"Planetary fantasy" is a complicated genre. It doesn't have strict boundaries, but the common theme is that these stories take place on planets other than Earth. One of my favourites, The Worm Ouroboros, takes place on Mercury; two very popular planetary fantasies, Out of the Silent Planet and Perelandra, take place on Mars and Venus. Naturally, you cannot actually live on these planets, and that is why these planetary fictions are fantasies: they build worlds and surroundings on these planets which are, of course, not supportable.
A planetary fantasy can include anything from Jules Verne to Tolkien's Roverandom, which are a good picture of the ends of the spectrum from science fiction to totally whimsical. In between, you may have works like C.S. Lewis' Space Trilogy (which is part science fiction, but also heavily philosophical and anthropological) and The Worm Ouroboros, which is very fantastic and has strong overtones of the mythological. Plenilune would fall between the Space Trilogy and The Worm Ouroboros in its tone: it has the casual life of The Worm Ouroboros, but also the philosophical, anthropological aspects of Lewis' works. Naturally, it is all my own creation, but if you wanted to know where it would lie in the spectrum, about there is about right.
essentially, planetary fantasy is what it says on the tin