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

May 2026 • 2026ApJ..1002L..41A

Authors • Adams Redai, Jéa • Chandra, Vedant • Yee, Samuel W. • DiTomasso, Victoria • Andrews, Sean • Öberg, Karin • Charbonneau, David • Woody, Rebecca • Latham, David W. • Bieryla, Allyson • Quinn, Samuel N. • Carmichael, Theron W. • Hsu, Chih-Chun • Vowell, Noah • Wang, Jason J. • Zieba, Sebastian • Benni, Paul • Collins, Karen A. • Ciardi, David R. • van Eyken, Julian • Fong, William • Lund, Michael B. • Tatarnikov, Andrei M.

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 61.3M_J ± 2.1M_J and radius of 0.82R_J ± 0.02R_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.

Links

• ELECTR https://doi.org/10.3847/2041-8213/ae4ec5
• PREPRINT http://arxiv.org/abs/2512.06069