  Planetesimals inevitably bear the signatures of their natal environment,
preserving in their composition a record of the metallicity of their system's
original gas and dust, albeit one altered by the formation process. When
planetesimals are dispersed from their system of origin, this record is carried
with them. As each star is likely to contribute at least $10^{12}$ interstellar
objects, the Galaxy's drifting population of interstellar objects (ISOs)
provides an overview of the properties of its stellar population through time.
Using the EAGLE cosmological simulation and models of protoplanetary formation,
our modelling predicts an ISO population with a bimodal distribution in their
water mass fraction. Objects formed in low-metallicity, typically older,
systems have a higher water fraction than their counterparts formed in
high-metallicity protoplanetary disks, and these water-rich objects comprise
the majority of the population. Both detected ISOs seem to belong to the lower
water fraction population; these results suggest they come from recently formed
systems. We show that the population of ISOs in galaxies with different star
formation histories will have different proportions of objects with high and
low water fractions. This work suggests that it is possible that the upcoming
Vera C. Rubin Observatory Legacy Survey of Space and Time will detect a large
enough population of ISOs to place useful constraints on models of
protoplanetary disks, as well as galactic structure and evolution.
