The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems. VIII. Molecular Mapping Performance With JWST/MIRI MRS: VHS 1256 b As A Case Study

VHS 1256 b was the first planetary-mass companion to be observed with the James Webb Space Telescope’s Mid-Infrared Instrument (JWST/MIRI) using the Medium-Resolution Spectrometer (MRS).

The MRS provides high-quality integral-field spectral data in the mid-infrared (IR) wavelengths from 4.9 to 18 um. This dataset serves as a testbed for applying cross-correlation techniques to characterize exoplanet atmospheres.

We implement the so-called molecular mapping approach, which consists of performing a spectral cross-correlation between each spectral pixel and atmospheric model templates.

We compare these results with those obtained from cross-correlation of the extracted spectrum. Using a self-consistent Exo-REM atmospheric model grid, we constrain the temperature, surface gravity, C/O ratio, and metallicity, finding values consistent with those obtained from other analysis methods. We detect CO (S/N ∼ 25) and H₂O (S/N ∼ 76), with tentative detections of NH₃ and CH₄ (S/N∼ 3). We test cross-correlation to measure trace-species abundances and isotopic ratios.

We measure a volume mixing ratio of [NH₃] =-5.73^{+0.15}_{-0.14} and an isotopic ratio ¹²C/¹³C=77.8+13−10, both consistent with free-chemistry retrievals. The derived NH₃ volume mixing ratio, combined with the measured temperature and radius, is consistent with VHS 1256 b having a mass above the deuterium-burning limit.

These results demonstrate the diagnostic power of mid-IR spectroscopy and highlight cross-correlation as a robust method for characterizing directly imaged exoplanets, even in future higher-contrast regimes where spectral extraction becomes challenging. Future MIRI MRS observations across a wider range of temperatures and masses will further expand our understanding of planetary atmospheric chemistry.

Mathilde Mâlin, Anthony Boccaletti, Benjamin Charnay, Laurent Pueyo, Alexis Bidot, Polychronis Patapis, Sasha Hinkley, Simon Petrus, Niall Whiteford, Marshall Perrin, Beth A. Biller, Gabriele Cugno, Thayne Currie, Camilla Danielski, Thomas Henning, Kielan K. W. Hoch, Markus Janson, Jens Kammerer, Elisabeth C Matthews, Evert Nasedkin, Paulina Palma-Bifani, Isabel Rebollido, Matthias Samland, Andrew Skemer, Jordan M. Stone, Genaro Suárez, Ben J. Sutlieff, Motohide Tamura, Christopher A. Theissen, Johanna M. Vos, Zhoujian Zhang, Alice Zurlo

Comments: Accepted in ApJ. Part of an ApJ Focus Issue from the ERS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2603.13543 [astro-ph.EP] (or arXiv:2603.13543v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2603.13543
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Submission history
From: Mathilde Mâlin
[v1] Fri, 13 Mar 2026 19:29:38 UTC (2,782 KB)
https://arxiv.org/abs/2603.13543

Astrobiology, Astrochemistry,