An ancient brown dwarf transits a metal‑poor star?

A new study has discovered TOI‑7019b, the first brown dwarf known to transit around a star belonging to the Milky Way's ancient thick disk. This host star is very old (about 12 billion years) and metal‑poor, with roughly one‑third the iron content of the Sun. The brown dwarf has a mass of about 61 Jupiter masses and a radius slightly larger than predicted by standard models for old, metal‑poor objects. This hints that magnetic fields may inhibit convection, inflating the dwarf beyond expectations. Finding a brown dwarf orbiting such an ancient, metal‑poor star more than doubles the metallicity range where these objects have been studied, making TOI‑7019b a unique benchmark for testing evolutionary models. Future observations of its atmosphere could show whether the dwarf's composition mirrors its host star's, as would be expected if they formed together like a hidden binary.

TOI‑7019 shows thick‑disk chemical signatures: [Fe/H] = –0.79 ± 0.05 dex, [α/Fe] = +0.26 ± 0.05 dex, [M/H] = –0.59 ± 0.06 dex. Its age is 12 ± 2 Gyr, derived from fitting the star's spectrum and SED to α‑enhanced isochrones and from the thick‑disk age‑metallicity relation. The brown dwarf has a mass of 61.3 ± 2.1 M_J and a radius of 0.82 ± 0.02 R_J, with an orbital period of 48.2592 ± 0.0001 days and eccentricity of 0.403 ± 0.002. The radius is ~12.3 % larger than standard evolutionary models predict for old, metal‑poor brown dwarfs, indicating missing physics such as magnetic inhibition of convection. This discovery pushes the explored metallicity regime for transiting brown dwarfs down by more than a factor of two, establishing TOI‑7019b as a benchmark system for testing evolutionary models in low‑metallicity environments. Future high‑resolution atmospheric studies could verify whether the brown dwarf's composition tracks its host star's abundances, as expected for binary‑like co‑formation.