Planet seeding through gas-assisted capture of interstellar objects

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 time-scales, not allowing, or requiring fine-tuned conditions for the efficient growth of ∼metre-sized objects. Here we show that ∼10⁴ interstellar objects such as the recently detected 1I/2017-U1 ('Oumuamua) could have been captured, and become part of the young Solar system, together with up to hundreds of ∼km-sized ones. The capture rates are robust even for conservative assumptions on the protoplanetary disc structure, local stellar environment, and planetesimal interstellar medium density. 'Seeding' of such planetesimals then catalyses further planetary growth into planetary embryos, and potentially alleviates the main challenges with the metre-sized growth barrier. The capture model is in synergy with the current leading planet formation theories, providing the missing link to the first planetesimals. Moreover, planetesimal capture provides a far more efficient route for lithopanspermia than previously thought.