Here is a Planescape-based theory of world types, expanding on the "elemental" theory in the SJ boxed set. Most planets and other celestial bodies are either creatures (constellates, starbeasts, etc. Liveworlds too perhaps) or elemental pockets. Worlds for the 18 inner planes are described here:
| Earth | An earth world is your basic planet or moon. Made of rock or earth, it will tend to be any size from pebble-sized up to about three times earth sized, and may have an atmosphere and/or support life. |
| Ooze | Ooze worlds are enormous heaps of slimy mud floating thru' the void. Inhabited mainly by ooze mephits and used as dumping grounds by spacefarers, they can develop into liveworlds if they support enough swamp-loving plant life. |
| Water | A water world is a globe of pure (ish) water in space. In spheres with sub-zero wildspaces, all water worlds will be near to hot bodies or have atmospheres. They support elemental water life, fish, and possibly floating islands. Popular as bases for pirates, who can moor large numbers of ships to floating stages on the surface. |
| Ice | Also called coldworlds, ice planets resemble water worlds but are frozen. They are found almost exclusively in spheres with subzero wildspace, and support ice mephits and cold-loving creatures like winter wolves and frost giants. Some have molten cores and are unstable, behaving like endless glaciers. |
| Air | Air worlds are pretty much as described in the SJ boxed set. Note however that air worlds have little or no prevailing gravity, and if they contain earth islands, these will support their own gravity planes and wells like normal asteroids do. Dragons love air worlds, as do elemental air creatures, aarakocra, avariel and tempests. |
| Smoke | Smoke worlds are rare, and tend to be barren and inhospitable. Gravitational conditions resemble those on air worlds, but the atmosphere is considered toxic. Only tempests and smoke mephits like it here, and perhaps a few antisocial dragons. |
| Fire | Fire worlds are usually stars of red, orange or yellow colours, tending to have a radiance component as they grow more yellow. For white stars, see radiance and positive, and for brown stars, see magma. Fire worlds are inhabited mainly by elemental fire creatures, as even red dragons find these worlds unpleasant for any great length of time. |
| Magma | Magma worlds vary in nature according to size. A small magma world will be a volcanic moon or planet (Io, the moon of Jupiter, is a good example), and will be inhabited by magma mephits and magmen. A larger one will more closely resemble a brown dwarf substellar object, or brown star. Light given out is minimal, but surface temperatures are in the region of 1000 degrees celsius, and heat-loving plants may inhabit the moons of such a body. |
| Positive Energy | Positive worlds are wite, blue and green stars. They are incredibly hot and bright, and tend to promote desert-like conditions on the surrounding planets. Approach with extreme caution. Uninhabitable. |
| Mineral | Mineral worlds are rare, but tend to be ore- and gem-rich earth-type worlds. They seldom support life, due to the purity and harshness of the local materials. Inhabitants may include gem dragons, mineral quasi-elementals, and the large number of quasi-elemental mineral creatures in a Dragon magazine whose number I forget that talked about ioun stones (which are found native occasionally on these worlds). |
| Mist | Mist worlds are like air worlds, but a lot damper. Inhabitants will be similar, with plenty of mist dragons and mist and steam mephits. |
| Radiance | Radiant worlds are cold, bright stars, like White Dwarf objects. Quasars may also (DM's option) be radiant worlds. Practically uninhabited, apart from by blind dragons and radiant mephits. |
| Lightning | Lightning worlds are unstable, and can only exist in the presence of mist or smoke worlds. Such agglomerations are called nebulae, and are dangerous to enter. |
| Ash | Ash worlds are warm, barren earth worlds, and support only dust mephits and a few other bizarre creatures. |
| Salt | Salt worlds superficially resemble mineral worlds, but tned to have moist, salty atmospheres. They support only salt mephits and are no use unless you need dirty salt. |
| Dust | Dust worlds are constantly shifting barren desert worlds. They have no stable surface, and landing on one is just asking to see your ship vanish in a dust storm. Dust mephits and a few lonely earth creatures hang out here. |
| Vacuum | Vacuum worlds are only theoretical objects, as they are usually indistinguishable from wildspace, and if they come into contact with other worlds, they are destroyed. Occasionally a vacuum pocket appears in an earth, salt or mineral world, where the local matter is not pliable enough to fill it. |
| Negative Energy | Negative worlds include black holes, which are obviously best left alone, and sargassos, described in Lost Ships. |
