Interstellar objects

ABSTRACT Since 2017, two macroscopic interstellar objects have been discovered in the inner Solar System, both of which are distinct in nature. The first interstellar object, 1I/‘Oumuamua, passed within lunar distances of the Earth, appeared asteroidal lacking detectable levels of gas or dust loss, yet exhibited a nongravitational acceleration. 1I/‘Oumuamua's brief visit left open questions regarding its provenance which has given rise to many theoretical hypotheses, including an icy comet lacking a dust coma, an elongated fragment of a planet or planetesimal that was tidally disrupted, and an ultra-porous fractal aggregate. The second interstellar object, 2I/Borisov, was distinct from 1I/‘Oumuamua in terms of its bulk physical properties and displayed a definitive cometary tail. We review the discoveries of these objects, the subsequent observations and characterisations, and the theoretical hypotheses regarding their origins. We describe 1I/‘Oumuamua and 2I/Borisov in the context of active asteroids and comets in the Solar System. The discovery of these two objects implies a galactic-wide population of similar bodies. Forthcoming observatories should detect many more interstellar planetesimals which may offer new insights into how planetary formation processes vary throughout the Galaxy.