Other (limited?) life forms, perhaps.
But not aliens.
Likewise, you can think it's possible but to convince others you need proof. So in terms of aliens, what proof is there that they exist?
When it comes to scientists or astrologists, they can spot meteors and asteroids or other objects coming 'close' to Earth, but funny how they never seem able to spot the aliens coming to our planet.
I'm not being obtuse or awkward here by the way. I just strongly believe that aliens don't exist and it doesn't even come into play with these 'balloons' appearing in USA/Canada.