  Star formation is spatially clustered across a range of environments, from
dense stellar clusters to unbound associations. As a result, radiative or
dynamical interactions with neighbouring stars disrupt (proto)planetary systems
and limit their radii, leaving a lasting impact on their potential
habitability. In the solar neighbourhood, we find that the vast majority of
stars form in unbound associations, such that the interaction of
(proto)planetary systems with neighbouring stars is limited to the densest
sub-regions. However, the fraction of star formation occurring in compact
clusters was considerably higher in the past, peaking at ~50% in the young
Milky Way at redshift z~2. These results demonstrate that the large-scale star
formation environment affects the demographics of planetary systems and the
occupation of the habitable zone. We show that planet formation is governed by
multi-scale physics, in which Mpc-scale events such as galaxy mergers affect
the AU-scale properties of (proto)planetary systems.
