The Interstellar Interlopers

Interstellar interlopers are bodies formed outside of the Solar System but observed passing through it. The first two identified interlopers, 1I/`Oumuamua and 2I/Borisov, exhibited unexpectedly different physical properties. 1I/`Oumuamua appeared unresolved and asteroid-like, whereas 2I/Borisov was a more comet-like source of both gas and dust. Both objects moved under the action of nongravitational acceleration. These interlopers and their divergent properties provide our only window so far onto an enormous and previously unknown galactic population. The number density of such objects is ∼0.1 AU^-3 which, if uniform across the galactic disk, would imply 10²⁵ to 10²⁶ similar objects in the Milky Way. The interlopers likely formed in, and were ejected from, the protoplanetary disks of young stars. However, we currently possess too little data to firmly reject other explanations.<label>■</label>1I/`Oumuamua and 2I/Borisov are both gravitationally unbound, subkilometer bodies showing nongravitational acceleration.<label>■</label>The acceleration of 1I/`Oumuamua in the absence of measurable mass loss requires either a strained explanation in terms of recoil from sublimating supervolatiles or the action of radiation pressure on a nucleus with an ultralow mass column density, ∼1 kg m^-2.<label>■</label>2I/Borisov is a strong source of CO and H₂O, which together account for its activity and nongravitational acceleration.<label>■</label>The interlopers are most likely planetesimals from the protoplanetary disks of other stars, ejected by gravitational scattering from planets. 1I/`Oumuamua and 2I/Borisov have dynamical ages ∼10⁸ and ∼10⁹ years, respectively.<label>■</label>Forthcoming observatories should detect interstellar interlopers every year, which will provide a rapid boost to our knowledge of the population.