For the first time, astronomers think they've spotted an object in our solar system that came from elsewhere in the galaxy.
A/2017 U1, as it's currently known, was first spotted on October 19 by the University of Hawaii's Pan-STARRS 1 telescope. It was travelling at 44km/second - so fast, it'll be on its way out of the solar system not long after it was first seen.
"Its motion could not be explained using either a normal solar system asteroid or comet orbit," said Rob Weryk, a postdoctoral researcher at the University of Hawaii Institute for Astronomy, who was the first to realise it wasn't any ordinary comet or asteroid.
"It is fairly certain we are dealing with our first truly identified alien visitor," Alan Fitzsimmons from Queen's University Belfast told The Guardian.
A/2017 U1 is estimated to be under 400m in diameter - which is still large enough to cause as much damage as 500 hydrogen bombs, according to Purdue University's 'Impact Earth' calculator.
"This is the most extreme orbit I have ever seen," added Davide Farnocchia of NASA's Center for Near-Earth Object Studies.
"It is going extremely fast and on such a trajectory that we can say with confidence that this object is on its way out of the solar system and not coming back."
While the planets and most other objects orbit the sun on a plane, A/2017 U1 came flying in from almost directly above. It passed inside Mercury's orbit on September 2, and was pulled into a hairpin turn by the Sun's gravity.
"We've never seen interstellar objects pass through before," said astronomer Karen Meech.
"We have been waiting for this day for decades," said Center for Near-Earth Object Studies manager Paul Chodas.
"It's long been theorized that such objects exist - asteroids or comets moving around between the stars and occasionally passing through our solar system - but this is the first such detection. So far, everything indicates this is likely an interstellar object, but more data would help to confirm it."