Harvard Physicist Avi Loeb Claims Alien Object Pierced Our Solar System in 2017

A renowned Harvard physicist has a bold theory -- a celestial object that hurled through our solar system in 2017 was actually not of our galaxy ... but from another, perhaps light-years away.

Avi Loeb -- whose new book, "Extraterrestrial: The First Sign of Intelligent Life Beyond Earth," delves more into his hypothesis -- came on "TMZ Live" Tuesday to explain why a massive, sleek unidentified piece of debris that crossed our path in 2017 might've been other-worldly.

Check it out ... Avi says when it was studied by physicists and other scientists at the time, it had none of the makings of a typical asteroid or meteor that we would see from a distant star in the Milky Way. On the contrary, he explains that it was actually propelled by an unknown source, and appears (to him, anyway) to have pierced through from a faraway place.

The Chairman of the Harvard Astronomy Dept. goes on to break down how he and his contemporaries think the object was catching flight ... it's a technology that's beyond our grasp at this point, but one that could describe how the 100-yard entity moved how it did. Think 'Star Wars' hyperspace ... and now, add light to the mix. That's pretty much what Avi believes happened here.

As for what exactly it could be ... Avi's said it might be nothing more than space junk. But, whose space junk is the key question. In his eyes, it ain't ours ... which is extraordinary.