The term "alien" refers to beings, usually sentient, that do not originate on the planet of the speaker. As such, it is entirely subjective depending on what planet the speaker is from. In the Lukassi galaxy, Humans are so common that they habitually and arrogantly refer to every non-Human sentient species as "aliens".
Most planets go through a phase of not knowing whether aliens actually exist, but people can't help wondering if they're alone in the universe and often attempt to make contact with anyone who's out there. Aliens almost certainly will come to visit every planet at some point, but unless they are captured or well-documented, they will usually be dismissed as hoaxes or the products of overactive imaginations.
Some people live in fear of aliens invading, killing or enslaving them all, and/or taking over their planet. Sometimes this fear turns out to be entirely justified.