Not the Birth Cluster: the Stellar Clustering that Shapes Planetary
  Systems is Generated by Galactic-Dynamical Perturbations
  Recent work has demonstrated that exoplanetary system properties correlate
strongly with ambient stellar clustering in six-dimensional stellar
position-velocity phase space, quantified by dividing planetary systems into
sub-samples with high or low phase space densities (`overdensity' and `field'
systems, respectively). We investigate the physical origins of the phase space
overdensities and, thereby, which environmental mechanisms may have impacted
the planetary systems. We consider the galactic-scale kinematic structure of
the Milky Way observed with Gaia and show that the overdensities correspond to
the well-known, kpc-scale kinematic ripples and streams in the Galactic disk,
which are thought to be generated by bar and spiral arm-driven resonances and
satellite galaxy passages. We also find indications that the planet
demographics may vary between individual phase space overdensities, which
potentially have differing physical origins and histories. Planetary systems
associated with the `phase space spiral' (a recent perturbation of the Galactic
disk) have a hot Jupiter-to-cold Jupiter ratio that is 10 times higher than in
field systems. Finally, the hot Jupiter-to-cold Jupiter ratio within
overdensities may increase with host stellar age over Gyr timescales. Because
the overdensities persist for several Gyr, we argue that late-time
perturbations of planetary systems most likely explain these trends, although
additional perturbations at birth may contribute too. This suggests that
planetary system properties are not just affected by stellar clustering in
their immediate surroundings, but by galaxy-scale processes throughout their
evolution. We conclude by discussing the main open questions towards
understanding the diversity of physical processes that together set planetary
system architectures.
