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.