Giant Planets and Eccentric Orbits are Common around Galactic Thick Disk Stars*

Planet formation in the Galactic thick disk is expected to be inefficient—low solid reservoirs, short disk lifetimes, and harsh irradiation environments should conspire to inhibit the assembly of planetary bodies—yet, planets are there, and they are stranger than we expected. Here, we present a homogeneous characterization of 32 exoplanetary systems orbiting chemically and kinematically confirmed thick disk stars, combining new detections with a systematic reassessment of archival systems, increasing the total number of exoplanets orbiting thick disk stars to 66. When planets form in the thick disk, a notable fraction are giants and move on more eccentric orbits than their thin disk counterparts—two results that challenge standard disk-evolution models. However, this should be interpreted with caution given detection biases and sample size. We also report TOI-1927 b and TOI-2643 b, the first puffy, low-density giant planets known to orbit thick disk stars, unexpected in old, metal-poor environments where planets should cool and contract efficiently. Together, these findings reveal an early Milky Way far more hospitable to planetary diversity than its harsh conditions would initially suggest.