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arxiv: 2604.26850 · v2 · pith:AADEMH3Knew · submitted 2026-04-29 · 🌌 astro-ph.EP

Oxygen and nitrogen isotopologues on cold COCONUTS-2b observed with MIRI/MRS

H. K\"uhnle , E. C. Matthews , P. Molli\`ere , P. Patapis , Z. Zhang , E. Nasedkin , D. Gasman , N. Whiteford
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H. S. Wang M. Ravet G. Chauvin M. Bonnefoy D. Barrado A. M. Glauser S. P. Quanz
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Pith reviewed 2026-05-21 00:16 UTC · model grok-4.3

classification 🌌 astro-ph.EP
keywords isotopologuesexoplanet atmosphereCOCONUTS-2bMIRI/MRSJWSToxygen isotopesammoniacold companion
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The pith

JWST MIRI/MRS spectra reveal three isotopologues including oxygen isotopes in water on the cold companion COCONUTS-2b.

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

The paper establishes that full-resolution atmospheric retrievals on MIRI/MRS data can isolate faint absorption lines from rare isotopologues in a 480 K planetary-mass object. A sympathetic reader would care because these isotopic ratios supply a new observable that can eventually be compared to the host star and used to distinguish formation pathways that were previously inaccessible for cold gas giants. The work shows the detection of 15NH3, H2^18O and H2^17O through leave-one-out removal and Bayes-factor tests, marking the first clear oxygen-isotope signature in water for such a cold world. This capability demonstration sets the stage for elemental and isotopic abundance measurements that link composition directly to formation history.

Core claim

We robustly detect three isotopologues, namely 15NH3, H2^18O and H2^17O in the atmosphere of COCONUTS-2b. We find the first clear evidence of oxygen isotopes in water in a cold companion. This data set demonstrates the capability of MIRI/MRS to characterize such cold planetary-mass companion's atmospheres with respect to their compositional and isotopic content.

What carries the argument

Atmospheric retrievals performed at the full spectral resolution of MIRI/MRS, combined with leave-one-out analysis and Bayes factor comparison to isolate faint isotopologue features.

Load-bearing premise

The retrieval models and statistical tests correctly attribute the observed spectral features to the isotopologues rather than to gaps in opacity data, residual systematics, or host-star contamination.

What would settle it

Re-running the retrievals with an independent opacity database or on a new MIRI/MRS visit that yields Bayes factors below the detection threshold for 15NH3, H2^18O or H2^17O.

Figures

Figures reproduced from arXiv: 2604.26850 by A. M. Glauser, D. Barrado, D. Gasman, E. C. Matthews, E. Nasedkin, G. Chauvin, H. K\"uhnle, H. S. Wang, M. Bonnefoy, M. Ravet, N. Whiteford, P. Molli\`ere, P. Patapis, S. P. Quanz, Z. Zhang.

