Planet Descriptors Legend

Planet groups dictate broadly speaking what kind of world inhabits an orbit.

Asteroid Belt

Dwarf Planet
Are celestial bodies in direct orbit of a star that are massive enough that their shape is controlled by gravitational rather than mechanical forces (and thus an ellipsoid in shape), but has not has not become gravitationally dominant.

Terrestrial Planet
Planets that are similar to Earth, with bodies largely composed of rock.

Helian Planet
Planets that are typically larger than terrestrial planets.

Jovian Planet
Planets largely composed of gaseous material and significantly more massive than terrestrials.

Planet types give a general idea of the profile of the planet.

Terrestrial Group, Telluric Class, Acheronian Type.
These are worlds that were directly affected by their primary’s transition from the main sequence; the atmosphere and oceans have been boiled away, leaving a scorched, dead planet.

Dwarf Group, GeoCyclic Class, Arean / Utgardian / Titanian Types.
These are worlds with little liquid, that move through a slow geological cycle of a gradual build-up, a short wet and clement period, and a long decline.

Terrestrial Group, Arid Class, Darwinian / Saganian / Asimovian Types.
These are worlds with limited amounts of surface liquid, that maintain an equilibrium with the help of their tectonic activity and their biosphere.

Helian Group, GeoHelian Class, Asphodelian Type.
These are worlds that were directly affected by their primary’s transition from the main sequence; their atmosphere has been boiled away, leaving the surface exposed.

Jovian Group, Chthonian Class.
These are worlds that were directly affected by their primary’s transition from the main sequence, or that have simply spent too long in a tight epistellar orbit; their atmospheres have been stripped away.

Dwarf Group, GeoTidal Class, Hebean / Idunnian Types.
These are highly active worlds, due to tidal flexing, but with some regions of stability; the larger ones may be able to maintain some atmosphere and surface liquid.

Helian Group, GeoHelian / Nebulous Classes.
These are typical helian or “subgiant” worlds – large enough to retain helium atmospheres.

Terrestrial Group, Epistellar Class, JaniLithic Type.
These worlds, tide-locked to the primary, are rocky, dry, and geologically active.

Jovian (Gas giants)
Jovian Group. If life-bearing, is most likely DwarfJovian Class, Brammain / Khonsonian Types.
These are huge worlds with helium-hydrogen envelopes and compressed cores; the largest emit more heat than they absorb.

Dwarf Group, GeoThermic Class, Phaethonic / Apollonian / Sethian Types, or GeoTidal Class, Hephaestian / Lokian Types.
These are dwarfs with molten or semi-molten surfaces, either from extreme tidal flexing, or extreme approach to a star.

Terrestrial Group, Oceanic Class, Pelagic / Nunnic / Teathic Types, or Tectonic Class, BathyGaian / BathyAmunian / BathyTartarian Types.
These are worlds with a continuous hydrological cycle and deep oceans, due to either dense greenhouse atmosphere or active plate tectonics.

Helian Group, Panthalassic Class.
These are massive worlds, aborted gas giants, largely composed of water and hydrogen.

Dwarf Group, GeoTidal Class, Promethean / Burian / Atlan Types.
These are worlds that, through tidal-flexing, have a geological cycle similar to plate tectonics, that supports surface liquid and atmosphere.

Dwarf Group, GeoPassive Class, Ferrinian / Lithic / Carbonian Types.
These are mostly dormant worlds, with surfaces largely unchanged since the early period of planetary formation.

Small Body
Small Body Group.
These are bodies too small to sustain hydrostatic equilibrium; nearly all asteroids and comets are small bodies.

Dwarf Group, GeoPassive Class, Gelidian Type, or GeoThermic Class, Erisian Type, or GeoTidal Class, Plutonian Type.
These worlds are composed of mostly ice and some rock. They may have varying degrees of activity, ranging from completely cold and still to cryo-volcanically active with extensive subsurface oceans.

Dwarf Group, GeoPassive Class, Stygian Type.
These are worlds that were directly affected by their primary’s transition from the main sequence; they are melted and blasted lumps.

Terrestrial Group, Tectonic Class, Gaian / Amunian / Tartarian Types.
These are worlds with active plate tectonics and large bodies of surface liquid, allowing for stable atmospheres and a high likelihood of life.

Terrestrial Group, Telluric Class, Phosphorian / Cytherean Types.
These are worlds with geoactivity but no hydrological cycle at all, leading to dense runaway-greenhouse atmospheres.

Dwarf Group, Epistellar Class, Vesperian Type.
These worlds are tide-locked to their primary, but at a distance that permits surface liquid and the development of life.

System generation rules, planet types and names taken from:

Planet Descriptors Legend

Traveller Shakey