2025/12/05 by Redai, Jéa Adams, Chandra, Vedant, Yee, Samuel W. +20 · 1 voice
#Earth and Planetary Astrophysics (astro-ph.EP) #FOS: Physical sciences #Solar and Stellar Astrophysics (astro-ph.SR)
paper · doi:10.48550/arxiv.2512.06069
We report the discovery of TOI-7019b, the first transiting brown dwarf (BD) known to orbit a star that is part of the Milky Way's ancient thick disk, as defined chemically ([Fe/H] = -0.79 ± 0.05 dex, [α/Fe] = +0.26 ± 0.05 dex, [M/H] = -0.59 ± 0.06 dex) and kinematically (v⊥ ≈ 150 ± 1 km s-1). We estimate a system age τ= 12 ± 2 Gyr by fitting the host star's spectrum and spectral energy distribution to alpha-enhanced isochrones, and independently using the age-metallicity relation of the thick disk. This makes TOI-7019 by far the most metal-poor and ancient BD host known to date. We measure a BD mass of 61.3 ± 2.1 M\rm J and radius of 0.82 ± 0.02 R\rm J from a joint analysis of transit photometry and radial velocity measurements, along with an orbital period of 48.2592 ± 0.0001 days and an orbital eccentricity of 0.403 ± 0.002. The measured radius appears 12.3% ± 2.8% larger than predicted relative to standard evolutionary models for old, metal-poor brown dwarfs, hinting at missing physics like the magnetic inhibition of convection. TOI-7019b lowers the probed metallicity regime for transiting BDs by over a factor of two, making it a benchmark system to test evolutionary models in the low-metallicity regime. Future measurements of TOI-7019b's atmosphere will test whether a brown dwarf's atmospheric composition tracks its host star's abundances, as expected for binary-like co-formation.