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
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2026
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| author | Mâlin, Mathilde Boccaletti, Anthony Charnay, Benjamin Pueyo, Laurent Bidot, Alexis Patapis, Polychronis Hinkley, Sasha Petrus, Simon Whiteford, Niall Perrin, Marshall Biller, Beth A. Cugno, Gabriele Currie, Thayne Danielski, Camilla Henning, Thomas Hoch, Kielan K. W. Janson, Markus Kammerer, Jens Matthews, Elisabeth C Nasedkin, Evert Palma-Bifani, Paulina Rebollido, Isabel Samland, Matthias Skemer, Andrew Stone, Jordan M. Suárez, Genaro Sutlieff, Ben J. Tamura, Motohide Theissen, Christopher A. Vos, Johanna M. Zhang, Zhoujian Zurlo, Alice |
| author_facet | Mâlin, Mathilde Boccaletti, Anthony Charnay, Benjamin Pueyo, Laurent Bidot, Alexis Patapis, Polychronis Hinkley, Sasha Petrus, Simon Whiteford, Niall Perrin, Marshall Biller, Beth A. Cugno, Gabriele Currie, Thayne Danielski, Camilla Henning, Thomas Hoch, Kielan K. W. Janson, Markus Kammerer, Jens Matthews, Elisabeth C Nasedkin, Evert Palma-Bifani, Paulina Rebollido, Isabel Samland, Matthias Skemer, Andrew Stone, Jordan M. Suárez, Genaro Sutlieff, Ben J. Tamura, Motohide Theissen, Christopher A. Vos, Johanna M. Zhang, Zhoujian Zurlo, Alice |
| contents | 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 $\sim$ 25) and H2O (S/N $\sim$ 76), with tentative detections of NH3 and CH4 (S/N$\sim$ 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 $^{12}\mathrm{C}/^{13}\mathrm{C}=77.8^{+13}_{-10}$, 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_13543 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | 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 Mâlin, Mathilde Boccaletti, Anthony Charnay, Benjamin Pueyo, Laurent Bidot, Alexis Patapis, Polychronis Hinkley, Sasha Petrus, Simon Whiteford, Niall Perrin, Marshall Biller, Beth A. Cugno, Gabriele Currie, Thayne Danielski, Camilla Henning, Thomas Hoch, Kielan K. W. Janson, Markus Kammerer, Jens Matthews, Elisabeth C Nasedkin, Evert Palma-Bifani, Paulina Rebollido, Isabel Samland, Matthias Skemer, Andrew Stone, Jordan M. Suárez, Genaro Sutlieff, Ben J. Tamura, Motohide Theissen, Christopher A. Vos, Johanna M. Zhang, Zhoujian Zurlo, Alice Earth and Planetary Astrophysics Instrumentation and Methods for Astrophysics 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 $\sim$ 25) and H2O (S/N $\sim$ 76), with tentative detections of NH3 and CH4 (S/N$\sim$ 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 $^{12}\mathrm{C}/^{13}\mathrm{C}=77.8^{+13}_{-10}$, 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. |
| title | 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 |
| topic | Earth and Planetary Astrophysics Instrumentation and Methods for Astrophysics |
| url | https://arxiv.org/abs/2603.13543 |