Who’s there? That’s up to you. This post concerns aliens in your science-fiction universe. Do they exist? If so, what are they like?
Many people don’t consider a story true science fiction unless it has aliens. My universe does not, as yet, have any non-human inhabitants. Nor do I plan to. Why, you ask? Because they’re not needed.
Before you include aliens, ask yourself what purpose they fill in your universe. Not from their point of view but from yours. In his classic Foundation Trilogy, Asimov had no aliens. They were not needed. Arguably, the Mule and the members of the Second Foundation might be considered alien but were not. Asimov did not need aliens to convey anything. Humans were alien enough for him.
In a similar vein, Herbert’s Guild Navigators, Bene Tleilax and even Bene Gesserit (Dune) might be considered alien but again, they were essentially human. In both of these cases the authors found sufficient alien-ness within humanity and didn’t need green-eyed monsters.
Now let’s shift gears. Star Trek would not have been Star Trek without its plethora of alien races. In this case, though, each alien race was carefully realized to emphasize both its uniqueness and its similarity to humanity. These beings both emphasized the fact that they were alien and they focused sharply some aspect in the mirror humanity holds up to itself.
Star Wars. Ahhh, Star Wars. More aliens than would fit into a Mos Eisley cantina. Most of them had very human aspects. The story, however, emphasized some of the most basic human conflicts and desires. The plethora of aliens served to demonstrate that some things – the struggle for freedom from tyranny – transcend such trivial things as the body’s exterior configuration.
Finally, consider Brin’s Uplift books. In these stories the aliens are ubiquitous throughout the universe and humanity is an endangered species. Some of the aliens are friends, some are enemies and others just don’t care. In this case the aliens define the struggle while humanity, with only unconventional unpredictability as its weapon, manages to succeed by the thinnest of margins.
Aliens can be fun, messengers, mirrors, obstacles or simply there. As with all things, the role they fill in your universe is up to you.