Then what is the basis for his statement that aliens do exist? Just some probability judgment?
The size of the universe or even our galaxy means that there is most likely life on other planets other than Earth.
They seem to be finding planets and moons fairly regularly with our limited scope of the Milky Way where there is a possibility of life.
The possibility that life form is advanced enough to visit here is the question. Unless they have some method to travel through space that defies our understanding of Speed of Light travel. Meaning some type of travel that we can not fathom. Making the distance between these planets something that can be done in much less than multiple Light Years.