St. Thomas Aquinas, in
Summa Theologica III q. 3 a. 7, "Whether one Divine Person can assume two human natures? (http://dhspriory.org/thomas/summa/TP/TP003.html#TPQ3A7THEP1)," leaves open the possibility that Christ could incarnate in other human (intelligent) natures, although St. Thomas thinks it is improbable:
QuoteOn Whether One Divine Person is Able to Assume Two Human Natures: That which is able [to do something] in one case and not in another has its power limited to one. The power of a divine person is, however, infinite, and it ought not be said that a divine person had assumed one human nature is such a manner that another could not be assumed to its personhood, for that is impossible, because an uncreated thing cannot be comprehended by a created thing. It is manifest therefore that whether we consider the divine person according to power, which is the principle of the union, or according to its personhood which is the term of the union, it must be said that the divine person besides a human nature which it has assumed, is able to assume another numerically different human nature.
Read the whole
The Thomist (http://www.thomist.org/jourl/explore.htm) article: "
Aquinas on Intelligent Extra-Terrestrial Life (http://www.thomist.org/jourl/2001/April/2001%20Apr%20A%20George%20web.htm)."
See Fr. Funes, S.J.'s interview "The Extraterrestrial Is My Brother (http://padrefunes.blogspot.com/2008/05/extraterrestrial-is-my-brother.html?m=1)."