Unidentified Flying Objects don't have to have come from an alien planet.
There are air force tests being conducted all the time that may not be "identified"
by either witnesses or the press.

Why flying saucers? Do aliens drink tea? Bertram Russell's teapot comes to mind.

The probability of space aliens visiting earth would substantiate Hawking's view that their germs would probably contaminate humanity and wipe 'em out.

I vote for the surreptitious visits from the air force or NASA.