Figure 1
Figure 1. Figure 1: Observation and cube detector images of channel 1A to 4A, where channel 4A gets neglected from further analysis (see text). The red cross corresponds to the position of the target and the blue circle to the aperture from which the flux has been extracted from. The two negative spots on the detector correspond to two nods originating from the background subtraction. The yellow circle shows the position of a… view at source ↗
Figure 2
Figure 2. Figure 2: Best-fit spectra of the free and constrained atmospheric retrievals in green and violet respectively compared to the data in black. Panel a) shows the full observation. In panel b) we show a subset at the Gemini wavelengths from 1 to 2.5 µm as indicated by the black box and in c) we show another subset for the strong NH3 feature between 8 and 12 µm. The spectra are shown in lower resolution λ ∆λ = 1000. Th… view at source ↗
Figure 3
Figure 3. Figure 3: Bulk parameter for the free and the constrained retrievals. In panel a) we show the effective temperature estimate in b) the retrieved metallicity, in c) the C/O ratio accounting for oxygen sequestration, in d) the radius, in e) the surface gravity and in f) the resulting mass based on the radius and gravity for the free and constrained retrieval in green and violet respectively. For panels a),b) and d)-f)… view at source ↗
Figure 4
Figure 4. Figure 4: Pressure-temperature profiles of the free and the constrained retrieval in green and violet respectively. In dark blue we show the PT structure of Jupiter. The H2O, NH4SH and NH3 condensation lines were taken from (Lodders & Fegley 2002) and the rest from petitRAD￾TRANS. Thicker lines in deeper atmospheric layers indicate the con￾vective part of the atmosphere calculated based on Eq. 1. 4.3. Bulk chemical … view at source ↗
Figure 5
Figure 5. Figure 5: Retrieved chemical abundances with the free and constrained retrievals in spectral resolution λ ∆λ = 1000 (in green and violet, respec￾tively), compared to the chemical equilibrium predictions for the free and the constrained retrievals (in dark and light grey, respectively). In Panel a) the abundances are taken at 0.28 bar corresponding to the max￾imum contribution in the free case. In Panel b) the variat… view at source ↗
Figure 6
Figure 6. Figure 6: Best-fit retrievals on full MIRI/MRS resolution data with and without including H18 2 O in blue and red, respectively, compared to the data in black In panel a)-c) we see the three different parts in the spectrum where the difference in the best-fits is visible. The residuals between the data and the corresponding best fits are shown in panels d)-f) and the differences in the fits in plots g)-i). Here, we … view at source ↗
Figure 7
Figure 7. Figure 7: Best-fit retrievals on full MIRI/MRS resolution data with and without including H17 2 O in blue and red, respectively, compared to the data in black with the same panel structure as in 6. We present the wavelength ranges with the largest, second and sixth largest differences in the models and more wavelength areas in the appendix in Fig. B.7 view at source ↗
Figure 8
Figure 8. Figure 8: Best-fit retrievals on full MIRI/MRS resolution data with and without including 15NH3 in blue and red, respectively, compared to the data in black with the same panel structure as in 6. Here, we present the wavelength ranges with the largest, second and third largest differences in the models and more wavelength areas in the appendix in Fig. B.5. 200 400 600 14N/ 15N 0 100 200 300 400 500 600 Number count … view at source ↗
Figure 9
Figure 9. Figure 9: Ratios of a) 14N/ 15N, b) 16O/ 18O, c) 16O/ 17O and d) 18O/ 17O based on the full resolution retrievals. We compare this to the values for the ISM (Oxygen: Wilson (1999), 14N/ 15N: Ritchey et al. (2015)) and solar values (16O/ 17O and 16O/ 18O: McKeegan et al. (2011), 18O/ 17O: Wilson (1999) and nitrogen: Marty et al. (2011)) as well to the values of WISE 1828 (Barrado et al. 2023), WISE 0855 (Kühnle et al… view at source ↗
Figure 10
Figure 10. Figure 10: Comparison of low and full resolution retrievals compared to the data. Panel a): The best-fit spectra for the low resolution in green and the full resolution retrievals, with or without 15NH3 in blue and red, re￾spectively. In panel b) we show the corresponding residuals. Only with the improvement of resolution we are able to detect the isotopologue. man et al. (2026) based on self-consistent grid-models,… view at source ↗
Figure 11
Figure 11. Figure 11: Comparison of the spectrum of COCONUTS-2b to WISE 0458 (Matthews et al. 2025). In panel a) both spectra are plotted on top of each other with the spectrum of WISE 0458 being scaled by a factor of 0.5. in panel b) the difference between COCONUTS-2b and WISE 0458 is shown. The purple and orange boxes indicate where in the spectrum the differences in absorption of C2H2 and HCN are located. We do not see the … view at source ↗
read the original abstract

Linking the composition of gas giant planets to their formation paths has long been a goal in exoplanet science. Especially, cold gas giants with temperatures below $\sim$500K have been out of reach for detailed atmospheric characterization. With JWST, however, we can reach high signal-to-noise (S/N) spectra for such cool worlds and can can measure not only their main trace gas abundances, but even their isotopic content unlocking new possibilities in linking them to their formation paths. In this study, we present the spectrum of one of the coldest planetary-mass companions COCONUTS-2b ($\mathrm{T_{eff}}\approx$480K, separation of $\sim$6400 au from its M dwarf host star) obtained with the Mid-InfraRed Instrument Medium Resolution Spectrometer (MIRI/MRS). Combining the MIRI and archival Gemini/FLAMINGOS-2 data sets, we aim to characterize the chemical composition and physical structure of its frigid atmosphere, setting the stage to uncover insights on the formation of COCONUTS-2b. For the first time on a MIRI/MRS data set, we use the full spectral resolution of MIRI/MRS and perform atmospheric retrievals to unlock the search for faint absorption features by rare molecules and isotopologues. The latter are identified using a leave-one-out analysis and Bayes factor comparison. We robustly detect three isotopologues, namely $^{15}$NH$_3$, H$_2^{18}$O and H$_2^{17}$O in the atmosphere of COCONUTS-2b. We find the first clear evidence of oxygen isotopes in water in a cold companion. This data set demonstrates the capability of MIRI/MRS to characterize such cold planetary-mass companion's atmospheres with respect to their compositional and isotopic content. In the future, the constrained elemental and isotope ratios provide a unique avenue in comparing with the host star's abundances and eventually in tracing formation scenarios.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 1 minor

Summary. The manuscript reports JWST MIRI/MRS spectroscopy of the cold (~480 K) planetary-mass companion COCONUTS-2b, combined with archival Gemini/FLAMINGOS-2 data. Atmospheric retrievals are performed at the full spectral resolution of MIRI/MRS to search for faint absorption features from rare isotopologues. These are identified via leave-one-out analysis and Bayes factor model comparison, yielding robust detections of 15NH3, H2^18O and H2^17O. The work claims the first clear evidence of oxygen isotopes in water for a cold companion and demonstrates MIRI/MRS capabilities for compositional and isotopic characterization of such objects.

