Planetary romance is a subgenre of science fiction in which the bulk of the action consists of adventures on one or more exotic alien planets, characterized by distinctive physical and cultural backgrounds. Some planetary romances take place against the background of a future culture where travel between worlds by spaceship is commonplace; others, particularly the earliest examples of the genre, do not, and invoke flying carpets, astral projection, or other methods of getting between planets. In either case, it is the planetside adventures which are the focus of the story, not the mode of travel. The Encyclopedia of Science Fiction mentions two caveats as to the usage of the term. First, while the setting may be in an alien world, if "the nature or description of this world has little bearing on the story being told," as in A Case of Conscience, then the book is not a planetary romance. Second, hard science fiction tales are excluded from this category, where an alien planet, while being a critical component of the plot, is just a background for a primarily scientific endeavor, such as Hal Clement's Mission of Gravity, possibly with embellishments. A significant precursor of the genre is Edwin L. Arnold's Lieut. Gullivar Jones: His Vacation (1905). In Science Fiction: The 100 Best Novels (1985), editor and critic David Pringle named Marion Zimmer Bradley and Anne McCaffrey two "leading practitioners nowadays" for the planetary romance type of science fiction. There is a significant overlap of the genre with that of sword and planet. Examples In fiction Almuric by Robert E. Howard Arrakis (in the Dune series) by Frank Herbert Barsoom (Mars) and Amtor (Venus) by Edgar Rice Burroughs Callisto by Lin Carter Darkover by Marion Zimmer Bradley Eldorado by Francis Carsac Several works by Alan Dean Foster: Midworld, Sentenced to Prism, Voyage to the City of the Dead, Drowning World, and most of the Flinx and Pip stories Gor by John Norman Hainish Cycle by Ursula K. Le Guin The Books of the Raksura by Martha Wells Helliconia by Brian Aldiss Jack of Shadows by Roger Zelazny Kregen by Kenneth Bulmer Krishna by L. Sprague de Camp Majipoor by Robert Silverberg Pern by Anne McCaffrey The Radio Man by Ralph Milne Farley Riverworld, The Green Odyssey and World of Tiers by Philip José Farmer The Saga of the Skolian Empire by Catherine Asaro, including the worlds of Raylicon, Balimul, Parthonia,