Listening to various sci-fi themes (examples included below), I can "hear" a certain similarity, but I can't really identify it. Something in the melody, a particular pattern of the strings, the color or the tone...
I am interested to know what these shared stylistic elements are, if they have names, and how they're constructed.
One of the three links was broken, but the lower two still work.
They both remind me of the X Files. What you've got harmonically in all three is:
The strong presence of a simple, 1,3,5 minor triad. This is a very clean sound so in a way it is elementary and childlike, but also eerie and suspenseful due to it being minor, kind of like the little kid pointing at the haunted house in the stereotypical horror movie trailer.
Then, the minor triad combined with a lot of synth and reverb in the timbre. A minor triad by itself in a classical instrumentation would sound more straightforwardly tragic, but make the timbre a little more spacey, and now the sound is misty and foggy.
Then superimposed on the minor triad in this foggy mist are a lot of fleeting major-chord associations. So while the minor triad is 1 3 5, I can tweak the top note to 1 3 6 of the minor scale, which will actually give me a major sound. Another way I could do this is to use 3 5 7, or flat 7 2 4. That fleeting major imposed on the minor is going to bring in a whole other-worldly effect, as the simple minor flits in and out of dramatically different emotions, but all in the same foggy mist.
It's difficult to define, because Sci-Fi is such a broad term.
When I Think Of Sci-Fi I think:
Dr Who, BladeRunner(and by extension Vangelis), Star Wars, Star Trek, War Of the Worlds, Thunder Birds,Journey to the Center of the Earth, Avatar, Metroid, Metal Gear Solid.
While containing a range of disperate musical styles, A common theme between the lot of them is a sense of the exotic. Whether It be through a classical medium or more bleeps bloops and synths, sci-fi's power is in taking you to another world, and thus it's music must reflect that.
In terms of tropes, again i think it's quite broad, so I'll stick to terms that I'd associate with the style. some won't fit with all the above ideas, but then again, Bach never wrote a Fugue.
Theremin, Synthesiser, Sliding Synth sound, echo.
I do reseach on automated music classification (among others) using data mining. You ask a very interesting question. At first glace I can feel a lot of built up tension in the songs and a very (fast) repetitive pattern underlying the main melody. Even the melody seems repetitive.
If we can get some midi examples or musicXML files, I might be able to let some pattern matching algorithms or other machine learning/classification techniques loose on this problem.
