Although nominally science fiction-based, sword-and-planet stories feature very little discussion on the scientific details of how any of this is possible. Can an Earthman breathe the atmosphere of Venus? Sure. Stop asking questions. How did he get there? Astral projection? Yes, fine.
Or this, apparently.
The prototypical sword-and-planet story is probably the Barsoom series of books by Edgar Rice Burroughs. These stories feature John Carter, a confederate officer who somehow makes his way to "Barsoom" (Mars) to stab lots of Martians. (You might remember this character from the movie version you didn't see.) And then there's The Outlaws of Mars by Otis Adelbert Kline, which features, no lie, Jerry of Earth, who travels to Mars only to find himself, within seconds of arriving, meeting a beautiful princess and murdering her pet.
Perhaps the greatest meet-cute ever written.
Sword-and-planet stories haven't completely faded away, of course, some of their principles living on in aspects of other science fiction (Star Wars' lightsabers, most notably). But the central premise of a "civilized" man arriving and fixing the natives' problems with his civilized brains and civilized brawn has faded away. Which is perhaps for the best.
Even if it means we've lost the art of pet-stabbing introductions.