The first visitor we ever had from interstellar space might not have been aliens, but it's turning out to be stranger than almost anything else scientists could have imagined.
'Oumuamua zipped by without warning in late 2017, and after months of analysis of what little data they managed to collect, scientists in 2018 declared it was probably a comet - despite its bizarre cigar-like shape.
But now they're not so sure. It was speeding up as it left our solar system, which asteroids simply can't do, and it had no tail, so it can't have been an icy comet.
A new study has concluded instead 'Oumuamua might have been little more than a conglomerate of dust and grains, so light it could be pushed through space by sunlight itself.
"I think if you hit this thing it would be a little bit like hitting a spider web," University of Oslo physicist Eirik Flekkøy told Popular Science.
He's co-authored a new paper, published last week, which suggests it's a kind of substance called an 'aerogel' - 100 times less dense than air, and lighter than anything scientists on Earth can make.
"It's not the kind of object that we've seen before in our solar system," said Flekkøy.
The second interstellar visitor we've ever noticed - 2I/Borisov - was determined to definitely be a comet. It's yet to arrive, so astronomers have had a lot more time to analyse it.