Have aliens already visited our planet? That is a very interesting question, but I would have to say that if they have already visited our planet, then they must not be that much more advanced than we are. The gap could literally be just a few hundred years of advancement. If you think about where we were as a species 500 years ago, or where we were even 100 years ago, compared to where we are today, the progress is mind blowing. In 1508 we had just finally managed to get to the “new world” and discover that yes there was something on the other side of the planet. Heck, we had finally figured out that the Earth was round, and had barely grasped the concept that it was not the center of our universe. In 1908 we had just begun to get off the ground in vehicles capable of flying through the air.
However, it took just sixty years to get from first flight to space flight. Overnight, we had managed to put probes on Mars, orbiters flying by Venus, Mercury, the outer solar system. With advanced technology of today, we not only see planets in our solar system, but planets outside of our own solar system, and soon we will be discovering other planets that will contain intelligent life much like our own. I think we will be cautious in making contact, yet would be very eager to do so. Perhaps, these beings are sending out the first signals that they are out there, and much like us here on Earth, don’t know where to start in terms of actual communication. Perhaps they are just as unsure, perhaps even a little bit scared of us as we would be of them. I have a feeling that our human nature would incline us to send spacecraft, unmanned, to these alien worlds as a first introduction.
The reason that I believe any civilization that would bother to visit us would be roughly the same age as our own, is because honestly, what is so interesting about Earth? There has to be so much more out there to be discovered, that unless we were the first planet that another civilization discovered that had life on it, we wouldn’t be much more than a species that had yet to explore its own solar system. I compare it to when we find a tribe in the amazon that we didn’t know about, its neat that we found them, and maybe we do some study, keep track of them, but do we really get that in depth? Will we learn much from them? Maybe, but I wouldn’t expect to. An advanced civilization would probably have little use for us other than a mild interest.
It could be said though, that the same spirit of exploration that drives humanity could be driving these alien civilizations. Perhaps finding Earth is of some value, perhaps making contact is beneficial to them, it makes them feel good, to know that they have added knowledge of another culture to their libraries, and have expanded their horizons. Perhaps an older civilization would appreciate a mentor like role that supposedly they gave to ancient peoples, or perhaps they would take the chance to test their scientific knowledge on a culture, much as we do on animals.
Whatever the reasons for aliens visiting Earth, I would say that either we are a mild curiousity, or that somebody is protecting us. Why else would they just fly around, cause a panic, and then leave without at least saying hi? Or the simple explanation is that they never visited at all?