A JWST MIRI spectrum of "the Accident" in our Galactic Halo

Brown dwarfs are crucial links connecting the lowest mass stars and Jovian worlds found in our solar system. The coldest brown dwarfs (Y dwarfs with Teff < 500 K), have atmospheric signatures that are akin to Jupiter and have received generous attention from the James Webb Space Telescope (JWST). One such object is WISEA J153429.75-104303, dubbed “the Accident” after being accidentally discovered. The Accident is a cold low-metallicity ([M/H] ~ -2.22) Y-type object with kinematics consistent with the Milky Way’s galactic halo. Recently, Faherty et al. 2025, revealed the JWST near-infrared spectrum for the Accident which showed strong absorption from methane, water, collision-induced absorption (CIA) from H2, and silane. The detection of silane by Faherty et al. (2025), marked the first detection of this molecule in a planetary body in or outside the solar system. In this presentation we will extend the analysis of this exciting object and present newly acquired JWST MIRI LRS spectra for the Accident. The full spectral energy distribution (SED) now covers 1-14 microns and yields parameters of (Teff=483+/-18 K, Logg=5.06+/-0.04, Mass=34+/-2 M_jup, Radius=0.855+/-0.01 R_jup). The mid infrared spectrum demonstrated clear molecular absorption from water and methane but no other secure molecule. Longward of 6 microns the spectrum is dominated by H2-H2 opacity hence no 10.5 micron ammonia nor secondary 11 micron peak of silane could be confidently confirmed. Given the extreme low metallicity of the Accident, we place its spectral peculiarities in context with equivalent temperature, field Y dwarfs as well as warmer low metallicity T dwarfs to evaluate how cool worlds may evolve through the cosmos. Deciphering the chemistry in the atmosphere of the accident will provide insights on primeval substellar evolution and formation at low-metallicities.

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Journal
Zenodo (CERN European Organization for Nuclear Research)
Published
2026-06-09
DOI
https://doi.org/10.5281/zenodo.20615532
Primary Topic
Stellar, planetary, and galactic studies
Type
article
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article

A JWST MIRI spectrum of "the Accident" in our Galactic Halo

Melanie J. Rowland, Dan Caselden, Pascal Tremblin, Channon Visscher et al.
Zenodo (CERN European Organization for Nuclear Research)
Stellar, planetary, and galactic studies
article

A JWST MIRI spectrum of "the Accident" in our Galactic Halo

Melanie J. Rowland, Dan Caselden, Pascal Tremblin, Channon Visscher, J. Davy Kirkpatrick, Ben Burningham, M. W. Phillips, B. Gauza, J.-Y. Zhang, E. L. Wright, N. Lodieu, Daniella C. Bardalez Gagliuffi, E. L. Martı́n, Michael Line, Marc Kuchner, P. Eisenhardt, Jacqueline Faherty, María Rosa Zapatero Osorio, Sarah Casewell, Austin Rothermich, Adam Schneider, Michael Cushing, Christopher Gelino, Eileen Gonzales, Sandy Leggett, Adam J. Burgasser, Sherelyn Alejandro Merchan, Jonathan Gagné, Genaro Suárez, Aaron Meisner, Federico Marocco, Víctor J. Sánchez Béjar
article en

Abstract

Brown dwarfs are crucial links connecting the lowest mass stars and Jovian worlds found in our solar system. The coldest brown dwarfs (Y dwarfs with Teff < 500 K), have atmospheric signatures that are akin to Jupiter and have received generous attention from the James Webb Space Telescope (JWST). One such object is WISEA J153429.75-104303, dubbed “the Accident” after being accidentally discovered. The Accident is a cold low-metallicity ([M/H] ~ -2.22) Y-type object with kinematics consistent with the Milky Way’s galactic halo. Recently, Faherty et al. 2025, revealed the JWST near-infrared spectrum for the Accident which showed strong absorption from methane, water, collision-induced absorption (CIA) from H2, and silane. The detection of silane by Faherty et al. (2025), marked the first detection of this molecule in a planetary body in or outside the solar system. In this presentation we will extend the analysis of this exciting object and present newly acquired JWST MIRI LRS spectra for the Accident. The full spectral energy distribution (SED) now covers 1-14 microns and yields parameters of (Teff=483+/-18 K, Logg=5.06+/-0.04, Mass=34+/-2 M_jup, Radius=0.855+/-0.01 R_jup). The mid infrared spectrum demonstrated clear molecular absorption from water and methane but no other secure molecule. Longward of 6 microns the spectrum is dominated by H2-H2 opacity hence no 10.5 micron ammonia nor secondary 11 micron peak of silane could be confidently confirmed. Given the extreme low metallicity of the Accident, we place its spectral peculiarities in context with equivalent temperature, field Y dwarfs as well as warmer low metallicity T dwarfs to evaluate how cool worlds may evolve through the cosmos. Deciphering the chemistry in the atmosphere of the accident will provide insights on primeval substellar evolution and formation at low-metallicities.

Zenodo (CERN European Organization for Nuclear Research)
The Graduate Center, CUNY (US), Western University (CA), Space Science Institute (US), Goddard Space Flight Center (US), Jet Propulsion Laboratory (US), American Museum of Natural History (US), University of Hertfordshire (GB), University of Leicester (GB), Universidad de La Laguna (ES), Amherst College (US), Commissariat à l'Énergie Atomique et aux Énergies Alternatives (FR), UCLA Health (US), CEA Paris-Saclay - Etablissement de Saclay (FR), Espace pour la vie (CA), University of California San Diego (US), NIU Observatory (US), Gemini South Observatory (CL), CEA Paris-Saclay (FR), Centro de Astrobiología (ES), Instituto de Astrofísica de Canarias (ES), Community Science and Data Center (US), NSF NOIRLab (US), United States Naval Observatory (US), Infrared Processing and Analysis Center (US), University of Zielona Góra (PL), Arizona State University (US), Université de Montréal (CA), San Francisco State University (US), The University of Texas at Austin (US), University of Toledo (US), University of Edinburgh (GB)
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Stellar, planetary, and galactic studies
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