Given what is now known about the populations of planets, and the generation, prevalence and distribution of complex organic molecules in space, it is now generally believed that it is unlikely that our civilization is alone in this galaxy. This idea is central to the concept of the Search for Extraterrestrial Intelligence (SETI), which has focused mainly on searching for radio signals originating from extraterrestrial communications since it has been imagined that extraterrestrial craft visiting Earth would be an extremely unlikely event. However, the fact that we have now observed two natural interstellar objects, asteroid 1I/2017 U1 (‘Oumuamua) and comet C/2019 Q4 (2I/Borisov), passing through our solar system and that we ourselves are now working toward sending probes to the Alpha Centauri system by 2069, a century after the first Moon landings, suggests that other civilizations may make similar efforts. Therefore, it is reasonable to consider what one can infer about an interstellar civilization that is known to have visited Earth.

To establish expectations for the general characteristics and capabilities of an interstellar civilization that has visited Earth, the travels of thousands of civilizations capable of interstellar travel were modeled and simulated. From the subset of those simulated interstellar civilizations that discover and visit Earth the distribution of their characteristics, such as the lifetime of the civilization, the distance of their civilized worlds from Earth, the number of star systems in their domain, and their basic ability to perform interstellar travel can be estimated. In addition to establishing realistic expectations about extraterrestrial visitors, these results also serve to inform about what characteristics and capabilities are required to create and maintain an interstellar presence.