An Ancient Brown Dwarf Transiting a Metal-poor Thick-disk Star

Abstract 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 M J ± 2.1 M J and radius of 0.82 R J ± 0.02 R 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 to be 12.3% ± 2.8% larger than predicted relative to standard evolutionary models for old, metal-poor BDs, as well as models that include stellar radiation, 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 BD’s atmospheric composition tracks its host star’s abundances, as expected for binary-like co-formation.