It is generally believed that it is unlikely that our civilization is alone in this galaxy. This belief is central to the premise of the Search for Extraterrestrial Intelligence (SETI), which has focused mainly on searching for radio signals originating from extraterrestrial communications, since it is believed that extraterrestrial craft visiting Earth would be an extremely unlikely event. However, the fact that we ourselves are currently working on developing probes to send to the Alpha Centauri system by 2069, strongly suggests that other civilizations may make similar, or more ambitious, efforts. Therefore, it is reasonable to inform our expectations by considering what characteristics and capabilities would be required for an interstellar civilization to find and visit Earth. In this paper, a physics-based analytic model of expanding interstellar civilizations is developed. A million civilizations that encounter Earth are simulated and their statistics are studied to determine their characteristics.

Advanced extraterrestrial civilizations may produce observable technosignatures when they migrate inside their planetary systems and to other planetary systems. Collectively, the search for technosignatures produced by migrating extraterrestrial civilizations can be described as the search for migrating extraterrestrial intelligence (SMETI). I propose that extraterrestrial civilizations may use free-floating planets as interstellar transportation to reach, explore and colonize planetary systems. I propose possible technosignatures and artefacts that may be produced by extraterrestrial civilizations using free-floating planets for interstellar migration and interstellar colonization, as well as strategies for the search for their technosignatures and artefacts. I discuss technosignatures that may be produced by extraterrestrial civilizations using other methods of interstellar migration and colonization. As an example, I discuss the star GJ 433, which experienced a close flyby a few thousand years ago. I divide possible technosignatures of migrating civilizations into groups to highlight similarities and differences among technosignatures produced by civilizations using different methods of migration and interstellar colonization. It follows from the comparison, for example, that interstellar migration using flybys of stars can blend with interstellar migration using free-floating planets.