Planets and their host stars carry a long-term memory of their origin in their chemical compositions. Thus, identifying planets formed in different environments improves our understating of planetary formation. Although restricted to detecting exoplanets within the solar vicinity, we might be able to detect planetary systems that formed in small external galaxies and later merged with the Milky Way. In fact, Gaia data have unequivocally shown that the Galaxy underwent several significant minor mergers during its first billion years of formation. The stellar debris of one of these mergers, Gaia-Enceladus (GE), is thought to have built up most of the stellar halo in the solar neighborhood. In this Letter, we investigate the origin of known planet-host stars combining data from the NASA Exoplanet Archive with Gaia EDR3 and large-scale spectroscopic surveys. We adopt a kinematic criterion and identify 42 stars associated with the Milky Way’s thick disk and one halo star. The only halo star identified, BD+20 2457, known to harbor two exoplanets, moves on a retrograde and highly eccentric orbit. Its chemical abundance pattern situates the star at the border between the thick disk, the old halo, and accreted populations. Given its orbital parameters and chemical properties, we suggest that BD+20 2457 is likely formed in the protodisk of the Galaxy, but we do not exclude the possibility of the star belonging to the debris of GE. Finally, we estimate a minimum age and mass limit for the star, which has implications for its planetary system and will be tested with future Transiting Exoplanet Survey Satellite observations.

行星及其宿主恒星在其化学成分中具有对它们起源的长期记忆。因此，识别在不同环境中形成的行星可以改善我们对行星形成的理解。虽然仅限于探测太阳附近的系外行星，但我们或许能够探测在小型外部星系中形成并随后与银河系合并的行星系统。事实上，盖亚数据已经明确表明，银河系在其形成的最初十亿年中经历了几次重大的小规模合并。这些合并之一的恒星碎片，盖亚-土卫二（GE），被认为已经在太阳附近形成了大部分的恒星晕。在这封信中，我们将来自 NASA 系外行星档案的数据与 Gaia EDR3 和大规模光谱调查相结合，调查了已知行星宿主恒星的起源。我们采用运动学标准，确定了与银河系厚盘相关的 42 颗恒星和一颗晕星。唯一确定的晕星BD+20 2457，已知拥有两颗系外行星，在逆行且高度偏心的轨道上运行。它的化学丰度模式将恒星置于厚圆盘、旧光环和吸积种群之间的边界处。考虑到它的轨道参数和化学性质，我们认为BD+20 2457很可能形成于银河系的原盘中，但不排除该恒星属于GE碎片的可能性。最后，我们估计了恒星的最小年龄和质量限制，