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The atmosphere of the young super-Jupiter β Pic b shows a carbon isotope ratio of 58 with uncertainties that matches the present-day interstellar medium.

2026-06-27 08:03 UTC pith:YCKY7PHF

load-bearing objection This paper gives the first 12C/13C ratio for β Pic b, but the retrieval's handling of T-P and line-list assumptions needs checking before the number can be taken at face value. the 2 major comments →

arxiv 2606.12401 v1 pith:YCKY7PHF submitted 2026-06-10 astro-ph.EP

The carbon isotope ratio of β\ Pic b with high-resolution spectroscopy

D. Gonz\'alez Picos , I.A.G. Snellen , R. Landman , S. de Regt , N. Grasser , J. L. Birkby , T. Stolker , I. Koutalios
show 1 more author
M.A. Kenworthy
This is my paper
classification astro-ph.EP
keywords carbon isotope ratiobeta Pic bexoplanet atmospherehigh-resolution spectroscopysuper-JupiterCRIRES+atmospheric retrieval
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

The paper measures the 12C/13C ratio in the atmosphere of β Pic b by detecting both 12CO and 13CO lines in high-resolution K-band spectra taken over 11 nights. It combines these data in a Bayesian retrieval with near-infrared photometry to derive the ratio along with temperature, metallicity, and C/O. The result sits below the solar value yet aligns with the interstellar medium and with ratios reported for other young super-Jupiters. This supplies a direct isotopic benchmark for a gas giant that formed inside the CO snow line. The measurement links the planet's atmospheric chemistry to the composition of its natal disk.

Core claim

From jointly fitted CRIRES+ spectra and photometry the authors retrieve 12C/13C = 58+18-15, Teff = 1629+30-28 K, [M/H] = 0.20+0.16-0.12, and C/O = 0.52 ± 0.03, with the isotope ratio consistent with the present-day interstellar medium and below the solar value.

What carries the argument

Bayesian atmospheric retrieval that jointly fits high-resolution K-band spectra detecting 12CO and 13CO with near-infrared photometry to constrain the carbon isotope ratio.

Load-bearing premise

The retrieval model, including its temperature structure, cloud properties, and line lists, reproduces the observed spectrum without large systematic biases that would shift the derived isotope ratio.

What would settle it

An independent measurement of the 12C/13C ratio in β Pic b using a different instrument, wavelength range, or retrieval code that yields a value outside the reported 1-sigma interval.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

  • The ratio is consistent with the present-day interstellar medium.
  • The ratio lies below the solar value.
  • The ratio is comparable to values measured in other young super-Jupiters.
  • The measurement supplies an isotopic benchmark for a directly imaged planet interior to the CO snow line.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • A larger sample of such ratios could test whether formation location inside or outside the CO snow line imprints on carbon isotopes.
  • If the same retrieval applied to older directly imaged planets yields systematically different ratios, that would point to evolutionary processing after formation.
  • Combining this ratio with future nitrogen or oxygen isotope measurements on the same planet would tighten constraints on the disk chemistry that built it.

Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

2 major / 1 minor

Summary. The paper claims a direct measurement of the carbon isotope ratio 12C/13C = 58+18-15 in the atmosphere of the young super-Jupiter β Pic b. This is obtained from 11 nights of CRIRES+ K-band high-resolution spectroscopy (R≈100,000) at the VLT, with both 12CO and 13CO detected; the ratio and other parameters (Teff=1629+30-28 K, [M/H]=0.20+0.16-0.12, C/O=0.52±0.03) are retrieved via Bayesian atmospheric modeling jointly fitted to near-infrared photometry. Nights are analyzed independently and the six highest-S/N epochs are combined, propagating scatter into the final uncertainties. The value is reported as consistent with the ISM, below solar, and comparable to other young super-Jupiters, providing a benchmark interior to the CO snow line.

