Disrupting Primordial Planet Signatures: The Close Encounter of Two
  Single-Planet Exosystems in the Galactic Disc
  During their main sequence lifetimes, the majority of all Galactic Disc field
stars must endure at least one stellar intruder passing within a few hundred
AU. Mounting observations of planet-star separations near or beyond this
distance suggest that these close encounters may fundamentally shape
currently-observed orbital architectures and hence obscure primordial orbital
features. We consider the commonly-occurring fast close encounters of two
single-planet systems in the Galactic Disc, and investigate the resulting
change in the planetary eccentricity and semimajor axis. We derive explicit
4-body analytical limits for these variations and present numerical
cross-sections which can be applied to localized regions of the Galaxy. We find
that each wide-orbit planet has a few percent chance of escape and an
eccentricity that will typically change by at least 0.1 due to these
encounters. The orbital properties established at formation of millions of
tight-orbit Milky Way exoplanets are likely to be disrupted.
