Interstellar objects and their potential role in planet formation
The formation and evolution of planetary systems commonly leads to the ejection of predominantly icy objects. The expectation, therefore, is that the Galaxy has a background population of interstellar planetesimals ejected from these systems. We argue that 1I/‘Oumuamua, the first interstellar interloper that has been detected entering the solar system, is unlikely representative of such a population of isotropically distributed objects, favoring the scenario that it originated from a nearby system. These interstellar planetesimals will also enter the environments where planet formation is taking place. We discuss the role of the gas drag and the gravitational potential in the trapping of these objects by a molecular cloud and a protostellar core kernel. We estimate the total number of interstellar planetesimals expected to be incorporated to the disk of each forming stellar system, considering uncertainties in their size distribution and different assumptions regarding their origin and expected background density. The interest of these trapped planetesimals is that they could have sizes large enough to rapidly grow into larger bodies, via the direct accretion of the sub-cm sized dust grains in the protoplanetary disk, before the planetesimals drift toward the star due to gas drag, overcoming the meter-size barrier that challenges the growth of cm-sized pebbles into km-sized objects, an unsolved problem in planet formation theories. We find that the number of interstellar planetesimals expected to be incorporated to the disk during these early stages of the star and planet formation process can be very significant and comparable to the that expected during the latter protoplanetary disk stage, suggesting that the trapping of interstellar bodies in these environments should be considered in future planet formation models. Our results, however, show a very wide range of possible values that will be narrowed down as the population of interstellar planetesimals becomes better characterized, particularly its background density, size, and velocity distributions.