Searching for Extragalactic Exoplanetary Systems: the Curious Case of
  BD+20 2457
  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.
