This article is not about undocumented immigrants; it is about extraterrestrials (ETs) and how to think about them. We have had a bizarre fascination with this topic and have even developed "cult" groups who are convinced that the government is concealing information about ETs. Hollywood has deluged us with movies alternatively portraying ETs as sympathetic and as cruel and threatening. Theories abound about how ETs would view the Earth and include concerns that they would strip us of precious metals and rare elements or exterminate us and colonize the planet.
What To Look For First - First of all, we must realize that any alien species would probably send sensors as a first step. After all, this is what we are doing as we explore the rest of our solar system. Sensors are much easier to transport, do not involve the risk of loss of life, and can gather much needed information for further exploration.
Who Are They? - Much more importantly, any species capable of getting sensors to the Earth and then actually traveling to the Earth would have to be much, much more advanced than we are. Distances are such that we currently have no feasible way of accessing even nearby solar systems, much less distant galaxies. There is talk of "wormholes" and other mechanisms but the truth is that we have no idea how travel over such distances could be accomplished. We are probably hundreds and maybe thousands of years away from being able to even develop a plausible plan. Travel over distances measured in thousands or millions of light years is so far beyond the reach of our current scientific knowledge that we would have to assume that a species that was advanced enough to get here would be far, far ahead of us in scientific knowledge and development. We would also have to assume that - through a combination of facility at dispute resolution and luck - such a society avoided the self-destruction which becomes available as soon as nuclear weapons are discovered. They must be smart enough and emotionally balanced enough to have dodged the nuclear bullet for thousands of years.
Why Would They Come Here? - This leads to the next hypothesis - which, again, grows from our own experience. A society which is that much more advanced than we are is not going to "need" the Earth as a source or rare minerals nor as a locus for colonization. They can almost certainly synthesize or locate elsewhere any materials they need and they can probably develop other planets as bases for colonization. I believe that their interest in the Earth would be very different. Bearing in mind our own interest in much less developed colonies of humans on this planet, I think that aliens would view the Earth as a kind of "park" in which they could observe and - to a limited degree - interact with a "species in development". Just as we take vacations to the Serengeti and study isolated tribes in the Upper Amazon or New Guinea, so too these advanced aliens would become fascinated with our planet and study it and perhaps use it as an expensive vacation venue. I can also imagine advanced aliens periodically abducting humans and other species to stock "zoos" on their home planet or planets. I can even imagine that the Earth would develop into an economically important resource because of tourism and research opportunities.
Would They Intervene? - When I was in Yellowstone this summer I was struck by the idealogy of non-intervention - the notion that we should simply let nature sort things out. This includes allowing wildfires to burn, allowing predators to kill other species, etc. I suspect that this would be the attitude of aliens to the Earth. They wouldn't want to "spoil" the pristine tourist and research attraction by "changing" evolution. However, this raises another issue. If they perceived that we were about to destroy ourselves, might they intervene to preserve a valuable tourist destination and/or research resource? My guess is that such a prospect would create a furious debate between "purists" who would want to let nature take its course and sentimentalists who would want to preserve the site. On balance, it is not impossible that they would intervene to preserve a valuable site. Of course, this raises the question of whether they have already intervened in human history. Our species came very, very close to extinction 70,000 years ago and we are all the descendants of some 10,000 survivors of a horrible volcano and resulting ice age. We also have had some "near misses" in terms of nuclear war. Could the aliens have put their thumb on the scale of history one or more of these times?
Conclusion - My ideas are way out of the mainstream because we just don't like to think of ourselves as someone else's "zoo animals" or primitive tribes. But - thinking it through - that's exactly what we would be from the point of view of a species advanced enough to actually travel to the Earth. The bad news is that we are not as big a deal as we think we are. The good news is that someone may actually be watching over us.