A Hypothesis for the Rapid Formation of Planets

Susanne Pfalzner1,2 and Michele T. Bannister3
Astrophysical Journal Letters 874, L34 Link to Article [DOI: 10.3847/2041-8213/ab0fa0]
1Jülich Supercomputing Center, Forschungszentrum Jülich, D-52428 Jülich, Germany
2Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
3Astrophysics Research Centre, Queen’s University Belfast, Belfast BT7 1NN, UK

The discovery of 1I/’Oumuamua confirmed that planetesimals must exist in great numbers in interstellar space. Originally generated during planet formation, they are scattered from their original systems and subsequently drift through interstellar space. As a consequence they should seed molecular clouds with at least hundred-meter-scale objects. We consider how the galactic background density of planetesimals, enriched from successive generations of star and system formation, can be incorporated into forming stellar systems. We find that at a minimum of the order of 107 ‘Oumuamua-sized and larger objects, plausibly including hundred-kilometer-scale objects, should be present in protoplanetary disks. At such initial sizes, the growth process of these seed planetesimals in the initial gas- and dust-rich protoplanetary disks is likely to be substantially accelerated. This could resolve the tension between accretionary timescales and the observed youth of fully fledged planetary systems. Our results strongly advocate that the population of interstellar planetesimals should be taken into account in future studies of planet formation. As not only the Galaxy’s stellar metallicity increased over time but also the density of interstellar objects, we hypothesize that this enriched seeding accelerates and enhances planetary formation after the first couple of generations of planetary systems.

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