Ok, question of all questions!!!

To be honest, based on the rules of the question, we don't even know if Pluto exists or not, most of us have never seen it in person. If Aliens came down to make their pressence known there would massive amounts of people (probably on ATS ) that would say it's all a lie.

Now, to answer the question I'd have to say large media coverage spanning several news angecies reporting the same "facts". But being from New York City I'll have to go with "I'll believe it when I see it"