Any
science fiction story or
novel whose primary venue is a
planet, and whose plot turns to a significant degree upon the nature of that venue, can be described as a
planetary romance. Think of those
SF novels where the
setting is almost as important as the characters or plot.
A famous example of a
planetary romance is the
Dune sequence by
Frank Herbert.
Jack Vance is a master of the sub-genre, writing dozens of novels set in numerous worlds. Other famous examples are
Marion Zimmer Bradley's
Darkover novels and
Anne McCaffrey's
Pern novels.
Part of the definition of this term comes from "The Encyclopedia of Science Fiction", ed. John Clute.