A Homogeneous Census of Thick Disc Planetary Systems

Is planet formation suppressed outside the Galactic thin disk? For the thick disk population, theoretical models point in that direction, due to low solid reservoirs, short protoplanetary disk lifetimes, and harsh irradiation environments, all of which hinder efficient planet formation. On the observational side, however, a growing number of thick disk stars hosting confirmed exoplanets—mostly gas giants—challenge this picture and suggest that planet formation and survival in the early Milky Way may have been more resilient and diverse than previously thought. In Ferreira et al. (2026, submitted), we present a homogeneous characterization of 38 exoplanetary systems orbiting bona fide thick disk stars, including a systematic reassessment of archival systems and 21 newly identified systems. This increases the total number of known thick disk planetary systems to 66. We combine detailed chemical abundances ([Mg/Fe], [alpha/Fe]) and orbital kinematics to robustly confirm thick disk membership. Our sample represents the oldest homogeneous population of known planetary systems, with ages spanning ~7 to 13.5 Gyr. Additionally, we also identify two systems hosting puffy exoplanets (TOI-1927 b and TOI-2643 b), the first remarkably low-density planets known to orbit thick disk stars, an unexpected result in low-metallicity environments ([Fe/H] = −0.3). Our sample provides a key empirical baseline for the comprehension of how planetary architectures form and evolve under the physical conditions of the early Milky Way thick disk.