Setting your story in space does not make it science fiction anymore than sticking wings on a squirrel makes it a bird.
“But there are flying squirrels, aren’t there?”
Flying squirrels are not birds. Try to stay with me, faithful reader.
I got into an argument. Big surprise, there. But the argument was over whether or not stories like Star Wars and Dune belonged in science fiction. My reply was, no.
Science fiction has to have an element of real science somewhere in the plot. From The Earth To the Moon, Jurassic Park and The Martian are examples of science fiction. They are based on the very real and peer-reviewed scientific knowledge of their respective eras. The fictional aspects of these stories does not detract from the science, even if the science of the book isn’t as accurate as a paper you might deliver to your professor at MIT.
Jules Verne and HG Wells are considered the fathers of science fiction. Although it should be noted that Jules Verne resented being compared to Wells, because unlike the former, the latter wasn’t as technical in his writing. Verne’s stories went the distance in showing the work whereas Wells mostly wanted you to just take him at his word that the science of his story worked.
To that end, the work of HG Wells is more accurately described as speculative fiction. While his stories do have some trace amounts of science, such as the science of lasers that is briefly touched upon in War of the Worlds, his stories tended to lean towards social commentary as opposed to making science fun and interesting.
So why is it that Star Wars and Dune are space opera and not science fiction?
Both stories are heavily influenced by mythology. Star Wars borrows from Greek mythology and Dune has it’s basis in Greek and Arabic myth. At the heart of both tales is the idea that faith in the all powerful religion of the respective universes will help a tiny band of rebels overthrow the evil empire.
Before the Prequels, The Force was mostly a spiritual path that allowed a Jedi to tap into this great storehouse of power in the universe. Even after The Phantom Menace introduced the idea of “midichlorians”, The Force was still mostly a fantasy because now the all powerful storehouse of power that gives you superpowers is a psychokinetic relationship with microorganisms.
Dune has a similar premise: Get high on this drug that can only be mined on one planet and you’ll be able to send spaceships from one part of the universe to another. I don’t know where to begin with that one.