It seems impossible, given how long humans have been studying the skies, but last October 19, a Canadian astrophysicist at the University of Hawaii observed the first confirmed interstellar object to travel through our solar system. The object is called ‘Oumuamua (the apostrophe is not a typo).

Wait a second. Isn’t there stuff barrelling through the solar system all the time?
“Interstellar object” is defined as an object in space that is not a star or substar and that is not bound by the gravitational field of any star. Before ‘Oumuamua, the closest object to an interstellar object that humans observed in our solar system was a comet called C/1980 E1. On December 9, 1980, this comet swung past Jupiter close enough for the latter’s gravity to alter the comet’s orbit, thereby changing its trajectory to one of ejection that was no longer bound to our Sun. The object is not expected to return — nor is ‘Oumuamua, which will leave our solar system in approximately 20,000 years.
You might ask: What about Haley’s Comet? Is that not interstellar? Although Haley’s orbit is gigantic and therefore seemingly interstellar, the comet is gravitationally bound to the Sun, meaning that, technically, it isn’t an interstellar object. It’s our solar system’s wayward space rock that comes home only every 75 years or so. Interstellar objects, by contrast, are just passing by. As if using our solar system as a virtual Spacebnb, they don’t come back.
When first noticed, ‘Oumuamua was classified as a comet. Upon further review, however, the object did not display a coma (the envelope of ice and dust around a comet’s nucleus). It then became the first comet ever to be reclassified as an asteroid. To one degree or another, there is still debate about category technicalities, namely whether it is actually some type of asteroid or comet after all, but bound by the gravity of another solar system’s star.
Scientists at Harvard even wanted to classify ‘Oumuamua as an alien spacecraft because of its odd long and thin shape and “wobbly” rotation. One way or another, we have an interesting guest for the next 20,000 years.
Check out a super cool TED talk about ‘Oumuamua here .