To get a reasonable idea as to how people would react to actual contact with extraterrestrials, we can look at all of the other times people thought they had contacted extraterrestrials. With the gift of hindsight, it's pretty easy to forget that people's responses to previous hoaxes and false alarms were, at the time, genuine. Of course, knowledge might have moved on a little in the scientific community, but the public reaction to first contact would likely be surprisingly similar. It has also been put forwards that we could gain a similar understanding of how us easily-spooked humans respond in a crisis by looking at the reactions of isolated societies when exposed to technological, global society and culture, because that is essentially what we would be. Of course, there's always the possibility that it would simply be No Big Deal after some time. As amazing as the prospect of discovering alien life seems to us now, we've discovered lots of things in the past. Nobody is particularly shocked and awed about the existence of the Americas anymore, but there was a time when that was the wonder if the age. If we did find a planet with a load of little green men on it, we'd probably be over it in the next 100 years or so.