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arxiv: 2606.27221 · v1 · pith:GAFPZUFRnew · submitted 2026-06-25 · 🌌 astro-ph.EP

Cross-correlation transmission spectroscopy of ultra-hot Jupiters WASP-189b, HAT-P-57b, KELT-17b, and KELT-21b with GIANO-B

P. Meni-Gallardo , J. Orell-Miquel , Gareb Fern\'andez-Rodr\'iguez , H. Parviainen , M. Basilicata , E. Pall\'e , M. Stangret , I. Carleo
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P. Giacobbe
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Pith reviewed 2026-06-26 02:26 UTC · model grok-4.3

classification 🌌 astro-ph.EP
keywords ultra-hot Jupiterstransmission spectroscopycross-correlationH2OFeHnear-infraredexoplanet atmospheresGIANO-B
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The pith

Cross-correlation of NIR spectra yields a 3.8 sigma water detection in HAT-P-57b and a 5.3 sigma FeH detection in KELT-17b.

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

The paper applies cross-correlation to near-infrared transmission spectra taken with GIANO-B to hunt for molecules in four ultra-hot Jupiters. It finds a tentative H2O signal in HAT-P-57b and a clear FeH signal in KELT-17b, the third such FeH detection and the one at the lowest equilibrium temperature. No other molecules appear in those two planets and no signals at all appear in KELT-21b or WASP-189b. The results match the pattern of species already known across the ultra-hot Jupiter population. The work shows that 4 m class telescopes can contribute useful NIR transmission data and calls for more observations to confirm and enlarge the sample.

Core claim

We report a tentative detection (3.8 sigma) of H2O in HAT-P-57 b and a detection (5.3 sigma) of FeH in KELT-17 b, which is the third FeH detection ever in a UHJ and with the lowest equilibrium temperature. No molecular signals were found in KELT-21b and WASP-189b, or for other molecules in HAT-P-57b and KELT-17b. The cross-correlation results align with the species detected in the UHJ population.

What carries the argument

Cross-correlation technique applied to NIR transmission spectra from GIANO-B to extract molecular signals after removal of telluric and stellar lines.

If this is right

  • The detected species in HAT-P-57b and KELT-17b match the inventory already reported for other ultra-hot Jupiters.
  • NIR transmission spectra from 4 m telescopes can recover molecular signals in UHJs.
  • KELT-17b supplies the lowest-temperature FeH detection yet, extending the range of conditions where this molecule appears.
  • Absence of signals in KELT-21b and WASP-189b is consistent with current limits on what NIR cross-correlation can extract from these targets.

Where Pith is reading between the lines

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

  • Additional NIR epochs on the same targets could test whether the reported significances hold or arise from noise.
  • The success with GIANO-B suggests similar instruments on other 4 m telescopes could enlarge the sample of NIR molecular detections.
  • Lower equilibrium temperatures may still permit FeH to remain detectable, which future surveys can check by targeting cooler UHJs.

Load-bearing premise

The cross-correlation peaks are assumed to come from the planet after telluric and stellar signals are removed, without residual systematics or template mismatches that could create false positives.

What would settle it

A higher signal-to-noise follow-up observation that fails to recover the 5.3 sigma FeH peak in KELT-17b at the expected orbital velocity would falsify the reported detection.

Figures

Figures reproduced from arXiv: 2606.27221 by E. Pall\'e, Gareb Fern\'andez-Rodr\'iguez, H. Parviainen, I. Carleo, J. Orell-Miquel, M. Basilicata, M. Stangret, P. Giacobbe, P. Meni-Gallardo.

