He received a brief response in the comments from a non-Cinerati member fan of Planetary Romance named Venusian that summarized the difference as, "there is no magic in planetary romance, and it's usually 'off planet.'" This definition is useful, as far as it goes, but it doesn't go deep enough to truly differentiate the two sub-genre from each other. It's also only half true. Add to this lack of specificity the particular -- and perculiar -- skepticism of a person like Tulkinghorn and it makes for a perfect topic for a longer post.
So...what is the difference between Planetary Romance and Sword and Sorcery fiction?
To begin, we must start by acknowledging that both of these sub-genre of fiction lie within the scope of Heroic Fantasy -- and sometimes Heroic Science Fiction -- which is itself a sub-genre of Fantasy literature.
[One could use this as an opportunity to advance the argument that in "speculative fiction" it is Fantasy that is the primary genre and all other classifications are sub-genre of Fantasy, but that is a discussion for another post. Let it merely be stated that I dislike the term "speculative fiction" as it seems to a) have an anti-fantasy bias, b) exhibit "embarrassment" with association with Fantasy, c)has a pro-Science Fiction bias (SF is the abbreviation for both), and is guilty of a litany of other sins including the theft of candy apples from small children at county fairs.]
Heroic Fantasy can be simply defined for the purposes of this discussion, it deserves a thorough examination itself, as narratives in which a heroic figure struggles against antagonists within an imagined setting which contains "impossible" or "improbable" elements. These elements can be magic, monsters, imagined science, or gobbledygook. Most of the fiction in modern Fantasy, epic or otherwise, is some form of Heroic Fantasy though some stories contain "mundane" protagonists or "anti-heroes." To be truly Heroic Fantasy, the protagonist must be larger than life; and this is even more true in the sub-genres of Planetary Romance and Sword and Sorcery.
To really discuss the differences between Planetary Romance and Sword and Sorcery, it is helpful to see how prior science fiction critics have defined the subject.
According to David Pringle (in John Clute and John Grant's Encyclopedia of Fantasy) Planetary Romance stories,
are stories of adventure set almost entirely on the surface of some alien world, with an emphasis on swordplay (or similar), monsters, telepathy or other under-explained "magic," and near-human alien civilizations which often resemble those of Earth's pre-technological past...The hero is usually from Earth, but the means of his or her "translation" to the far planet is often supernatural rather than technological, involving flying carpets, astral projection, angel-power and kindred devices. Spaceships are sometimes mentioned, but the complete lack of interest shown in the mechanics of space travel is one of the principal features distinguishing PR from space opera...; super-scientific spacecraft and other mighty machines are central to space opera, but rarely feature in planetary romance.
The same volume includes a definition of Sword and Sorcery written by John Clute, David Langford, and Roz Kaveney which claims,
In 1961 Michael Moorcock requested a term to describe the fantasy subgenre featuring muscular Heroes in violent conflict with a variety of Villains, chiefly Wizards, Witches, evil Spirits, and other creatures whose powers are -- unlike the hero's -- supernatural in origin. Fritz Leiber suggested "Sword and Sorcery", and this term stuck.
I think these two definitions are extremely useful and one might argue that the Pringle and Clute definitions provide us with sufficient data to provide us with a clear understanding of these two genre, but I am not quite satisfied with Clute's definition of Sword and Sorcery. Certainly, the Pringle definition of Planetary Romance gives us a strong sense of the kind of story one might expect if one were to call it Planetary Romance. It also provides ammunition against Venusian's claim that Planetary Romance doesn't feature magic. This is important because one of the things that makes Planetary Romance so special is that way that it walks the tightrope between Fantasy and Science Fiction. It is a wonderful crossover genre.
Some brief examples of the "magic" featured in tales of Planetary Romance include the telepathic language of the Martians of Barsoom, the psychic hounds of Leigh Brackett's Skaith novels, and the "Force" in the Star Wars films. The Star Wars films being a wonderful filmic example of Planetary Romance. Planetary Romance tales feature magic, but it is not a necessary condition for the tale and is often merely a means to an ends. What is fairly universal is the inclusion of fallen empires, dying worlds, and the ruins of once great civilizations.
The obsession with fallen empires, dying worlds, and ruins of once great civilizations is one shared with the Sword and Sorcery genre. The dying planet of Barsoom shares a great deal with Robert E. Howard's presentation of Hyperborea. Though one should note that the empires of Sword and Sorcery are dead empires for the reader, they are usually living (though dying) empires for the characters within the tale. In Planetary Romance, the fallen civilizations are often artifacts from a "more noble" time. In Sword and Sorcery, civilization itself must fall as it corrupts the natural man with its decadence. This is one distinction between the genre, the 19th century moral clarity of Planetary Romance is often in direct opposition to the 20th century pessimism (almost nihilism) of Sword and Sorcery fiction.
