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

TL;DR

This study characterizes 38 exoplanet systems around thick disc stars, revealing unexpected low-density planets in metal-poor environments, challenging existing models of planet formation and atmospheric retention in the early Milky Way.

Contribution

It provides the first homogeneous analysis of thick disc exoplanet hosts, identifying new low-density planets and expanding understanding of planet formation in ancient, metal-poor galactic environments.

Findings

Discovery of two low-density, inflated planets around thick disc stars.

First detection of puffy planets in metal-poor environments.

Establishment of a new empirical baseline for planetary architectures in the early Milky Way.

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.