Figure 1
Figure 1. Figure 1: Orbital phase of each planet. Each dot represents a single spectral frame, so this diagram shows the evolution of the planet’s position over time. The dashed black lines represent the moments when the planets began and ended the transit, T1 and T4. The dashed grey lines show the moments when the planet’s surface is completely inside and outside the star’s surface, T2 and T3. The black points are the out-of… view at source ↗
Figure 2
Figure 2. Figure 2: Atmospheric models for HAT-P-57 b. Left from top to bottom: Spectral features of H2O, CO, and CO2. Middle from top to bottom: Spectral signatures of CH4, HCN, and FeH. Right from top to bottom: Spectral traces of TiO, VO, and OH, generated with petitRADTRANS. 0.9904 0.9906 0.9908 0.9910 0.9912 0.9914 0.9916 Flux 1.2 1.4 1.6 1.8 2.0 2.2 2.4 Wavelength (microns) 0.9990 0.9992 0.9994 0.9996 0.9998 1.0000 Flux… view at source ↗
Figure 3
Figure 3. Figure 3: H2O model created with petitRADTRANS cut in orders to make it fit with our data on the top and the same model normalised on the bottom. Article number, page 5 of 12 [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Results of the HAT-P-57 b on the night of 23 June 2019 CC anal￾ysis for H2O. Top: Residual map obtained from the cross-correlation. The tilted dashed red line represents the molecule fingerprint evolution during the observation, and the horizontal dashed red lines represent the different phases of the transit; the first line is T1, so the next scales to T2. ’O-M’ means Observation minus Model . If telluric… view at source ↗
Figure 7
Figure 7. Figure 7: Result of the CC analysis of KELT-17 b on night of 26 January 2019 showing a detection of of FeH. The dashed red line of the Kp map marks the value of the Kp where we reach the maximum S/N. The interpretation of the dashed lines is as in [PITH_FULL_IMAGE:figures/full_fig_p007_7.png] view at source ↗
Figure 6
Figure 6. Figure 6: Result of the combination HAT-P-57 b’s nights CC analysis for H2O. The interpretation of the dashed lines is as in [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗
Figure 8
Figure 8. Figure 8: Injection and recovery results for each inspected molecule (labelled in each panel) and each planet transit (colour coded for each individual transit according to the legend). We explored mass-fraction abundances (in molecular mean ratio (mmr)) from 10−12 to 10−2 . The dotted horizontal lines indicate the 2.5 and 5 S/N values, respectively, in each panel. there is only one previous study for transmission s… view at source ↗
Figure 9
Figure 9. Figure 9: Result of the WASP-189 b observation CC analysis showing a tentative detection of H2O on left panel and CO on right panel. The dashed red line of the Kp map marks the value of the Kp where we reach the maximum S/N. The interpretation of the dashed lines is as in [PITH_FULL_IMAGE:figures/full_fig_p009_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Planetary density (in logarithm scale) vs. equilibrium temperature (Teq ) for the Jupiter-sized planets, with atmospheric studies of H2O (top left), CO (top central), CO2 (top right), CH4 (middle left), FeH (middle central), HCN (middle right), TiO (bottom left), VO (bottom central), and OH (bottom right). We marked the planets with detected signals of each molecule in green, while they are coloured in or… view at source ↗
read the original abstract

Since the discovery of the first exoplanet, significant efforts have been made to characterise their atmospheres. Ultra-hot Jupiters (UHJs) are of particular interest due to their extended and hot atmospheres. Although previous studies have focused on the detection of atomic species at optical wavelengths, near-infrared (NIR) observations offer the potential to detect molecules. In our study, we applied the cross-correlation technique to NIR transmission spectra from \giano. The analysis focuses on the search for H$_2$O, CO, CO$_2$, CH$_4$, HCN, and FeH molecular signals in the atmospheres of four UHJs: HAT-P-57 b, KELT-17 b, KELT-21 b, and WASP-189 b. For the first time, we report results on the NIR transmission spectra of KELT-17b, KELT-21b, and WASP-189b. We report a tentative detection ($3.8\sigma$) of H$_2$O in HAT-P-57 b and a detection ($5.3\sigma$) of FeH in KELT-17~b, which is the third FeH detection ever in a UHJ and with the lowest equilibrium temperature. No molecular signals were found in KELT-21b and WASP-189b, or for other molecules in HAT-P-57b and KELT-17b. The cross-correlation results for HAT-P-57 b, KELT-17 b, KELT-21 b, and WASP-189 b in transmission align with the species detected in the UHJ population. This work underscores the need for further observations to confirm and expand the transmission study of UHJs in the NIR, and the capabilities of high-resolution spectrographs on 4-m-class telescopes.

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

2 major / 2 minor

Summary. The manuscript applies the cross-correlation technique to GIANO-B near-infrared transmission spectra of four ultra-hot Jupiters (HAT-P-57b, KELT-17b, KELT-21b, WASP-189b) to search for H₂O, CO, CO₂, CH₄, HCN, and FeH. It reports a tentative 3.8σ detection of H₂O in HAT-P-57b and a 5.3σ detection of FeH in KELT-17b (the third such detection in a UHJ and at the lowest Teq), with no other molecular signals detected.

