Hidden Companions of the Early Milky Way I. New alpha-Enhanced Exoplanet Hosts

TL;DR

This study delivers a homogeneous catalogue of exoplanets orbiting alpha-enhanced thick-disc stars, integrating new detections with reanalyses of archival data to produce self-consistent stellar and planetary parameters. By combining high-precision RVs, space-based photometry, and uniform spectroscopy, it strengthens thick-disc membership via chemical and kinematic criteria and reveals two unusually inflated planets, TOI-1927 b and TOI-2643 b, that challenge expectations for metal-poor environments. The work shows that giant and inflated planets can form and survive in the early Milky Way, hinting at alternative formation pathways such as disc instability and highlighting the need to study planetary architectures within Galactic archaeology contexts. These results establish a robust empirical baseline for how planetary systems assemble and evolve under the depleted, short-lived discs of the ancient Galaxy, with further papers planned to compare thick- and thin-disc populations and to explore detailed chemical signatures.

Abstract

Planet formation in the chemically ancient, dynamically heated Galactic thick disc remains poorly constrained, owing to the expectation that its low solid reservoirs, short disc lifetimes, and harsh irradiation environments inhibit efficient assembly of planetary bodies. However, an increasing number of confirmed thick disc planet hosts now challenge this view, indicating that planetary formation and survival in the early Milky Way may have been more resilient -- and more diverse -- than standard disc-evolution models suggest. Here we present a homogeneous characterisation of 38 exoplanetary systems orbiting bona fide thick disc stars, combining new detections with a systematic reassessment of archival systems. High-precision radial velocities and space-based transit photometry, combined with uniform high-resolution spectroscopy, yield self-consistent stellar and planetary parameters, and thick disc membership is secured via joint chemical and kinematic criteria. Among these systems, we identify two remarkably low-density, inflated planets -- TOI-1927 b and TOI-2643 b -- representing the first puffy planets known to orbit thick disc stars, and an outcome that is highly unexpected in metal-poor environments, thereby challenging current models of atmospheric retention and thermal inflation at low metallicity. This consolidated sample establishes a new empirical baseline for understanding how planetary architectures emerge under the depleted, short-lived discs characteristic of the early Milky Way.

Figures (18)

• Figure 1: Toomre diagram for our sample of stars with thick disc-like kinematics. Green stars indicate thin disc host stars from NASA Exoplanet Archive 2013PASP..125..989A, whilst blue circles and red triangles represent stars hosting TOIs and confirmed planets, respectively.
• Figure 2: Left panel: [Mg/Fe] as a function of [Fe/H] for our confirmed thick disc stars (triangles). Thin disc stars from our Inti sample 2021MNRAS.504.1873Y are shown as circles, whilst the known thick disc stars are represented by diamonds. The blue line marks the thin/thick disc boundary defined by Griffith:2019ApJ...886...84G, whilst the blue shaded region shows the offset between our measurements and those from GALAH. Right panel: Same as the left panel, but with the blue line representing the thin/thick disc boundary defined by Adibekyan:2011AA...535L..11A as a function of [$\alpha$/Fe].
• Figure 3: Period-Mass (left) and Period-Radius (right) for thick disc exoplanets. Grey points represent the full NASA Exoplanet Archive population. Thick disc planets are shown as star symbols, colour-coded by the stellar mass (left) and scaled semi-major axis (right). Solar System planets are labelled with their initial letters. The dashed blue line indicates the brown dwarf mass/radius limit ($13~M_{\rm Jup}$ / $25~R_{\oplus}$; 2011ApJ...727...57S2022MNRAS.511.3133H), and the shaded degenerate zone highlights where massive planets and brown dwarfs are indistinguishable by radius alone, further requiring mass measurements for proper classification. In the left panel, dashed lines of constant semi-amplitude $K = 1,~5,~10,~20,~{\rm and}~50$ m s$^{-1}$ are indicated assuming $M_\star = 1~M_\odot$. In the right panel, the dashed black lines delineate the Neptune Desert boundaries following 2024AA...691A.233C.
• Figure 4: Periodograms and best-fit models over HARPS radial velocities for the three planets: HD 4308 $b$ (first panel), HD 4308 $c$ (second panel), and HD 4308 $d$. The binned data correspond to 20 uniformly spaced bins (also applied for all phase diagrams presented in this work). Residuals for each fit are displayed beneath the corresponding panel. In the last panel, the planet's signal is represented in green, stellar activity modulations in yellow, and the joint model in blue.
• Figure 5: Periodogram (first panel) and best-fit model over HARPS radial velocities for HD 150433 $b$ (second and third panel). Dashed horizontal grey line in the first panel indicates 0.01 false-alarm probability levels, and the periodogram peak is indicated by the blue horizontal line. In the second panel, the planet's signal is represented in green, stellar activity modulations in yellow, and the joint model in blue. Residuals in the third panel are shown in the lower panel.