The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems. VII. Molecular Mapping Performance with JWST/MIRI MRS: VHS 1256 b as a Case Study

Abstract 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 4.9–18 μ m. This data set 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 H 2 O (S/N ∼ 76), with tentative detections of NH 3 and CH 4 (S/N ∼ 3). We test cross-correlation to measure trace-species abundances and isotopic ratios. We measure a volume mixing ratio (VMR) of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">[</mml:mo> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">NH</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msub> <mml:mo stretchy="false">]</mml:mo> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>5.7</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.14</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.15</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> and an isotopic ratio <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mn>12</mml:mn> </mml:mrow> </mml:msup> <mml:mi mathvariant="normal">C</mml:mi> <mml:msup> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mrow> <mml:mn>13</mml:mn> </mml:mrow> </mml:msup> <mml:mi mathvariant="normal">C</mml:mi> <mml:mo>=</mml:mo> <mml:mn>77</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>13</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , both consistent with free-chemistry retrievals. The derived NH 3 VMR, 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.

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Journal
The Astronomical Journal
Published
2026-04-16
DOI
https://doi.org/10.3847/1538-3881/ae5240
Primary Topic
Stellar, planetary, and galactic studies
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article
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The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems. VII. Molecular Mapping Performance with JWST/MIRI MRS: VHS 1256 b as a Case Study

P. Palma-Bifani, Jens Kammerer, M. Janson, Andrew Skemer et al.
The Astronomical Journal
Stellar, planetary, and galactic studies
article

The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems. VII. Molecular Mapping Performance with JWST/MIRI MRS: VHS 1256 b as a Case Study

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

Abstract

Abstract 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 4.9–18 μ m. This data set 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 H 2 O (S/N ∼ 76), with tentative detections of NH 3 and CH 4 (S/N ∼ 3). We test cross-correlation to measure trace-species abundances and isotopic ratios. We measure a volume mixing ratio (VMR) of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">[</mml:mo> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">NH</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msub> <mml:mo stretchy="false">]</mml:mo> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>5.7</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.14</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.15</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> and an isotopic ratio <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mn>12</mml:mn> </mml:mrow> </mml:msup> <mml:mi mathvariant="normal">C</mml:mi> <mml:msup> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mrow> <mml:mn>13</mml:mn> </mml:mrow> </mml:msup> <mml:mi mathvariant="normal">C</mml:mi> <mml:mo>=</mml:mo> <mml:mn>77</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>13</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , both consistent with free-chemistry retrievals. The derived NH 3 VMR, 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.

The Astronomical JournalVol. 171(5)
Osaka Gakuin University (JP), United States Naval Research Laboratory (US), Centre National de la Recherche Scientifique (FR), Space Telescope Science Institute (US), Goddard Space Flight Center (US), European Southern Observatory (DE), American Museum of Natural History (US), Johns Hopkins University (US), Université de Bordeaux (FR), Universitat de València (ES), Stockholm University (SE), Délégation Paris 7 (FR), University of California, Santa Cruz (US), University of Zurich (CH), Université Paris Cité (FR), Trinity College Dublin (IE), University of Exeter (GB), Arcetri Astrophysical Observatory (IT), ETH Zurich (CH), University of California San Diego (US), Sorbonne Université (FR), Sorbonne Paris Cité (FR), Laboratoire d'Astrophysique de Bordeaux (FR), Max Planck Institute for Astronomy (DE), Astrobiology Center (JP), Université Bourgogne Franche-Comté (FR), Scottish Universities Physics Alliance (GB), Centro de Astrobiología (ES), AlbaNova (SE), Astronomical Observatory (RS), Naval Research Laboratory Remote Sensing Division, The University of Texas at San Antonio (US), University of Rochester (US), University of Baltimore (US), The University of Tokyo (JP), Universidad Diego Portales (CL), The University of Osaka (JP), University of Edinburgh (GB)
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Stellar, planetary, and galactic studies
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