Interstellar worlds: the emerging picture of 1I/‘Oumuamua and 2I/Borisov

The arrivals of 1I/ʻOumuamua and 2I/Borisov led to a flurry of intense observational and theoretical studies, and a sustained interest in the broader populations of these small planetesimals from other planetary systems. The community’s efforts in data collection are now effectively entirely published for the briefly ground-observable visit of 1I in 2017. Increasing amounts of extended characterisation have now been reported from across the multi-month-observable visit of 2I in 2019-2020. With this, we can piece together an overview of the physical properties of these two small worlds. The first observed decametre-scale interstellar objects can be considered in context relative to the Solar System small-body populations, with which they share some common features — and a delightful array of variation, hinting at potential differences in their planetary disks of origin. The broader context of the populations of interstellar objects across the Galaxy continues to provide a rich theoretical scope, complementing the prospects of further ISO discoveries in upcoming sky surveys.