Alien immigration is a form of immigration involving the movement of aliens entering human-controlled territory.
Reasons for undergoing alien immigration vary greatly between individuals and species, but common motivations are to avoid persecution, to escape conflict, religious and ideological freedom, the desire to find work, to escape environmental issues, and being sent into exile.
Due to the size of space, borders between space-faring civilizations are often ambiguous, and thus unintentional or illegal alien immigration has remained common throughout history.
Since alien immigration is dependent on a species' ability to travel interstellar distances, humans did not encounter the issue of alien immigration until centuries after developing faster-than-light technology. The earliest recordings of alien immigration occurring within human space was during the 3100s, when the young Confederacy of Humanity passed multiple progressive acts allowing aliens to coexist with humans within minor colonies and outposts. Over centuries more rights were granted to aliens living within human territory, such as the ability to become a citizen of the Confederacy.
Of course, such extensive rights created controversy. Especially during the 3500s alien immigration was a popular subject in human politics and culture. This was fueled by conflicts between humans and aliens living together, which at the time was a concept not yet fully realized. Following centuries would create more rights for aliens while revoking others, eventually creating a more fair and practical approach to the subject.
Major instances of alien immigration controversy include the Earth Homestead Acts, the Tazt Incident, and immigration during major interstellar conflicts like the Second Galactic War and the War of the Decamillenial Transition.
Like typical immigration, the concept of alien immigration is based on the idea of push and pull forces, which drive individuals from their places of origin and into some place else. These forces can be political, cultural, or economic in nature, and often have a large impact on both where an immigrant is leaving from and entering, given enough individuals are doing so.
Often most alien immigrants seek permanent residence in whichever state they enter, but some exceptions exist. For example, the Centauri Confederacy is among the few human space nations that allows alien passage through designated "fringe territory," where individuals of any species and citizenship may enter and leave at will for any reason, so long as no Confederate laws are broken during their time within their borders. Furthermore, the Confederacy of Humanity is one of many civilizations to allow alien refugees escaping war, famine, or plague to find shelter on Confederate space stations until their home is safe.
Since humanity possesses colonies across much of the known universe, alien immigration is always occurring. For this reason exact numbers are difficult to obtain. Recent estimates show roughly two trillion registered aliens pass through human territory in the Milky Way in one year, suspected to be one the highest of all human-inhabited galaxies. Most of these alien immigrants relocated to "space hubs," worlds that are entirely devoted to producing and transporting a handful of products to other nearby colonies.
On the other hand, the worlds to receive the least amount of alien immigrants were those deep within human space, such as Sol, which is notorious for its nearly nonexistent alien population. This is due to the fact that these worlds are highly diverse, having housed humans for millennia, and thus have a longstanding tradition of human superiority. Combined with equally diverse and unpredictable culture and economics, these worlds are unappealing for most alien immigrants. Only 300 million immigrants relocated to Earth, Venus, and Mars in 999900.