PERSPECTIVE: COUNTING ALIENSThe Inferred Abundance of Interstellar Objects of Technological Origin

Five years ago, astronomers noticed a strange-looking object—maybe a thousand feet long, oblong, shiny and fast—streaking across space, tens of millions of miles from Earth. The course and speed of the object indicated it originated from outside the solar system.

David Axe writes in the Daily Beast that astronomers, who dubbed the thing ‘Oumuamua—Hawaiian for “scout”—started arguing about it.

The overwhelming majority of scientists said they do not know what ‘Oumuamua is, but that they are unwilling to speculate as to what it might be.

But a much smaller group of scientists, led by Harvard astronomer Avi Loeb, argued that we should at least consider the possibility that ‘Oumuamua is an alien spacecraft.

Axe notes that 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In a new study which appeared online on 22 September, and which has not yet been peer-reviewed, Loeb and his coauthor Carson Ezell, also a Harvard astronomer, concluded there are as many as 4,000,000,000,000,000,000 (or 4 quintillion) of them.

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

Here are the Introduction and Conclusion of the paper by Ezella and Loeb:

Introduction
Recent surveys have allowed for the detection of the first four known interstellar objects over the past decade: the interstellar meteors CNEOS 2014-01-08 [1] and CNEOS 2017-03-09 [2], the interstellar object ‘Oumuamua [3, 4] and the interstellar comet Borisov [5]. The rate of detection of interstellar objects depends on detection sensitivity, given the objects’ size and distance. One can use recent rates of detection of interstellar objects and known capabilities to estimate the density of similar objects in the solar neighborhood and the total number of such objects bound by the thin disk of the Milky Way [1].

Estimates for density and quantity of naturally occurring interstellar objects bound by the Milky Way’s thin disk assume that interstellar objects are initially ejected from their host stars in random directions [6], and that they are vertically distributed with respect to the galactic plane based on the scale height of their parent stars. Such assumptions still hold if the objects are artificial and undirected, such as space debris from extraterrestrial technological civilizations (ETCs).