  The first confirmed interstellar interloper in our Solar System,
1I/`Oumuamua, is likely to be a minor body ejected from another star, but its
brief flyby and faintness made it difficult to study. Two remarkable properties
are its large (up to 2.5 mag) rotational variability and its motion relative to
the Sun before encounter. The former suggests an extremely elongated shape
(aspect ratio ~10) and the latter an origin from the protoplanetary disk of a
young star in a nearby association. Against expectations, it is also not
comet-like. 1I/`Oumuamua's variability can also be explained if it is a contact
binary composed of near-equilibrium ellipsoidal components and heterogeneous
surfaces, i.e. brighter, dust-mantled inner-facing hemispheres and darker,
dust-free outer-facing poles. Such shapes are a plausible outcome of radiation,
tides and collisions in systems where planets are clearing planetesimal disks.
The probability that 1I/`Oumuamua has the same motion as a young (<~100 Myr)
stellar association by coincidence is <1%. If it is young, its detection vs.
more numerous, older counterparts could be explained as a selection effect due
to darkening of surfaces by Galactic cosmic rays and loss of dust.
1I/`Oumuamua's apparent lack of ices can be explained if ejected rocky
planetesimals are characteristically smaller and thus far more numerous than
their icy counterparts: the Solar System may currently host several such
objects captured by the combined gravity of Jupiter and the Sun.
