Stellar clustering shapes the architectures of planetary systems
  Planet formation is generally described in terms of a system containing the
host star and a protoplanetary disc, of which the internal properties (e.g.
mass and metallicity) determine the properties of the resulting planetary
system. However, (proto)planetary systems are predicted and observed to be
affected by the spatially-clustered stellar formation environment, either
through dynamical star-star interactions or external photoevaporation by nearby
massive stars. It is challenging to quantify how the architecture of planetary
systems is affected by these environmental processes, because stellar groups
spatially disperse within <1 billion years, well below the ages of most known
exoplanets. Here we identify old, co-moving stellar groups around exoplanet
host stars in the astrometric data from the Gaia satellite and demonstrate that
the architecture of planetary systems exhibits a strong dependence on local
stellar clustering in position-velocity phase space, implying a dependence on
their formation or evolution environment. After controlling for host stellar
age, mass, metallicity, and distance from the Sun, we obtain highly significant
differences (with $p$-values of $10^{-5}{-}10^{-2}$) in planetary (system)
properties between phase space overdensities and the field. The median
semi-major axis and orbital period of planets in overdensities are 0.087 au and
9.6 days, respectively, compared to 0.81 au and 154 days for planets around
field stars. 'Hot Jupiters' (massive, close-in planets) predominantly exist in
stellar phase space overdensities, strongly suggesting that their extreme
orbits originate from environmental perturbations rather than internal
migration or planet-planet scattering. Our findings reveal that stellar
clustering is a key factor setting the architectures of planetary systems.
