In today’s Legal Theory Lexicon entry on “Persons and Personhood”, Larry Solum suggests that if
an intelligent alien species were to arrive on Earth … [and] the members of the aliens displayed evidence of human-like intelligence and could communicate with us (e.g. were able to master a human natural language, such as English), then we might be tempted to treat members of this species as morally and/or legally entitled to the same rights as humans.
He also gives the example of “Chewbacca and Yoda in the Star Wars movies. Neither Chewbacca nor Yoda is a member of the species homo sapiens, yet both are treated as the moral and legal equivalents of humans in the Star Wars universe.”
As it happens, the issue whether aliens able to communicate with us should be entitled to legal personality has in fact been raised in a Canadian Court. In the case of Joly v. Pelletier, [1999] O.J. No. 1728 (QL), Justice Epstein of the Superior Court of Ontario granted the defendants’ motion to dismiss on the ground that the plaintiff, who claimed that he was not a human being but rather a Martian whose DNA test results were being tampered with by the CIA, Bill Clinton, and sundry others, was not a person, and therefore not capable of being a “plaintiff” within the meaning of Ontario’s Rules of Civil Procedure!
For what it’s worth, I think that Professor Solum is right, and Justice Epstein, rather too formalistic, albeit quite amusing. But this raises further questions. If an individual intelligent alien is a person, what about collective intelligences, whether made up of insects (as in Isaac Asimov’s short story “Hallucination“) or bacteria (as in Asimov’s novel Nemesis)? What about a collective artificial intelligence (as in Stanislaw Lem’s novel The Invincible)? And perhaps most importantly: could any alien crazy enough to turn up on Earth these days be considered intelligent?
Double Aspect 
The last point is an amusing Catch-22 in the truest sense of that term. As to the point about person, I think the question is quite thorny. A profoundly mentally retarded human (with cognitive ability say 3 SD below the mean) is still regarded as a person, even though at that point said person’s cognitive function is within shooting distance of an intelligent chimpanzee. Although we might be inclined to afford the intelligent chimpanzee more consideration than his brethren, I do not think we would seriously suggest the chimpanzee is a ‘person’ or worthy of as much consideration as the aforesaid human. Is this a kind of prejudice? Perhaps. But I’m not sure it’s a bad prejudice. At the very least, it’s a prejudice that has deep evolutionary roots.