In this case, the researchers say it could be "debris from an advanced technological equipment" before suggesting the much wilder, "more exotic scenario" that Oumuamua may be an operational probe sent by aliens.
If Oumuamua is a lightsail, he added, one possibility is that it was floating in interstellar space when our solar system ran into it, "like a ship bumping into a buoy on the surface of the ocean", NBC News reported.
It was moving at 59,030mph when it was first tracked by scientists.
That means the object would be made of some thin material that could absorb radiation from the sun - either a naturally created material we've never seen before, or something made by aliens.
That high speed is what the Harvard astronomers are basing their theory on.
In operation, the pair theorizes that the velocity of the object and its unusual trajectory can be a result of the fact that it is no longer working.
"It is impossible to guess the objective behind Oumuamua without more data", Loeb was quoted as saying.
The paper, written by Abraham Loeb, professor and chair of astronomy, and Shmuel Bialy, a postdoctoral scholar, at the Harvard Smithsonian Center for Astrophysics, points out that comparable light-sails already exist on earth.
"Like most scientists, I would love there to be convincing evidence of alien life, but this isn't it", said Alan Fitzsimmons, an astrophysicist at Queens University, Belfast.
Coryn Bailer-Jones, an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany, said: "In science, we must ask ourselves 'Where is the evidence?"
"Recent observational and theoretical studies imply that 'Oumuamua is not an active comet", the researchers said in a draft version of their study.
An animation from NASA's Jet Propulsion Laboratory in Pasadena shows the path of A/2017 U1 as it traveled through our solar system in September and October 2017.
Another alternative? It was an actual alien spacecraft sent to observe Earth. In order for it to be effective, Oumuamua needs to be less than a millimeter in thickness, like a sail. "You have to understand that for scientists, the craziest idea is always publishable, as long as there is a tiny chance that it is not wrong", she wrote on Twitter.
Solar sails also can't change course after being launched, so if 'Oumuamua was truly a solar sail, it would be traceable back to its origin.
'But until every other possibility has been exhausted dozen times over, even the authors probably don't believe it'. It is expected to shoot past Saturn and leave the solar system in early 2019, according to the agency. "Bailer-Jones" paper on possible origin sites for 'Oumuamua was accepted by the Astrophysical Journal in September.