Hmm... definitely Science Fiction! Just because there are cows, horses and slug pistols don't make it not so!
Lets see, from what I recall -> they travel in spaceships... wear space suits (with magnetic soles?)... use 3D neural imagers... have technology based 'floaty islands'... space stations (with robotic machinery)... heat shielding... atmo engines... life support... air locks and decompression options... space shuttles (and even a star wars ship IIRC!)... sonic weapons... 'levitating' trains...
Sure, it also has elements of fantasy (as does most Sci-Fi)... though at least they got the no sound transmission in space thing right (one of my pet peeves). And there is no FTL travel or FTL comms (though realtime like radio transmissions - explained in the commentary as actually having delays which were simply not shown - and in several episodes that actually appear to exist). 'Course, one has to accept multi-year (decade+) interstellar travel of sizeable populations (though one could go with embryo transport as well...).
The terra-forming bit might be a bit over the top... but not totally inconceivable actually (other than the gravity part - but they could also be extremely dense bodies... we just don't know how common such things might or might not be).
For those who think science fiction must be 'hard science', the greatest aberrations I recall are the 'gravitics' and 'psionics'. Unlike Traveller, which tosses in FTL (jump) tech, aliens (several of the uplifted household pet variety...) and mass independent physics!
Oh, and multiple stars orbiting a central star or system of stars actually do exist. I believe the current max is six stars (in various orbital arrangements - typically involving multiple binaries...) and there are quite a number of five star systems known. And we knew of over 170 moons in our single star system circa 2009! (Though most would not hold a terraformed atmo from any current extrapolated tech...).
Ultimately, like any entertaining fiction, it revolves around people which the audience can relate to in some fashion. And, like all such things, has fans and critics!