The nearest star to the sun is 4 light years away, but in the fastest spacecraft so far made by man, it would take 80,000 years to get there, so if aliens ever have reached Earth, they must have really souped up space ships, or just have a lot of time on their hands.
Then again, isn't there a school of thought that Jesus Christ was an alien? I can see the logic - surviving an execution, being raised up in a cloud and being visited by men with bright rings on the top of their heads and all that....