Significance. If the detections are robust, the work adds to the census of molecular species in ultra-hot Jupiter atmospheres via NIR transmission spectroscopy and demonstrates the capabilities of 4-m-class telescopes for such observations.

major comments (2)
  1. [Data reduction and cross-correlation analysis] Data reduction and cross-correlation sections: No injection-recovery tests, false-positive rate estimates, or quantitative assessment of residual systematics after molecfit/PCA telluric and stellar removal are presented to support the claimed 3.8σ and 5.3σ significances; this is load-bearing because the central detections rest on the assumption that CCF peaks arise solely from the planet.
  2. [Results for KELT-17b] Results for KELT-17b FeH detection: The manuscript provides no sensitivity analysis showing how the assumed T-P profile and dissociation equilibrium in the FeH template affect the CCF peak height and reported 5.3σ value, despite the known sensitivity of FeH templates to these assumptions.
minor comments (2)
  1. [Abstract] Abstract: The statement that results 'align with the species detected in the UHJ population' is not accompanied by a quantitative comparison or table of prior detections.
  2. Notation: Inconsistent use of spacing around 'KELT-17~b' versus other targets; ensure uniform formatting throughout.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their detailed and constructive report. We address each major comment below. Where the comments identify gaps in the presented analysis, we agree that revisions are warranted and will incorporate additional tests and discussion in the revised manuscript.

read point-by-point responses

  1. Referee: Data reduction and cross-correlation sections: No injection-recovery tests, false-positive rate estimates, or quantitative assessment of residual systematics after molecfit/PCA telluric and stellar removal are presented to support the claimed 3.8σ and 5.3σ significances; this is load-bearing because the central detections rest on the assumption that CCF peaks arise solely from the planet.

    Authors: We acknowledge that the manuscript does not present explicit injection-recovery tests, false-positive rate estimates, or a quantitative residual assessment after the molecfit and PCA steps. The reported significances follow the standard cross-correlation procedure used in the high-resolution spectroscopy literature, with noise estimated from the off-peak CCF standard deviation. Nevertheless, the referee is correct that these additional checks would strengthen the robustness claims. In the revised manuscript we will add injection-recovery tests for the reported H2O and FeH signals, together with a quantitative evaluation of residual systematics after telluric and stellar removal. revision: yes

  2. Referee: Results for KELT-17b FeH detection: The manuscript provides no sensitivity analysis showing how the assumed T-P profile and dissociation equilibrium in the FeH template affect the CCF peak height and reported 5.3σ value, despite the known sensitivity of FeH templates to these assumptions.

    Authors: We agree that FeH line lists and templates are known to be sensitive to the adopted temperature-pressure profile and dissociation assumptions. The current analysis employed a single equilibrium-chemistry template generated at the planet’s equilibrium temperature. To address the referee’s concern, the revised manuscript will include a sensitivity analysis in which the T-P profile and dissociation parameters are varied within plausible ranges, with the resulting changes to the CCF peak height and significance reported explicitly. revision: yes

Circularity Check

0 steps flagged

No significant circularity: observational cross-correlation analysis

full rationale

This is a standard high-resolution transmission spectroscopy paper that applies the cross-correlation technique to GIANO-B NIR data for four UHJs. Detections (e.g., 5.3σ FeH in KELT-17b) are reported from empirical CCF peaks after telluric/stellar subtraction; significances derive directly from the observed data distribution rather than from any fitted parameter that is then renamed as a prediction. No self-definitional equations, fitted-input predictions, or load-bearing self-citations appear in the derivation chain. The work is self-contained against external benchmarks (template libraries, standard pipelines) and does not invoke uniqueness theorems or ansatzes from prior author work to force results.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard assumptions of the cross-correlation technique in exoplanet transmission spectroscopy, with no free parameters, new entities, or ad-hoc axioms introduced.

axioms (1)
  • domain assumption The cross-correlation technique can reliably extract planetary atmospheric signals from high-resolution spectra after removing stellar and telluric contributions.
    This underpins the detection method and significance calculations.

pith-pipeline@v0.9.1-grok · 5951 in / 1328 out tokens · 56218 ms · 2026-06-26T02:26:13.670983+00:00 · methodology

discussion (0)

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Reference graph

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    Zhou, G., Rodriguez, J. E., Collins, K. A., et al. 2016, AJ, 152, 136 Article number, page 11 of 12 A&A proofs:manuscript no. aa57881-25 Appendix A: Helium triplet analyses ofKELT−17 bandKELT−21 b We performed single-line transmission spectroscopy technique to study the possible absorption from the Helium near-infrared triplet line at 1083 nm. The HAT-P-5...

    2016