Five years ago a very strange object—maybe a thousand feet long, oblong, shiny and fast—streaked across space, tens of millions of miles from Earth. Its course and speed indicated it had come from outside the solar system. A visitor from another star.

Astronomers dubbed the thing ‘Oumuamua—Hawaiian for “scout”—and started arguing about it.

On one side are an overwhelming majority of scientists who don’t know what Oumuamua is, but aren’t willing to speculate as to what it might be.

On the other side, are a much smaller camp led by Harvard astronomer Avi Loeb, who argues that we should at least consider the possibility that ‘Oumuamua is an alien spacecraft.

Now Loeb is asking the next logical question. How many other ‘Oumuamuas could there be in and around the solar system? In a new study that appeared online on Sept. 22 and hasn’t yet been peer-reviewed, Loeb and his coauthor Carson Ezell, also a Harvard astronomer, concluded there are as many as 4,000,000,000,000,000,000 (or 4 quintillion) of them.

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Each is a visitor from another star, and each, possibly, artificially created.

That might seem like a lot. But the solar system is vast. And the space between our star system and our closest neighbor, Proxima Centauri, is even more vast. Actually finding any of those 4 quintillion possible mysterious objects for closer study could be really, really hard.

To be clear, Loeb isn’t claiming there are quintillions of alien craft zooming around our corner of the Milky Way. After all, he’s never said that ‘Oumuamua is definitely a robotic probe or crewed craft—just that we should be open to the possibility.

Read more: The Daily Beast