Meaning of "alien"

The free dictionary ( http://www.thefreedictionary.com/alien ) says:

1. An unnaturalized foreign resident of a country. Also called noncitizen.
2. A person from another and very different family, people, or place.
3. A person who is not included in a group; an outsider.
4. A creature from outer space.

The scope is "strange, doesn't belong in the group." The term alien for an outer space creature takes from that meaning.

By the way: the answer to the hypothetical question is "yes and no." "Yes" because the baby is indeed a creature from outer space (it is arguable whether "creature" means "non-human creature"). "No" because the baby is born of presumably at least one US citizen and therefore not an alien in the context of US citizenship.

To answer your question on the scope of the word alien: I've always used 'alien' synonymously with 'foreigner' or to describe anything that is not native to a specified area.

Merriam-Webster defines alien as:

Belonging or relating to another person, place, or thing

Alien can be used to mean extraterrestrial (as in this case) or foreigner.

In this case it would mean extraterrestrial because the baby would born outside the Earth.

In this case it simply means "someone from outer space". But it also has the added flavor of "no country of origin" because technically the American baby would not be born on American soil.