I've always been partial to the moniker "Planetary Romance". A genre which also includes the likes of Flash Gordon, Buck Rodgers, etc.
Aside: keep in mind when these sorts of stories were written. John Carter hit the scene at a time when physicists were just comming to grips with radiation. So inventing "rays" that powered the characters anti-grav tech was prety good. No, I'm not saying that he was writting hard sci-fi. I'm saying that he paid lip service to the science of his time while writting in plot devices that were essentially magical. Much the same way that Star Trek writters do today (quick, its the third act! Re-align the deflector array to "plot resolutioun"

).
Back to topic. In its more highly developed form Planetary Romance gives way to Space Opera (Star Wars). The defining trait here being that nearly all that lip service to science disapears in favor of an intentional mimicry of mythic cycles (here comes the messiah folks, and he's carrying a laser-sword).