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"I never get tired of looking at that when I come home."
-- frequent sentiment of many individuals returning to a home planet
A "planet" in the Spelljammer setting is a large celestial body that (generally) fits all of the following criteria, though there are exceptions:
Is large enough to possess an independent gravity field (standard rules do not apply; the gravity fields of stellar objects are almost always extremely small for the size of the object)
Is not a strange object that forms a tail when its extremely oval orbit brings it close to a primary (that is, not a comet)
A planet usually meet the following secondary criteria, as well:
Orbits a primary in such a manner that the orbit is circular, or nearly so
Is shaped like a sphere, a cube, or other recognizable shape, including irregular (as opposed to dust clouds, nebulae, and visible sargassos that frequently have no recognizable boundaries, or at least, no permanent boundaries)
Is at least 500 miles in diameter; smaller objects are usually called asteroids (but smaller objects might be called planets, according to local traditions, and this criterion definitely does not apply to cluster worlds)
Is not located inside an asteroid belt (exceptions exist; these are almost always dwarf planets)
As noted above, cluster worlds are planets. Although the individual bodies of the cluster are typically far too small to be planets, the entire cluster qualifies as a planet
Typical Qualities Edit
Most planets have atmospheres, but some are void worlds, with no air of any kind. The air is usually breathable, but again, not always
Some planets have smaller worlds that orbit them, called moons
Some planets also have rings around them
In rare cases, a moon may have a moon or rings, or even both
Let the space traveler beware Edit
Some cultures may call something a planet that, objectively, is most certainly not; it is usually best to respect, or at least ignore and refrain from commenting upon, local customs!
Physical description of what a planet is: All worlds described in the Lorebook of the Void fit the rough criteria given above
Atmosphere references: Concordance of Arcane Space
Example of a planet being the primary of a crystal shell: Lorebook of the Void, Greyspace
The large asteroid Ceres in Greyspace is not listed in Greyspace as a planet