chi^2$, and 1D polynomial fits to the projected $\Delta\chi^2$ produce the reported parameter values and limits.

What would settle it

A high-signal JWST NIRSpec G395H observation resolving the 4–5 $\mu$m CO and CO2 bands, analyzed with a pressure-temperature profile recalculated for each grid point, would settle it: if secure CO2 near 1 percent and H2O above roughly $10^{-5}$ at 1 mbar are required to fit, the C/O ratio sits near solar and the >2.1 lower limit breaks.

Watch

Extended reading notes

Core claim

On the paper's own terms, the discovery is that exploring metallicity (0.1–1000 times solar), C/O (0.1–100), and $K_{zz}$ ($10^5$–$10^{10}$ cm$^2$/s) together collapses K2-18 b's chemical state to a high-metallicity, high-C/O corner of parameter space. The grid best fit sits at metallicity 280.7, C/O 90.0, and $K_{zz}=10^6$ cm$^2$/s; polynomial fits to the projected $\Delta\chi^2$ give a 2$\sigma$ metallicity interval of $266^{+291}_{-104}$ and a 2$\sigma$ lower limit on C/O of 2.1. CH4 is the only secure absorber, with $\log_{10}[\mathrm{CH_4}] = -0.3^{+0.1}_{-1.7}$ at 1 mbar, while CO2 can fall below $10^{-6}$, consistent with the recent reanalysis doubting its detection. The paper emphasizes that minor species such as CO, H2O, and NH3 may still be present at observable-to-ppm levels but are masked by CH4 and noise, and that a high C/O ratio favors aerosol formation. It also finds that the equilibrium retrieval, with metallicity $71^{+58}_{-20}$ and C/O $6.78^{+5.8}_{-2.0}$, fits the data marginally better ($\Delta\chi^2=-2$), so the case for disequilibrium rests on physical expectation rather than on statistical preference.

Load-bearing premise

The paper assumes that one fixed temperature-pressure profile, originally fitted to a different composition, stays valid over the entire model grid; if the planet's thermal structure changes with metallicity or C/O, the retrieved abundances and the C/O lower limit could shift enough to break the central claim.

Editorial extensions

If this is right

If the high-metallicity, high-C/O result is right, K2-18 b's missing water is expected: oxygen is tied up in CO and CO2, so the absence of a secure H2O detection is not evidence against water deeper down.
Methane becomes the only safe spectroscopic anchor; CO2, H2O, NH3, and CO claims from constant-abundance retrievals will need rechecking against chemistry-coupled models.
A C/O ratio above 2.1 favors carbon-rich aerosol and haze formation, which could explain the steep short-wavelength slope that the current models do not capture.
Because $K_{zz}$ is unconstrained, vertical mixing cannot be measured from CH4; useful constraints will come from species whose abundances change strongly with quenching depth.
Higher-signal 4–5 $\mu$m spectroscopy is the concrete next step: CO and CO2 bands there are the leverage points for turning the C/O lower limit into a measurement.

Reading between the lines

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

Editorial inference: The fixed pressure-temperature profile is the unquantified hinge; a self-consistent P-T grid could move the C/O limit by more than the quoted 1.4 shift from excluding H2O opacity, so 2.1 should be read as model-dependent.
Editorial inference: The bimodal metallicity behavior hints that K2-18 b's data currently permit two chemical families—metal-rich with near-solar C/O, and moderately metal-rich with very high C/O—so the next observation should target CO, not just CH4, to separate them.
Editorial inference: If the paper's picture holds, habitability arguments based on a liquid-water ocean at the surface must be reconsidered, since a carbon-rich, H2O-poor observable atmosphere is what the chemistry predicts.
Editorial inference: The result that $K_{zz}$ is unconstrained despite large non-equilibrium abundance changes suggests that 'disequilibrium' detections in similar planets will need abundance pairs with strong quenching sensitivity, not just the presence of CH4.

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Editorial analysis

A structured set of objections, weighed in public.

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

Desk Editor's Note

read the letter

Colleague,

You should know this paper before the next modeling meeting. It runs the FRECKLL non-equilibrium chemistry model over a much wider joint grid (metallicity 0.1–1000x solar, C/O 0.1–100, Kzz 1e5–1e10) than previous disequilibrium analyses of K2-18 b, and compares each model to the JWST spectrum. That grid is the real contribution. It lets them see a metallicity bimodality between low and high C/O regimes and puts a lower limit on C/O around 2.1 at 2σ, broadly consistent with Schmidt et al. (2025) but from a more complete parameter search.