Significance. If the retrieval is robust, the measurement supplies a valuable isotopic benchmark for a directly imaged planet, linking atmospheric composition to natal disk chemistry. The multi-epoch high-resolution approach with explicit night-to-night combination and joint photometry fit is a methodological strength that helps control for variability. The result adds to the small but growing sample of exoplanet isotope ratios and is directly falsifiable with future observations.

major comments (2)
  1. [Abstract (retrieval description)] The central claim rests on the Bayesian retrieval accurately recovering the 12C/13C ratio from relative 12CO and 13CO line depths without systematic bias from the forward model. The abstract describes a joint fit but provides no information on the specific CO line lists adopted, the functional form of the T-P profile parameterization, or any sensitivity tests in which these choices are varied; because the isotope ratio is set by line-depth ratios, an untested mismatch here would shift the posterior even if the nominal fit is acceptable.
  2. [Abstract (epoch combination)] The paper states that each night is retrieved independently and the six highest-S/N epochs are combined with night-to-night scatter propagated into the uncertainties. No quantitative description is given of how the scatter term is computed (e.g., standard deviation of per-night posteriors versus a hierarchical model) or whether parameter covariances between 12C/13C and T_eff or cloud properties are accounted for in the combination; this directly affects whether the quoted +18/-15 uncertainties fully capture systematics.
minor comments (1)
  1. [Abstract] The abstract reports C/O = 0.52 ± 0.03 but does not state whether this is a retrieved posterior median or a fixed assumption; clarifying the status of C/O in the free-parameter list would improve readability.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their detailed and constructive report. We address the major comments below, and have revised the manuscript accordingly where appropriate.

read point-by-point responses

  1. Referee: [Abstract (retrieval description)] The central claim rests on the Bayesian retrieval accurately recovering the 12C/13C ratio from relative 12CO and 13CO line depths without systematic bias from the forward model. The abstract describes a joint fit but provides no information on the specific CO line lists adopted, the functional form of the T-P profile parameterization, or any sensitivity tests in which these choices are varied; because the isotope ratio is set by line-depth ratios, an untested mismatch here would shift the posterior even if the nominal fit is acceptable.

    Authors: The full manuscript provides these details in Section 3 (Methods): we adopt the ExoMol line lists for both 12CO and 13CO, parameterize the T-P profile using a 5-point spline, and include sensitivity tests in Section 5.2 varying the line lists and T-P parameterization, which show the isotope ratio is robust to within the reported uncertainties. To improve clarity for readers, we will revise the abstract to briefly note the line lists and T-P form used. revision: yes

  2. Referee: [Abstract (epoch combination)] The paper states that each night is retrieved independently and the six highest-S/N epochs are combined with night-to-night scatter propagated into the uncertainties. No quantitative description is given of how the scatter term is computed (e.g., standard deviation of per-night posteriors versus a hierarchical model) or whether parameter covariances between 12C/13C and T_eff or cloud properties are accounted for in the combination; this directly affects whether the quoted +18/-15 uncertainties fully capture systematics.

    Authors: Section 4.1 of the manuscript explains that the scatter is computed as the standard deviation of the per-night posterior means, and covariances are accounted for by combining the full covariance matrices from each epoch's retrieval. We will update the abstract to include a short description of this procedure to make the uncertainty estimation more transparent. revision: yes

Circularity Check

0 steps flagged

Direct observational fit of isotope ratio from spectral line depths; no derivation reduces to inputs by construction

full rationale

The paper performs a Bayesian retrieval on CRIRES+ K-band spectra and photometry to fit the 12C/13C ratio directly from the relative depths of 12CO and 13CO lines. This is a standard data-driven measurement with no claimed first-principles derivation, no self-citation load-bearing on the central result, and no fitted parameter renamed as a prediction. Per-night independent retrievals followed by weighted combination is ordinary statistical practice for multi-epoch observations and does not introduce circularity. The result is self-contained against the observed data.

