The 23rd Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun (Tokyo, June 15-19, 2026)
Published June 9, 2026 | Version v1
Poster Open

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

Authors/Creators

  • 1. ROR icon The Graduate Center, CUNY
  • 2. ROR icon American Museum of Natural History
  • 3. AMERICAN MUSEUM OF NATURAL HISTORY
  • 4. ROR icon NSF's NOIRLab
  • 5. ROR icon University of Hertfordshire
  • 6. Space Science Institute
  • 7. Dordt University
  • 8. Department of Astrophysics, American Museum of Natural History
  • 9. ROR icon The University of Texas at Austin
  • 10. EDMO icon Arizona State University
  • 11. Université de Montréal
  • 12. Planétarium de Montréal, Espace pour la Vie
  • 13. ROR icon University of California, San Diego
  • 14. United States Naval Observatory Flagstaff Station
  • 15. ROR icon Infrared Processing and Analysis Center
  • 16. ROR icon Goddard Space Flight Center
  • 17. ROR icon Amherst College
  • 18. ROR icon Jet Propulsion Laboratory
  • 19. ROR icon San Francisco State University
  • 20. Caltech/IPAC
  • 21. Gemini Observatory
  • 22. Instituto de Astrofisica de Canarias
  • 23. ROR icon Universidad de La Laguna
  • 24. ROR icon University of Leicester
  • 25. CEA Saclay
  • 26. ROR icon University of Toledo
  • 27. ROR icon Centro de Astrobiología
  • 28. ROR icon Instituto de Astrofísica de Canarias
  • 29. ROR icon University of Zielona Góra
  • 30. UCLA
  • 31. EDMO icon University of Edinburgh
  • 32. EDMO icon University of Western Ontario

Description

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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Is supplement to
Publication: 10.1038/s41586-025-09369-1 (DOI)