Illegal Aliens

I know the intent but can't quite get my thought process around the phrase illegal alien. Does this mean if a person from another planet came here they would be called illegal aliens? Does this mean that there are legal aliens? I am not for giving jobs to people, feeding them, educating them or giving houses to people who won't become American citizens but I am for allowing anybody who wants to be an American to do so. I understand being here illegally but if an alien from another planet came here, would they be illegal or legal ?