Notice how I said at this point in time? It maybe possible that our planet is stumbled upon when our race is long dead. But, today, at this moment. Impossible.
Why is it impossible? Maybe we don't understand everything about physics everywhere and forever, eh? So why not?

The real question regarding these so called sightings is: Why would aliens reveal themselves to loonies and hicks? Surely if they wanted to make an impact or talk to us they would have identified our leaders and approached them. If they have the technology to get here in the first place they surely have the technology to find out who the important people on our planet are.
You make the mistake of attributing human motivations to them.

Surely, impossible, 100%, 0, definitely, never: stop that.