What evidence do they have that aliens are demons?

From Norman Geisler's deposition for McLean v. Arkansas Board of Education November 14, 1981

Q. Do you have any professional opinion, as to the existence of UFO’s

A. Yes. I believe that UFO’s exist.

Q. And how are they connected with Satan?

A. I believe that they are part of a mass deception attempt, that they are means by which Satan deceives because he is a deceiver and he is trying to deceive people. He did it from the beginning in the Garden of Eden, and he has been doing it now through the years. And this is one of the ways that he is deceiving people.

Q. Your description of demons, I take it, are those other angels that fell with Satan. And they exist now?

A. Yes.

Q. And your experience with the occult — how did that relate to your view of the existence of Satan?

A. I think occult phenomena, such as the moving of physical objects through the air, and so forth, such as is manifest, for example, in the Empire Strikes Back. The Luke Skywalker, the ability to move physical objects. I think that this is a demonic power that you get by occult practices. It has been done in time immemorial. It is condemned in the Bible. And it is still manifest in the world today.[1]