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 non-gravitational 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 which, if uniform across the galactic disk, would imply 10 to
10 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.