Interstellar planetesimals: potential seeds for planet formation?
  We investigate the trapping of interstellar objects during the early stages
of star and planet formation. Our results show a very wide range of possible
values that will be narrowed down as the population of interstellar objects
becomes better characterized. When assuming a background number density of
2$\cdot$10$^{15}$ pc$^{-3}$ (based on 1I/'Oumuamua detection), a velocity
dispersion of 30 km/s and an equilibrium size distribution, the number of
interstellar objects captured by a molecular cloud and expected to be
incorporated to each protoplanetary disk during its formation is O(10$^{9}$)
(50 cm-5 m), O(10$^{5}$) (5 m-50 m), O(10$^{2}$) (50 m-500 m), O(10$^{-2}$)
(500 m-5 km). After the disk formed, the number of interstellar objects it
could capture from the ISM during its lifetime is 6$\cdot$10$^{11}$ (50 cm-5
m), 2$\cdot$10$^{8}$ (5 m-50 m), 6$\cdot$10$^{4}$ (50 m-500 m), 20 (500 m-5
km); in an open cluster where 1% of stars have undergone planet formation,
these values increase by a factor of O(10$^{2}$-10$^{3}$). These trapped
interstellar objects might be large enough to rapidly grow into larger
planetesimals via the direct accretion of the sub-cm sized dust grains in the
protoplanetary disk before they drift in due to gas drag, helping overcome the
meter-size barrier, acting as "seeds" for planet formation. They should be
considered in future star and planet formation models and in the potential
spread of biological material across the Galaxy.
