What we’re talking about are species from other worlds coming to Earth. Life on other planets may take many forms. Regardless, what I want to discuss are those life forms capable of coming to our planet.

I can conceive of non intelligent life forms being capable of doing this in a variety of ways. Spores drifting through space for millennia, for example. Or, a natural ability to teleport or bend space.

But the real interest is in the intelligent life forms that are, or may, come to our planet. I believe that they will, do, fall into the following broad categories:

The prime requirement for all of these is that they possess the ability to leave their planet and travel to ours. We have that ability, now, though we could only send life at the microscopic level, and it would take thousands of years. We cannot send ourselves at this time. So, our visitors must have one or more technologies or abilities that we do not have.

They may only have the one technology. We may discover a starship filled with vacuum tubes, but with an engine that enables star travel. Or, they may have a device that we cannot even recognize as a starship.

I won't spend a lot of time discussing beings that have the innate ability to move between the stars. We don't have it, and probably never will. It's not currently in our biological makeup. And, if any of the reports about "UFO's" are correct, we aren't being visited in any way that we can notice by such beings. People see starships, though I could suggest that they expect to see starships.

Barbarians are intelligent beings that do not have our overall technology but have acquired the technical means or possess the native ability to bridge the stars. They found a starship with idiot proof controls. They have the ability to teleport. Whatever. The first example that springs to mind would be Viking like beings, looting and raiding their way across the stars. Certainly, primative, peace loving agrarians may also fall into this category. While we might expect that such beings may visit us, I do not believe that they already have in any significant manner. Unless the technology is very fool proof, Barbarians will have a fairly high casulty rate. Opening the starship door on a new planet exposes the star traveller to a tremendous number of risks. Disease, environmental incompatabilities, unfriendly natives, and the simple inability to reopen the door when you want to get back inside. We don't have a lot of reports suggesting either "raiding" or aliens wandering around bedazzled by our lifestyle wonders.

Superiors are technologically ahead of us. They build starships. The have anti gravity. They cure cancer. If we are visited by these aliens, we must remember what happened to the Incas and Aztecs when the Spanish landed. The Spanish weren't that far ahead, just advanced enought to kick ass. The aliens can range from just barely advanced from us to near god like advancements. These would be the aliens that threaten us the most. They are in a position to impose. Most of the existing reports of "close encounters of the third kind" would be in this category. Alien pregnancies, mysterious implants in your nose, etc.

Equals have the general technology level that we have. They have made one or two key advances that allow themselves star travel, or have that as an innate ability. Or, they found a starship with idiot proof controls. We might be getting these folks as visitors. They would want to study us, covertly. They would be less likely to fall prey to the immediate dangers of landing on an unknown planet.

Unrecognizables are beings that are so advanced that we do not recognize them as advanced. All technology is magic to a primitive people, and these beings are so far ahead of us that we may view them as gods. If they have visited us, we call it something else. These are the "who built the pyramids" aliens.