The origin of interstellar asteroidal objects like 1I/2017 U1 'Oumuamua

We study the origin of the interstellar object 1I/2017 U1 'Oumuamua by juxtaposing estimates based on the observations with simulations. We speculate that objects like 'Oumuamua are formed in the debris disc as left over from the star and planet formation process, and subsequently liberated. The liberation process is mediated either by interaction with other stars in the parental star-cluster, by resonant interactions within the planetesimal disc or by the relatively sudden mass loss when the host star becomes a compact object. Integrating backward in time in the Galactic potential together with stars from the Gaia-TGAS catalogue we find that about 1.3Myr ago 'Oumuamua passed the nearby star HIP 17288 within a mean distance of $1.3$pc. By comparing nearby observed L-dwarfs with simulations of the Galaxy we conclude that the kinematics of 'Oumuamua is consistent with relatively young objects of $1.1$--$1.7$Gyr. We just met 'Oumuamua by chance, and with a derived mean Galactic density of $\sim 3\times 10^{5}$ similarly sized objects within 100\,au from the Sun or $\sim 10^{14}$ per cubic parsec we expect about 2 to 12 such visitors per year within 1au from the Sun.