Clearly, D&D has borrowed a lot from sword & planet stories over the years. The Dark Sun setting was specifically designed to be similar (in some ways) to Barsoom. Creatures like the gorillon are quite explicitly the same as Barsoomian white apes. The Pathfinder setting book Distant Worlds is clearly designed to be the Leigh Brackett solar system, complete with Barsoom-like Mars and Amtor-like Venus, turned into fantasy (it's actually a bit more science fiction and less planetary romance than I expected.) Other, non-D&D games have explored the territory even more thoroughly; Space: 1889 has a strong planetary romance angle, Hollow Earth Expedition has a Mars add-on that's just like Barsoom (given that normally the setting is just like Pellucidar, it's fair to say that Jeff Combos is a Burroughs fan...) Lizard wrote what may be the most popular (if my own completely unscientific observations about it and people asking about it online are any guide) of the d20 mini-games in Iron Lords of Jupiter.
|White ape of Barsoom? Or gorillon of D&D?|
I said a few days ago that sword & sorcery and sword & planet are already almost indistinguishable save for the rather arbitrary distinction that the former uses magic, and the latter uses pseudo super science to essentially do the same thing. This was a rather facile over-simplification, so let's have a look at what is really different, shall we?
1) First off, the races. While you could conceivably come up with some way to make fantasy races fit into planetary romance (Pathfinder's elven kingdom on Venus actually works pretty well, for instance) you're better off changing them. The classic sword & planet setting has humans—although of some exotic ethnicity unlike any on earth, usually, and then a bunch of really kind of gonzo non-human races. The Green Men of Baroom, for instance, are ten feet tall, have chameleon-like eyes, huge porcelain tusks, and four arms. Lin Carter's Yathoon on Callisto are giant insect people. Barsoom also has kangaroo-like hopping people, and gigantic head-like spiders that ride headless humanoid bodies as steeds. Amtor has bird people, fish people. people that grow as buds from plants, and people that reproduce via mitosis, etc. Look for some really gonzo, unusual races, mix that up with regularish human people with green skin and blue hair or something like that, and your racial selection is good to go.
2) Secondly, classes: A lot of D&D classes (at least in most editions) are related to some kind of magic; either divine or arcane. While I can imagine a planetary romance that uses magic, normally it does not. Curiously, psychic (or psionic) powers is not at all out of place; in the first John Carter novel, Burroughs goes on and on about the telepathy that is common on Mars (although it's rather conveniently forgotten and rarely mentioned again later; kind of like midichlorians that way...) and on Callisto, the Mind Wizards are another example of weird psychic mind-readers and mental manipulators. Some of the more colorful tricks of a psion in D&D are essentially indistinguishable from magic, other than the mechanics and the pseudo-science names attached to the powers. I think something a bit more toned down than actual wizards is desirable. Something more like the Jedi would work.
3) Many fantasy stories, even sword & sorcery, don't really emphasize the "pulp aesthetic"—things needs to be gee, gosh, exotic, exciting, over the top: gratuitously exciting in a boy's adventure tale kind of way. One fascinating feature of planetary romance (actually, Burroughs, who created the genre, did this in most of his other books too) is that just over the next rise or hill, just on the other side of the next sea, or whatever, is always some really bizarre, unguessed, exotic location. Although the action is fast and furious, there's also a sense of the travelogue about these books; it's also about showcasing the imagination. Quiet little villages and farmlands are never as exciting as abandoned cities protected by illusion-projecting primitives who everyone assumed were extinct, or valleys filled with weird cyclopean carnivorous plant people, or a sea made of red mist instead of water, or... y'know. Whatever.
4) To be really true to the genre, you might think about being the PCs be Earth people stranded in a strange land. While it's not required (there were Barsoom, and other settings, books that featured native characters from time to time) it's customary. Burroughs did it, Carter did it, Resnick did it, Kline did it, Akers did it, Moorcock did it, Howard did it, Norman did it, etc. With a pedigree like that, you've got to give it some serious thought.
5) While there certainly are super-science elements to a sword & planet campaign, your scientific stuff can't be so effective that it eliminates a) big, angry, primitive monsters that are basically equivalent to alien dinosaurs, or b) the effectiveness of hand-to-hand combat with swords and stuff. Make sure that your rayguns (or whatever kinds of guns you have) aren't so good that naturally everyone will switch to using them exclusively as their weapons of choice, for example.
Along those same lines, while it was really only (mostly) Barsoom that had naked people running around, this is supposed to be a swashbuckling kind of genre. Plonking around in heavy armor doesn't seem to really be the way things are done. D&D has some optional rules for scaling armor class (as to hit scales) without needing armor; I'd either adopt these, or find some others that do more or less the same thing.
6) In a swashbuckling combat game, lots of dungeon-crawling doesn't really seem appropriate. While certainly John Carter or some of the other guys occasionally got into a fight with someone or other in a cave, the entire paradigm of dungeon-crawling seems highly inappropriate for this genre. Zipping across the planet in a flying ship, sneaking into an enemy city at night to rescue a supremely hot space princess or eliminate a political rival—that I can see. There's lots of politics, gladiatorial games, mass combat of mounted and/or flying troops, and the discovery of strange and exotic cultures, races or terrain while lost out in terra incognita. If there's an old school model that you need in order to run the game, the hex-crawl makes a lot more sense than the dungeon-crawl.