Interstellar ObjectsOPEN ACCESS 

Darryl Z. Seligman, Amaya Moro-Martín

invited review for Contemporary Physics, comments welcome


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 ∼63 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 characterizations, 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 ∼1026 similar bodies. Forthcoming observatories should detect many more interstellar planetesimals which may offer new insights into how planetary formation processes vary throughout the Galaxy.