Searching for Extragalactic Exoplanetary Systems: the Curious Case of BD+20 2457
H\'elio D. Perottoni, Jo\~ao A. S. Amarante, Guilherme Limberg, Helio, J. Rocha-Pinto, Silvia Rossi, Friedrich Anders, and Lais Borbolato

TL;DR
This study investigates the origins of a known exoplanet-hosting star, BD+20 2457, using Gaia and spectroscopic data, suggesting it may have formed in the Milky Way or been accreted from an external galaxy, impacting our understanding of planetary formation.
Contribution
The paper combines kinematic and chemical analyses to explore the extragalactic origin of a planet-host star, highlighting its potential formation in the Galaxy's protodisk or as debris from a merger event.
Findings
BD+20 2457 is likely formed in the Milky Way's protodisk.
The star's orbit and chemical properties suggest possible extragalactic origin.
Implications for planetary system evolution and future observations.
Abstract
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…
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