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The first confirmed interstellar interloper in our Solar System,1I/`Oumuamua, is likely to be a minor body ejected from another star, but itsbrief flyby and faintness made it difficult to study. Two remarkable propertiesare its large (up to 2.5 mag) rotational variability and its motion relative tothe Sun before encounter. The former suggests an extremely elongated shape(aspect ratio ~10) and the latter an origin from the protoplanetary disk of ayoung star in a nearby association. Against expectations, it is also notcomet-like. 1I/`Oumuamua's variability can also be explained if it is a contactbinary composed of near-equilibrium ellipsoidal components and heterogeneoussurfaces, 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 dueto darkening of surfaces by Galactic cosmic rays and loss of dust.1I/`Oumuamua's apparent lack of ices can be explained if ejected rockyplanetesimals are characteristically smaller and thus far more numerous thantheir icy counterparts: the Solar System may currently host several suchobjects captured by the combined gravity of Jupiter and the Sun.

What and Whence 1I/`Oumuamua: A Contact Binary from the Debris of a Young Planetary System?