What they do well: the paper is transparent. It states clearly that the P-T profile is fixed from Blain et al. (2021), that condensation is not included, that the Kzz dimension is unconstrained, and that the equilibrium retrieval fits slightly better than the non-equilibrium grid (Δχ² = −2). That last point is the one to keep in mind: the title says "non-equilibrium chemistry" but the data do not favor it, and the authors admit it. The CH4 detection is robust, and the CO2 non-detection is consistent with Schmidt et al.

The soft spots are real but not fatal. The fixed P-T profile is load-bearing for the high-metallicity/high-C/O result. At 1000x solar and C/O 100 the thermal structure will not be the same as the 175x solar, C/O 0.13 profile they inherited. They note "some uncertainty" but don't quantify it; that is the main thing a revision should address, either with a self-consistent P-T iteration or a sensitivity test. The significance claims (4σ over flat line) ignore model complexity and the fact that the grid is not a proper retrieval; the Δχ² thresholds are applied after a polynomial fit to a projection, not a marginalization. That makes the 2σ numbers optimistic. The C/O lower limit also drops from 2.1 to 1.4 when H2O opacity is removed, so the constraint is softer than the abstract implies.

Still, the paper is a useful exploration. It is worth citing for the grid range and for the behavior of minor species, and it deserves a serious referee. I would send it out with a request to clarify the CH4 abundance convention, add a sensitivity test on the P-T profile, and tone down the non-equilibrium framing to match the actual evidence.

Referee Report

3 major / 5 minor

Summary. The paper compares a grid of 1D non-equilibrium chemical models computed with FRECKLL to the JWST transmission spectrum of K2-18 b, exploring metallicity (0.1–1000× solar), C/O (0.1–100), and Kzz (10^5–10^10 cm^2/s) under a fixed P–T profile taken from Blain et al. (2021). Using a chi-square grid search and polynomial fits to projected Δχ², it reports a best-fit metallicity of 266 (+291/−104) at 2σ, a C/O lower limit of 2.1 at 2σ, a robust CH4 detection (log10[CH4] ≈ −0.3 at 1 mbar), and no constraint on Kzz. A complementary equilibrium retrieval with TauREx yields a similar overall fit (metallicity ≈ 71, C/O ≈ 6.8) with a slightly lower χ². The paper concludes that the data favor a metal-rich, carbon-rich atmosphere but that non-equilibrium chemistry cannot be conclusively proven from these data.

Significance. If the metallicity and C/O constraints survive a self-consistent treatment, they would be important anchors for sub-Neptune formation and atmospheric evolution, and the demonstration that constant-abundance retrievals are insufficient is valuable. The study's strengths are the unusually wide C/O exploration (0.1–100), the use of a full kinetic network with photochemistry, and the candid acknowledgment of model limitations (fixed P–T, no condensation). The paper does not oversell the detection of minor species and explicitly states that only CH4 is robustly detected. However, the headline parameter constraints are conditional on modeling assumptions that are not fully tested, and the statistical reporting needs more care.

major comments (3)

[§2.3, Table 4] The grid spans metallicity 0.1–1000 and C/O 0.1–100, but every model uses the single fixed P–T profile from Blain et al. (2021), which is itself the best fit for metallicity = 175, C/O = 0.13, Kzz = 10^6. The paper states that composition-induced temperature changes are expected to be small, but provides no quantitative test; the retrieved solutions at metallicity ≈ 266 and C/O ≳ 2 are far from the fiducial composition, and the equilibrium retrieval of Section 3.2 uses the same fixed profile, so it does not validate the assumption. Since CH4, CO, and H2O abundances and the C/O constraint depend on quench temperatures, the headline 2σ C/O lower limit of 2.1 is not robust until the P–T sensitivity is quantified, for example with a self-consistent radiative-equilibrium grid or at least a perturbed-profile test.
[§2.4] The confidence intervals in Table 4 are derived by collapsing the 3D grid over Kzz via minimum χ², projecting the 2D Δχ² onto 1D, and fitting a polynomial to the projected values. This is neither a Bayesian marginalization nor a profile likelihood with stated coverage, and the reported intervals (e.g., metallicity 266 +291/−104) should not be presented as standard 1σ/2σ/3σ confidence intervals. The >4σ significance relative to a flat line also does not account for the number of grid points (30×58×6) or the implicit model selection. The paper should either re-derive the constraints with a statistically principled method (e.g., nested sampling over an interpolated forward model) or explicitly label the numbers as approximate grid-based ranges without coverage guarantees.
[§3.5] The central claim 'C/O ≥ 2.1 at 2σ' is highly sensitive to the treatment of H2O: when H2O opacity is excluded (a crude proxy for condensation, which FRECKLL does not model), the 2σ limit drops to 1.4 and the 3σ limit to 0.57. Because the default model likely overestimates gas-phase H2O in the cool upper atmosphere, the high-C/O conclusion is conditional on an unmodeled process. The paper should either implement a condensation/rainout scheme or present the C/O lower limit as explicitly model-dependent rather than as a robust atmospheric constraint.

