As a hypothetical argument, I'm not really certain why any Earth government would want to keep knowledge of extraterrestrial life secret--outside of the rather frequent sci-fi narrative in which the military obtains, incorporates, and reverse engineers alien technology. I don't think that the mere proof of extraterrestrial life would inspire panic and rioting in the streets.More seriously, I'm quite skeptical that any government on Earth is capable of keeping something as significant as affirmative proof of extraterrestrial life a secret for the same reasons that I find most conspiracy theories involving Earth governments rather hard to believe. Our governments are barely compete enough to go about the actual, stated business of governing, maintaining infrastructure and social programs, etc. Moreover, the more people that are involved in a conspiracy: the more likely that there'll be a leak and it'll be exposed. After all, it only takes the likes of a single individual like Edward Snowden or Bradley Manning to expose a conspiracy and leak the evidence to the press, WikiLeaks, or what-have-you.