  The recently discovered minor body 1I/2017 U1 (`Oumuamua) is the first known
object in our Solar System that is not bound by the Sun's gravity. Its
hyperbolic orbit (eccentricity greater than unity) strongly suggests that it
originated outside our Solar System; its red color is consistent with
substantial space weathering experienced over a long interstellar journey. We
carry out an simple calculation of the probability of detecting such an object.
We find that the observed detection rate of 1I-like objects can be satisfied if
the average mass of ejected material from nearby stars during the process of
planetary formation is ~20 Earth masses, similar to the expected value for our
Solar System. The current detection rate of such interstellar interlopers is
estimated to be 0.2/year, and the expected number of detections over the past
few years is almost exactly one. When the Large Synoptic Survey Telescope
begins its wide, fast, deep all-sky survey the detection rate will increase to
1/year. Those expected detections will provide further constraints on nearby
planetary system formation through a better estimate of the number and
properties of interstellar objects.
