Planet formation begins with collisional-growth of small planetesimals accumulating into larger-ones. Such growth occurs while planetesimals are embedded in a gaseous protoplanetary-disc. However, small-planetesimals experience collisions and gas-drag that lead to their destruction on short timescales not allowing (or requiring fine-tuned conditions) for the efficient growth to ~metre-size  larger objects. Here we show that small (up-to 0.1 - 10 km-size) unbound interstellar-objects passing through a gaseous protoplanetary-disc can be efficiently captured to become embedded in the disc. In particular, as many as 10^4 interstellar objects such as the recently detected 1I/2017-U1 ('Oumuamua) could have been captured, and become part of the young Solar System. 'Seeding' of such planetesimals then catalyze further planetary-growth into planetary embryos, and potentially alleviate the main-challenges with the meter-size growth-"barrier".