Significance. If the detections hold, the result is significant because it extends isotopic measurements to cold exoplanet atmospheres below 500 K, opening a route to compare elemental and isotope ratios with the host star and thereby constrain formation pathways. The leave-one-out procedure together with Bayes factor comparison supplies an internal consistency check that is independent of the final quoted abundances and strengthens the statistical support for the faint features.

major comments (1)
  1. [Atmospheric retrieval and isotopologue identification] The central claim that the three isotopologues are robustly detected rests on the assumption that the forward-model opacity database is complete for all relevant species at ~480 K and that no residual MIRI/MRS systematics (fringing, calibration artifacts, or host-star contamination) remain at the 1–2 % level after data reduction. The manuscript does not provide explicit tests or documentation of opacity completeness or of the impact of possible unmodeled continuum or line features on the leave-one-out and Bayes-factor results; any such feature could produce spurious support for an isotopologue once the dominant NH3 and H2O bands are near saturation.
minor comments (1)
  1. [Abstract] Abstract contains a typographical error: 'can can measure' should read 'can measure'.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive review and for recognizing the significance of extending isotopic measurements to cold planetary-mass companions. We address the major comment on the assumptions underlying our isotopologue detections below.

read point-by-point responses

  1. Referee: [Atmospheric retrieval and isotopologue identification] The central claim that the three isotopologues are robustly detected rests on the assumption that the forward-model opacity database is complete for all relevant species at ~480 K and that no residual MIRI/MRS systematics (fringing, calibration artifacts, or host-star contamination) remain at the 1–2 % level after data reduction. The manuscript does not provide explicit tests or documentation of opacity completeness or of the impact of possible unmodeled continuum or line features on the leave-one-out and Bayes-factor results; any such feature could produce spurious support for an isotopologue once the dominant NH3 and H2O bands are near saturation.

    Authors: We agree that the manuscript would be strengthened by additional explicit documentation. Our retrievals employ line lists from ExoMol for NH3 and H2O isotopologues, which are the most complete available and have been applied successfully in similar temperature regimes. The leave-one-out analysis combined with Bayes factor comparison is intended to isolate the contribution of each faint isotopologue and to guard against spurious signals from unmodeled features or residuals. Nevertheless, we acknowledge that direct tests of opacity completeness at 480 K and quantitative assessment of residual systematics at the 1–2 % level were not presented in detail. In the revised manuscript we will add a new subsection describing the opacity sources, their validation range, and supplementary tests including residual maps, sensitivity to injected continuum offsets, and checks for fringing or calibration artifacts. These revisions will be incorporated prior to resubmission. revision: yes

Circularity Check

0 steps flagged

No circularity: detections rely on direct spectral comparison and independent statistical tests

full rationale

The paper's central result is an observational detection of isotopologues (15NH3, H2^18O, H2^17O) obtained by performing atmospheric retrievals on MIRI/MRS spectra, followed by leave-one-out model comparisons and Bayes factor evaluation. These steps compare forward-model spectra (with versus without each isotopologue) against the observed data; the Bayes factor is computed from the evidence difference and is not algebraically equivalent to any fitted abundance parameter. No equation or procedure in the manuscript reduces the claimed detection to a self-definition, a renamed fit, or a self-citation chain. The analysis is therefore self-contained against external spectral data and standard Bayesian model selection.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard exoplanet atmospheric modeling assumptions and the reliability of statistical tests for faint spectral features. No new physical entities are postulated.

free parameters (1)
  • isotopologue abundances
    Abundances of the rare isotopologues are fitted parameters within the atmospheric retrieval to match the observed spectrum.
axioms (1)
  • domain assumption The atmospheric model accurately represents the temperature-pressure profile, chemistry, and opacities of the object.
    Invoked when performing retrievals and when using leave-one-out analysis to identify isotopologues.

pith-pipeline@v0.9.0 · 5973 in / 1624 out tokens · 87959 ms · 2026-05-21T00:16:18.383359+00:00 · methodology

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Works this paper leans on

2 extracted references · 2 canonical work pages

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    F., Spiegelman, F., & Kielkopf, J

    Allard, N. F., Spiegelman, F., & Kielkopf, J. F. 2016, A&A, 589, A21 Ando, T. 2007, Biometrika, 94, 443 Ando, T. 2011, American Journal of Mathematical and Management Sciences, 31, 13 Argyriou, I., Glasse, A., Law, D. R., et al. 2023, A&A, 675 Azzam, A. A. A., Tennyson, J., Yurchenko, S. N., & Naumenko, O. V . 2016, MNRAS, 460, 4063 Bailer-Jones, C. A. L....

    work page arXiv 2016

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    Article number, page 23 of 26 A&A proofs:manuscript no

    Here we show the part of the spectrum with the fourth to eight largest differences between the best-fits. Article number, page 23 of 26 A&A proofs:manuscript no. COCONUTS2b_MIRImrs 3.5 3.4 3.3 3.2 CH4 0 200 400 600 800 1000 1200Number count [#] 7 6 13CH4 0 200 400 600 800 1000 11 10 9 8 CH3D 0 50 100 150 200 250 300 350 3.1 3.0 2.9 2.8 2.7 H2O 0 200 400 6...

    work page 2000