Now, a new paper by researchers at the Harvard Smithsonian Center for Astrophysics raises the possibility that the elongated dark-red object, which is 10 times as long as it is wide and traveling at speeds of 196,000 miles per hour, might have an "artificial origin".
In a paper to be published November 12 in The Astrophysical Journal Letters, the pair declare that the reddish, elongated, stadium-sized object "may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization", NBC News reported.
It was moving at 59,030mph when it was first tracked by scientists.
Oumuamua's unusual trajectory and high speed sets it aside from other space objects such as asteroids and comets.
The researchers are not saying outright that Oumuamua is a sign of extraterrestrial life.
"It is impossible to guess the objective behind Oumuamua without more data", Avi Loeb, chairman of Harvard's astronomy department and a co-author of the paper, told NBC News.
If the object is a lightsail, the paper adds it might have been floating in interstellar space when our solar system ran into it "like a ship bumping into a buoy on the surface of the ocean".
Alongside Shmuel Bialy, a postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics, acknowledges that the alien spacecraft theory is an "exotic" one.
"It's certainly ingenious to show that an object the size of Oumuamua might be sent by aliens to another star system with nothing but a solar sail for power", Seth Shostak, a senior astronomer at the SETI Institute in Mountain View, Calif., told NBC News.
"In science", he said in an email, "we must ask ourselves, "Where is the evidence?, not "Where is the lack of evidence so that I can fit in any hypothesis that I like?"
Another scientist, also studying Oumuamua - and who identified four dwarf stars that could be Oumuamua's point of origin - says the object moved too haphazardly to collect any valuable information, even if it was a probe.
"Why send a spacecraft which is doing this?" he said.
The object shot past our Sun last October, being observed as something literally from out of this world (i.e. not from our solar system).
Mr Loeb called his findings "purely scientific and evidence-based" and added: "I follow the maxim of Sherlock Holmes: when you have excluded the impossible, whatever remains, however improbable, must be the truth".
The Oumuamua was spotted late past year as it drifted into the solar system and passed close enough to the sun that it could be seen on observational telescopes.