Who is they? You forgot to mention that, and it kind of totally matters.
Alien doesn't mean "from another planet", it means "not from around here". Synonyms for alien include words like foreign, non-native, nonindigenous, strange, from afar, and incongruous.
The technical term for space aliens is extraterrestrial life form, which includes any form of life that is not native to our planet. But you're not interested in the technical science stuff, you're more interested in "social science", which makes any real scientist puke in revulsion.