In late October, 2017, astronomers noticed a faint dot of light which was moving through the sky. This happens every night, as telescopes detect the hundreds of thousands of asteroids which live in our Solar System. But measurements over the next few days showed something unusual -- this object had an orbit unlike any other. It was NOT orbiting our Sun, but instead simply passing through the Solar System. It was the first visitor from interstellar space to be observed thoroughly and confirmed as extrasolar in origin.
The IAU has designated the object by the name of 1I/2017 U1 ‘Oumuamua. You can see its motion through our Solar System using Tony Dunn's orbit simulator (which was used to create the diagram shown above).