The aliens may indeed be fallen angels.
The very first Christian symposium on aliens will address this idea July 3-5, organized by Guy Malone, author and co-founder of Alien Resistence - an organization that studies Biblical ideas on the UFO and alien abduction phenomena.
Malone's "In a Nutshell" web page at alienresistence.org walks readers through a quick explanation - citting Biblical chapter and verse.
"The Bible contains relevant information about what many believe to be a recent phenomenon, that neither most modern churches, nor most modern ufologists, are well informed about," Malone says on his web site.
Scoff if you must, but I think one would be foolish to casually dismiss the mythological similarities between "angels" and "aliens." I tend to doubt that either label reflects a genuine understanding of the archetypal contact experience; perhaps we are all blind men examining different parts of the same elephant.