Could ETI be in the Solar System?

In Freitas (1983), the author does a deep dive into where we should look and what ETI objects we should look for in our Solar System.

The paper takes a series of interesting assumptions in order to start quantifying the search it eventually describes. From the beginning, all ETI objects are categorized into 3 categories. “(I) Objects intended to be found, (II) objects intended not to be found, and (III) objects for which detection by us is irrelevant or unimportant”. He argues that objects in classes I don’t exist because the alien technology is so advanced, if they wanted it to be found, it would be. I don’t really agree with this assertion as most all communication methods rely at least somewhat on the technology level of the receiving civilization. Unless they had probes looking for population centers and landing directly next to groups of people, there is no guarantee that we find anything, especially if we aren’t spending a lot of effort looking. He also argues that objects of class II are impossible to observe. I will accept that class II objects are not worth looking for as if they do exist, I could easily see them having some advanced stealth technology that makes them nearly impossible to detect (at least at our current tech level).

Once he decides that we are looking for “objects for which detection by us is irrelevant or unimportant”, he places these objects in geocentric or selenocentric (moon-centric) orbit, likely at the L4 or L5 Lagrange points, as they are the only stable ones. Then, it is decided that an optical ground resolution of <10 m “is required for unambiguous visual detection from orbit of intelligent activity on the surface of the Earth”. I have no idea how he came up with this number, but if we can see totally unambiguous evidence of intelligent meddling on Mar’s surface with only 50m/pixel, I don’t see why we need this resolution.

After these assumptions, he compares his proposal to other proposed search spaces in a reasonable fashion.