The recently newly discovered asteroid that, in cosmic distances, barely missed the earth just a week or so ago is a prime example that things exist that we have no clue actually exists.....until we first notice them.
-
"Planet X" exists....out beyond Pluto...in the minds of scientists who theorize its existence based on unexplained gravitational pulls that, according to them, is best explained by the existence of the yet-too-actually-be discovered Planet X. 
-
"Could" a "Planet Nibiru" or a "Really Big Asteroid Nibiru" really exist? Yes, it "could." "Could" a "Nibiru," if it exists, come close enough to the earth to cause "disturbances".....especially if it actually crashed into the earth? "Yes," it "could."
-
The question is twofold: (1) Would anyone see it with anymore warning than when we discovered the recent asteroid that just barely missed earth? (2) If it was on a collision course, would any government or any scientist tell us?
-
And the answer to both questions is "No."