minor comments (5)

[Abstract] The CH4 abundance is reported as log10[CH4] = −0.3 +0.1/−1.7 without specifying the confidence level; Table 4 shows these are 3σ uncertainties, so the abstract should state this to avoid implying a 1σ range.
[§3.2 / Abstract] Because the equilibrium retrieval has a slightly lower χ² than the best non-equilibrium model (Δχ² = −2.0), the statement 'Non-equilibrium models exceed 4σ confidence over a flat line' should clarify that this establishes the presence of spectral features, not disequilibrium chemistry; the title and abstract should be rephrased to avoid overclaiming.
[Table 4] The Kzz entry 'All within 1σ' is ambiguous; please specify that all grid values are within 1σ of the best fit, i.e., Kzz is unconstrained by the data.
[§2.3] The phrase 'consistent with the findings of Blain et al. (2021), where the impact was shown to be small' lacks a quantitative reference; please cite the specific figure or section of Blain et al. that supports this claim.
[Throughout] Typographical artifacts such as 'su fficient', 'di fficult', 'W ASP-39 b', and inconsistent spacing in 'C /O' should be corrected in a final proofread.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: grid search against external JWST data; fixed P-T is an acknowledged limitation, not a circular input.

full rationale

The paper's derivation chain is a forward grid search: FRECKLL non-equilibrium chemistry is run over metallicity, C/O, and Kzz, spectra are generated with TauREx, and the models are compared via chi-square to the independent JWST data of Madhusudhan et al. (2023). The retrieved metallicity, C/O ratio, and Kzz are best-fit parameters from that comparison, so they are constraints rather than predictions, and no fitted parameter is renamed as a prediction. The methane 'robust detection' is a spectral fit result, not a quantity defined in terms of itself. The fixed P-T profile from Blain et al. (2021) is an input assumption, and the paper explicitly acknowledges that this introduces 'some uncertainty in the derived values' and that constraints 'should be viewed as indicative of chemical trends rather than precise determinations' (Sections 2.3, 4.3, and 4.5). That is a model limitation and a correctness risk, but it is not circular because the P-T profile is not defined in terms of the retrieved abundances or C/O limit. The self-citations, including the FRECKLL code (Al-Refaie et al. 2024, with coauthor Venot), the Venot et al. (2020) chemical network, and Jaziri et al. (2024) for ACE, are citations to prior code, networks, or tests; they do not supply the target result and do not force the retrieved parameters. The H2O-condensation sensitivity test, which lowers the 2-sigma C/O limit from 2.1 to 1.4, is a robustness check rather than a circular step. No equation in the paper reduces a prediction to an input by construction, and no load-bearing uniqueness claim is imported from the authors' prior work. The central claims therefore have independent content and are benchmarked against external observations.

Assumptions & free parameters 4 free parameters · 5 assumptions · 0 invented entities

The central constraints depend on the chemistry model (Venot network within FRECKLL), the assumed thermal profile, the stellar proxy, and the observational data, all taken from prior literature. No new physical entities are introduced.

free parameters (4)

Metallicity = 266 (+291/-104) at 2 sigma; grid best 280.7
Grid parameter fit to JWST spectrum via chi-square; central to the high-metallicity claim.
C/O ratio = lower limit 2.1 at 2 sigma; grid best 90.0
Grid parameter fit to JWST spectrum; a lower limit, no upper limit, is the central C/O claim.
Kzz (eddy diffusion) = unconstrained; grid best 1e6 cm2/s
Grid parameter, found to have no significant impact on the fit.
Planet radius = not reported explicitly; adjusted to match spectra
The radius is retrieved to align model spectra with observations, adding a free parameter not included in the grid statistics.

