An Ancient Brown Dwarf Transiting a Metal-Poor Thick Disk Star

TL;DR

TOI-7019b is a transiting brown dwarf orbiting an ancient, metal-poor thick-disk star, providing a uniquely old and metal-poor benchmark for substellar evolution. A joint analysis of TESS transits, ground-based photometry, and high-precision TRES radial velocities yields M_b = 61.3 ± 2.1 M_J and R_b = 0.821 ± 0.015 R_J with P = 48.2592 ± 0.0001 days and e = 0.403 ± 0.002, while the host star shows [Fe/H] ≈ -0.79 and [α/Fe] ≈ 0.26, placing it in the Milky Way’s thick, high‑α disk with an age of τ ≈ 12 ± 2 Gyr. The BD’s radius is ~12% larger than current old-age, metal-poor BD models predict, hinting at missing physics or composition effects. The system extends the metallicity baseline for transiting BDs and supports gravitational-instability-like formation scenarios over core accretion for high-mass substellar companions. This discovery offers a critical empirical anchor for calibrating cooling and atmospheric models in the low-metallicity regime and motivates atmospheric follow-up to test whether the brown dwarf’s composition tracks its metal-poor host.

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] $= -0.79 \pm 0.05$ dex, [$α$/Fe] $= +0.26 \pm 0.05$ dex, [M/H] $= -0.59 \pm 0.06$ dex) and kinematically ($v_{\perp} \approx 150 \pm 1$ km s$^{-1}$). We estimate a system age $τ= 12 \pm 2$ 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 $61.3 \pm 2.1$ $M_{\rm J}$ and radius of $0.82 \pm 0.02$ $R_{\rm J}$ from a joint analysis of transit photometry and radial velocity measurements, along with an orbital period of $48.2592 \pm 0.0001$ days and an orbital eccentricity of $0.403 \pm 0.002$. The measured radius appears $12.3\% \pm 2.8\%$ 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.

Figures (11)

• Figure 1: Left: Gaia XP metallicities from Andrae2023 for TESS objects of interest, versus the Galactocentric azimuthal velocity. The Sun is shown for reference. Whereas most TOIs are thin disk stars in the Solar neighborhood, TOI-7019 stands out as an extreme outlier in this space. Right: The Toomre diagram, showing the Galactocentric azimuthal velocity versus the perpendicular (radial and vertical) velocity. Dashed lines indicate constant total Galactocentric velocity. TOI-7019 lies in a sparsely populated regime of the Toomre diagram typically associated with the kinematic thick disk.
• Figure 2: TRES spectrum, orders 23 and 24, as used with the stellar characterization code UberMS. Data shown in black and best-fit model shown in red. Residuals divided by the spectral uncertainties are plotted in the lower panel, with horizontal dashed lines marking $\chi=1$ and $\chi=-1$.
• Figure 3: $\mathrm{[Fe/H]}\xspace$ and $\mathrm{[}\alpha\mathrm{/Fe]}\xspace$ of TOI-7019 in comparison to the allStarLite-dr17 APOGEE sample. We have shifted the APOGEE sample by +0.04 in $\mathrm{[}\alpha\mathrm{/Fe]}\xspace$ to account for a systematic offset between the $\mathrm{[}\alpha\mathrm{/Fe]}\xspace$ from APOGEE and that measured by UberMSPass2025_UberMS. The dashed blue line indicates a typical selection that separates the high-$\alpha$ (thick) and low-$\alpha$ (thin) disk.
• Figure 4: The observational HR diagram --- effective temperature versus surface gravity --- showing the UberMS fit to TOI-7019's TRES spectrum and broadband photometry. Black contours delineate the $1\sigma-3\sigma$ statistical error regimes. The Andrae2023 measurement using low-resolution Gaia XP data is also shown. Gaia XP metallicites have a reported uncertainty of 0.1 dex Andrae2023. MIST isochrones are overlaid for a range of stellar ages, with other parameters fixed to the adopted values for TOI-7019.
• Figure 5: The age-metallicity relation (AMR) of the Milky Way's high-$\alpha$ disk, using subgiant stars from Xiang2022. The row-normalized black histogram shows the marginal distribution of stellar ages at each metallicity. The shaded red band indicates the metallicity of TOI-7019. Marginal age distributions are shown in the top panel for the entire subgiant sample, the high-$\alpha$ disk selection, and high-$\alpha$ disk stars with metallicities similar to TOI-7019.