Interstellar Objects as Catalysts for Rapid Planet Formation Around Massive Stars

The discoveries of 1I/ʻOumuamua, 2I/Borisov, and 3I/Atlas have provided strong evidence that interstellar objects (ISOs) are widespread throughout our galaxy. Their prevalence suggests that these objects are not only passively drifting through space but may also play an active role in important astrophysical processes. In particular, ISOs are likely to be drawn into collapsing molecular clouds, where they can become incorporated into the material that forms protoplanetary disks around young stars. Within these disks, ISOs may serve as large initial building blocks, overcoming the significant obstacle known as the "1-meter barrier"—a stage in the standard accretion model where growth by sticking becomes inefficient. Based on this, we propose that two different modes of planet formation—traditional core accretion and ISO-seeded formation—can operate side by side. Our analysis indicates that during the collapse of molecular clouds, ISOs are preferentially captured by regions forming higher-mass stars. This makes the ISO-seeded mode of planet formation particularly likely to occur around stars with masses greater than 1 Msun. Planets around such stars would form more rapidly and efficiently because the presence of ISOs provides a head start to the accretion process. Observational evidence supports these ideas: higher-mass stars are found to have much shorter-lived protoplanetary disks, yet they host giant planets more frequently and at larger masses compared to lower-mass stars. The tendency for ISO-seeded planet formation to be favoured around higher-mass stars could therefore offer a natural explanation for why massive stars can form large planets within the brief lifespans of their disks, addressing a key challenge in our understanding of planetary system formation.