Aliens are Fallen Angels, Chuck Missler explains in detail Gen 6:4 & Jude 1:6 in relation to Aliens/Fallen Angels (sons of God)
Genesis 6:4 (KJV) 4 There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
Jude 1:6 (KJV) 6 And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.
Numbers 13:33 (KJV) 33 And there we saw the giants, the sons of Anak, which come of the giants: and we were in our own sight as grasshoppers, and so we were in their sight.
Deuteronomy 1:28 (KJV) 28 Whither shall we go up? our brethren have discouraged our heart, saying, The people is greater and taller than we; the cities are great and walled up to heaven; and moreover we have seen the sons of the Anakims there.