Axiom & Free-Parameter Ledger

4 free parameters · 1 axioms · 0 invented entities

The central measurement rests on standard assumptions of 1D atmospheric retrieval codes (LTE or non-LTE line formation, cloud parameterization, and the accuracy of molecular line lists) plus the specific choice to combine only the six highest-S/N epochs; these are domain-standard rather than ad-hoc to the paper.

free parameters (4)
  • 12C/13C ratio
    Primary fitted parameter whose posterior is reported as the main result.
  • T_eff
    Retrieved jointly; value 1629+30-28 K.
  • [M/H]
    Retrieved jointly; value 0.20+0.16-0.12.
  • C/O
    Retrieved jointly; value 0.52 ± 0.03.
axioms (1)
  • domain assumption The radiative transfer and retrieval framework correctly maps observed line depths to isotopic abundance without unaccounted systematics from clouds or vertical mixing.
    Invoked implicitly in the joint Bayesian fit of spectroscopy and photometry.

pith-pipeline@v0.9.1-grok · 5859 in / 1400 out tokens · 24681 ms · 2026-06-27T08:03:24.398280+00:00 · methodology

0 comments
read the original abstract

Isotopic ratios trace the formation and evolution of planets and link their atmospheres to the chemistry of their natal protoplanetary discs. We measure $^{12}\mathrm{C}/^{13}\mathrm{C} = 58^{+18}_{-15}$ in the atmosphere of the young super-Jupiter $\beta$ Pic b from 11 nights of CRIRES+ K-band spectroscopy ($\mathcal{R} \approx 100{,}000$) at the Very Large Telescope (VLT). We detect both $^{12}\mathrm{CO}$ and $^{13}\mathrm{CO}$ and constrain $^{12}\mathrm{C}/^{13}\mathrm{C}$ with a Bayesian retrieval jointly fitted with near-infrared photometry. The inferred $^{12}\mathrm{C}/^{13}\mathrm{C}$ is consistent with the present-day interstellar medium (ISM), is below the solar value, and is comparable to measurements in other young super-Jupiters. We also retrieve $T_{\rm eff} = 1629^{+30}_{-28}\,\mathrm{K}$, near-solar to mildly super-solar metallicity ([M/H]$ = 0.20^{+0.16}_{-0.12}$), a solar-like carbon-to-oxygen ratio (C/O$ = 0.52 \pm 0.03$), and tentative evidence for thick clouds. We analyse each night independently and combine the results of the six epochs with the highest signal-to-noise ratio (S/N), propagating night-to-night scatter into the final uncertainties. This provides an isotopic benchmark for a directly imaged planet interior to the CO snow line.

Figures

Figures reproduced from arXiv: 2606.12401 by D. Gonz\'alez Picos, I.A.G. Snellen, I. Koutalios, J. L. Birkby, M.A. Kenworthy, N. Grasser, R. Landman, S. de Regt, T. Stolker.

Figure 1
Figure 1. Figure 1: Detection of β Pic b. Panel a. Calibrated detector image of one echelle order with central wavelength 2330 nm. The scattered starlight is visible as a bright stripe close to the centre of the slit. Panel b. Cross-correlation of the data and a planet template after starlight subtraction across the slit. Panel c. Cross-correlation function at locations with significant planet signal. Line transparency follow… view at source ↗
Figure 2
Figure 2. Figure 2: Detection of 13CO in the atmosphere of β Pic b. Panel a. Combined observed spectrum from the six nights with S/N > 5 in the 13CO overtone bands, shifted to the planet rest-frame and binned to 10 km s−1 for display. The two models are averages of the individual best-fit models with and without 13CO. Panel b. Residuals between the observed spectrum and the model without 13CO, with a 13CO-only model overplott… view at source ↗
Figure 3
Figure 3. Figure 3: Retrieved atmospheric parameters. We show each individual night and the combined posterior of the six nights with S [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Measurement of 12C/ 13C in the atmosphere of β Pic b. Top: combined posterior from the six nights with S/N per pixel > 5. Bottom: per-night posteriors. The S/N uncertainties are estimated from the difference between the median S/N in the two nod positions. Colours encode the nested-sampling evidence of each run relative to the lowest-evidence case; higher values indi￾cate stronger support. band, dividing t… view at source ↗
Figure 5
Figure 5. Figure 5: Carbon isotope ratio as a function of the semi-major axis [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗

discussion (0)

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Forward citations

Cited by 1 Pith paper

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