I do believe that our planet has been visited by an alien civilization. Why they came or how often they have come, I have no idea. They might have just been looking around the milky way? Why? Because they can. The same way we can go hiking and bird watching.
While it is impossible to say for sure I think that aliens if they do exist have never visited earth, if they had it would have been able to be tracked by our extensive tracking technology both in space and on ground and we would have solid proof that they had.
The universe is so vast that it is likely aliens exist. As a matter of fact, it is almost a certainty.that they exist, but our world is relatively young. Earth has existed for a fraction of a second compared to some parts of space. Therefore, even if other species did exist they wouldn't have had the opportunity to discover us.
President Bill Clinton doesn't think that aliens have visited Earth already. However, he thinks that it's possible aliens might some day visit. He also believes that there are planetary objects with similar properties to Earth's, so that he feels it's possible. However, this debate question asks if aliens have visited Earth already, and I would say no. There really is no evidence of them ever having visited here, or even that such quick travel would be possible.