But it is more than a pessimistic world view that separates the two genres. Sword and Sorcery tales contain within them elements of the Weird Horror tale. When Michael Moorcock, a master of Sword and Sorcery whose Elric character perfectly embodies the Sword and Sorcery obsessions with cultural decadence and Weird Supernatural Horror, describes Conan's relation to his world (and to prior Heroic Fantasy characters) he writes, "If the form of Howard's stories was borrowed at third and fourth hand from Scott and Fenimore Cooper, the supernatural element from Poe and others, the barbarian hero of the Conan stories owed a great deal to Tarzan and other Burroughs primatives. Given to impulsive violent action, sudden rough affection and bouts of melancholy...Conan mistrusted civilization. He was forever at odds both with the respectable world and the occult world; forever detecting plots to seduce him." [emphasis mine]
In Heroic Fantasy magic can be a tool that is neutral in its use. The "Force" has both a light side and a dark side, the telepathy of Martians isn't in itself corrupting. In Sword and Sorcery tales magic is by its nature a corrupting force. Conan fears and opposes magic, even the anti-Conan Elric eschews its use whenever possible and the use of magic rituals often comes with a great cost.
Notice the use of the word "fear" when describing Conan's reaction to magic and the supernatural. Howard's invincible barbarian is sometimes as deathly afraid as the most frail Lovecraftian protagonist when it comes to things that lurk in the spaces between. Though the supernatural beast, "neither a hound nor a baboon," that attacks him in The Phoenix and the Sword "rouse[s] in the Cimmerian a frenzied fury akin to madness," a creature similar to Tsathaggua leaves him "frozen with nauseated horror." What is this creature that so frightens Conan, the man beyond fear? It is an "amorphous bulk...Its unstable outlines somewhat suggested an octopus, but its malformed tentacles were too short for its size, and its substance was a quaking, jelly-like stuff which made him physically sick to look at... among this loathsome gelid mass reared up a frog-like head." The creature is either Shoggoth or Tsathaggua (the fact that the creature's summoner is named Tsotha hints at the second), but it is certainly beyond the abilities of our champion to defeat this "blasphemy agains the eternal laws of nature." This is the kind of creature one would not expect to find in the Planetary Romance fiction of Brackett or Burroughs, but that is perfectly at home in the "dreams" of Lovecraftian horror. Horrific creatures abound in the Conan fiction, and in Sword and Sorcery generally. Fritz Leiber's Lankhmar stories have the "Gods of Lankhmar," Michael Moorcock's Elric tales feature all kinds of Weird Horror from the gods of chaos to much smaller beings.
Planetary Romance is a hopeful fantasy where heroes strive valiantly and where the hero chooses good over evil -- even at personal expense. Sword and Sorcery is a dark and nihilistic genre with a dark view of human nature where the hero often chooses self-interest over the Good. It is his firm command of this single feature distinguishing Sword and Sorcery from other Heroic Fantasy (that of the incorporation of the Weird Horror tale into Heroic Fantasy) that makes Michael Moorcock's anti-Conan stories about the tragic albino Elric so ingenious. Moorcock simultaneously deconstructs the character of Conan while writing a story that embodies the conventions -- even while it expands them -- of the Sword and Sorcery tale.
The first words readers of Howard's Conan read as a description of the archetypal character are, "Hither came Conan, the Cimmerian, black-haired, sullen-eyed, sword in hand, a thief, a reaver, a slayer, with gigantic melancholies and gigantic mirth, to tread the jeweled thrones of the Earth under his sandalled feet."
The first words readers read as a description of Michael Moorcock's Elric are, "His name was Elric of Melnibone king of ruins, lord of a scattered race that had once ruled the ancient world. Elric, sorcerer and swordsman, slayer of kin, despoiler of his homeland, white-faced albino, last of his line."
Both quotes are from the first published stories of the respective character, and both stories take place toward the end of the character's life. It is exquisite the way that Moorcock inverts almost every aspect of the Conan character in the creation of his anti-hero. He inverts every aspect save one, both men are prone to gigantic melancholies. One might think due to the fact that Moorcock's Elric tales are a deconstruction of the Conan character, or possibly an adult version of an adolescent character, that Moorcock would use the deconstruction as an opportunity to attack the genre itself. Moorcock doesn't. He uses it as an opportunity to refine the genre and expand it. By removing the aspects of the genre that are adolescent wish fulfillment and focusing on the central concepts of Sword and Sorcery, Moorcock allows us to see the literary merit of the conventions of the genre free from the constraints of whimsy. The young reader, seeing the power of Conan, might miss the criticisms of society and the dark presentation of human nature. The reader of Elric's stories cannot avoid them for their terror and their beauty. In writing fiction that is a negative image of the original, possibly to criticize the original, Moorcock created a lens that allows readers to more greatly appreciate what Robert E. Howard has done with his Conan tales -- something that the Lin Carter and L Sprague deCamp pastiches missed -- the demonstration of how fiery human nature reacts when faced with supernatural horror. Conan often fights against the darkness, but he often flees as well.
John Carter would never flee from the giant white ape of Barsoom. He might feel some twinge of fear before he grapples with the beast and defeats it. When translucent skinned invaders from Jupiter attack, horrifying visage and all, it is John Carter who flies of to their home world to defeat them -- fearless in the face of the unnatural or the evil. Luke, when captured by Vader in Return of the Jedi, doesn't succumb to despair. Instead he sees "the good" in his father and fights to redeem a lost father. In Planetary Romance Evil can be defeated. In Sword and Sorcery some Evil is best left in the pit where you found it.