belonging to a foreign country.
"an alien culture"synonyms: foreign, overseas, non-native, external, distant, remote More"the study of alien cultures promotes self-awareness"
(of a plant or animal species) introduced from another country and later naturalized.
"many food chains are based upon alien plants"
unfamiliar and disturbing or distasteful.
"principles that are alien to them"synonyms: unfamiliar, unknown, unheard of, foreign; Morestrange, peculiar, odd, bizarre, outlandish;
remote, exotic, novel
"emerging from the station in the City, they found themselves in an alien landscape"
incompatible with, unusual for, opposed to, conflicting with, contrary to, adverse to, in conflict with, at variance with, antagonistic to;
unacceptable to, repugnant to, hostile to, inimical to;
"he has been asked to adopt a vicious role alien to his nature"
supposedly from another world; extraterrestrial.
"alien beings"synonyms: extraterrestrial, other-worldly, unearthly; MoreMartian, Venutian, Jovian
"alien beings have landed on Earth"
Let's focus on the last one. A being from another world could be anything. Seeing as the word 'alien' can be applied to an incredibly broad range of beings, such as plants, amoebas, and so on, I think it would be ludicrous to assume that in the millions and billions of galaxies in our universe our planet is the only one that has any form of life on it.
I fully believe that the first extraterrestrial life will be confirmed within the next 50 years.