Let's think this one through and see what happens. On planet earth every single space probe that was intentionally launched at another planet did one of four things 1)Flyby with pictures being taken 2) it went into permanent orbit or 3) it landed either successfully or catastrophically. 4) Or a landing and return sampling mission.

So there might be UFO's that satisfied number 1) at some point in earth's history an Alien race flew probe by when earth was a frozen ball back in the cambrian/precambrian decided nothing of interest and never returned. 2) Did they send one that went into permanent orbit? Not so far. 3) Did they land something on earth? So far all of the earth landers have left a lot of debris behind when they landed on other planets. But not with UFO's on earth. But with UFO's we have a fifth category; They don't flyby. They don't go into orbit and they don't land. They always seem to stay at about the 20-50000 level never running out of fuel and always staying tantalizing out of range of every single jet and plane in the vicinity. On top of that they apparently emit no IR, no radio waves, no sound waves, and can't be picked up on radar, but are in the visible light range of most cameras and people. There have been millions of sightings but not a single bit of evidence that stands up to scientific scrutiny.

Let's apply this to an earth probe and see what happens we send a probe to mars. It crosses the millions of miles in its two year journey enters the martian atmosphere and stops right at the 20-50000 foot ceiling. It never lands, it never leaves, it apparently never runs out of fuel it just flys around baffling whatever martians are there and never crashes. Why in the world would we do that? We wouldn't. Of course we can see the absurdity of that. So Occam's razor says more than likely UFO's don't exist and the sightings are due to natural phenomena of somekind.