That's a lot!
Loeb isn't outright saying 'Oumuamua was aliens per se, but he is saying we should be open to that possibility. In light of that outlook, he's basically asking what "respectable" scientists would never deign to: how many possible 'Oumuamuas could there be in our solar system that go unnoticed?
First, Loeb and fellow Harvard astronomer Carson Ezell looked at how many interstellar visitors we've already spotted. "One can use recent rates of detection of interstellar objects and known capabilities to estimate the density of similar objects in the solar neighborhood," the astronomers wrote in the study, as quoted bySince 'Oumuamua, there have been three more interstellar objects detected by astronomers, making it four in eight years.
That number is brought down to the more humble 4 quintillion figure when limiting the scope to the "habitable zone" near the Sun, which is potentially exciting since, if some of themSome of Loeb's theories are boundary-pushing, but he never fails to raise some fascinating points. Even if the majority of 4 quintillion interstellar objects turn out to just be bits of space rock, which Loeb admits is likely, that still leaves quite a bit of room for some of them to be alien spacecraft. Fingers crossed, at least.