assumptions (5)

domain assumption The fixed P-T profile from Blain et al. (2021), derived for metallicity=175, C/O=0.13, Kzz=1e6, is representative of the atmosphere across the entire explored grid.
Invoked in Section 2.3; if the thermal structure changes significantly with metallicity or C/O, the chemical output and constraints would shift.
domain assumption The Venot et al. (2020) chemical network accurately represents the neutral C/H/O/N chemistry of a temperate sub-Neptune.
The network is the core of FRECKLL; rate uncertainties and missing C2+ species are not propagated into the quoted uncertainties.
domain assumption The MUSCLES GJ 436 stellar spectrum is a suitable proxy for K2-18's UV irradiation for photochemistry.
Used for photolysis rates; a different stellar spectrum would change photochemical product abundances.
domain assumption The H2-rich background assumption remains valid for the forward model even at the best-fit high metallicity and C/O.
The reported CH4 mole fraction of about 0.5 at 1 mbar is difficult to reconcile with an H2-dominated atmosphere, yet the mean molecular weight and scale height are not recomputed self-consistently.
domain assumption The observed spectrum and adopted error bars from Madhusudhan et al. (2023) are accurate, including the -41 ppm offset between NIRISS and NIRSpec.
All chi-square values and confidence intervals depend on these data and errors.

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Cite this review

Pith. "Pith review of Unraveling the non-equilibrium chemistry of the temperate sub-Neptune K2-18 b." pith.science (2026). https://pith.science/paper/NQ4L6XPD

@misc{pith2026250714983,
  author       = {Pith},
  title        = {Pith review of: Unraveling the non-equilibrium chemistry of the temperate sub-Neptune K2-18 b},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/NQ4L6XPD}},
  note         = {Machine review of arXiv:2507.14983}
}

read the original abstract

The search for habitable, Earth-like exoplanets faces major observational challenges due to their small size and faint signals. M-dwarf stars offer a promising avenue to detect and study smaller planets, especially sub-Neptunes-among the most common exoplanet types. K2-18 b, a temperate sub-Neptune in an M-dwarf habitable zone, has been observed with HST and JWST, revealing an H2-rich atmosphere with CH4 and possible CO2. Conflicting interpretations highlight the importance of non-equilibrium chemistry, which is critical for constraining atmospheric parameters like metallicity, C/O ratio, and vertical mixing (Kzz). This study explores the parameter space of metallicity, C/O ratio, and Kzz for K2-18 b using the non-equilibrium chemistry model FRECKLL and JWST data. We generated spectra from a 3D grid of models and compared them to observations to refine atmospheric constraints. A fixed pressure-temperature profile was used to capture first-order chemical trends, acknowledging some uncertainties. Our best-fit model favors high metallicity (266^{+291}_{-104} at 2 sigma) and high C/O ratio (C/O > 2.1 at 2 sigma). CH4 is robustly detected (log10[CH4] = -0.3^{+0.1}_{-1.7} at 1 mbar), while CO2 remains uncertain due to spectral noise. Kzz has no clear impact on the fit and remains unconstrained. Non-equilibrium models outperform flat spectra at > 4 sigma confidence, confirming atmospheric features. Minor species, such as H2O and NH3, may be present but are likely masked by dominant absorbers. Our results highlight the limits of constant-abundance retrievals. The atmosphere has a high C/O ratio suggesting possible aerosol formation. Better constraints require higher-precision data. Future JWST NIRSpec G395H and ELT/ANDES observations will be critical for probing habitability and refining models.

Figures

Figures reproduced from arXiv: 2507.14983 by the authors.

**Figure 2.** Transit spectra of the best fit non-equilibrium 1D model at resolution 200 (black solid line). This is the approximate best [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗

**Figure 3.** Species profiles of the best fit of non-equilibrium 1D [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗

Figures from the paper (2 more)

**Figure 4.** Figure 4: Evolution of ∆χ 2 with metallicity or C/O ratio. The χ 2 values are computed by comparing 1D chemical models to the K2-18 b observations from Madhusudhan et al. (2023). The ∆χ 2 values represent the difference from the best-fit model. The 1D projection is obtained by s…

**Figure 6.** Figure 6: Species profiles of non-equilibrium 1D models within 3 [PITH_FULL_IMAGE:figures/full_fig_p007_6.png]

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