The ArcBuilder Universe


The Planetary Classification List


The Planetary Classification List began years ago as an attempt by myself to categorize both the Solar System's and my own fictional worlds into a neat and tidy system of identification.  Ironically, a year or two later the astronomical community began to find the first extrasolar worlds, and it quickly became clear that no planet was going to fit neatly into a pre-set system.  In fact, as time has gone on, our own Solar System has demonstrated this even more.

The PCL has, thus, gone through a number of iterations, sometimes having even been started from scratch due to my own impatience.  Still, it has become clear to me that any sort of attempt at organizing worlds into specific types is inherently flawed.  The Galaxy is so huge, so varied, that we could never possibly hope to pigeonhole these worlds in any but the most general of ways.  And yet I continue on with the PCL; it has become an integral part of my Setting, and one that may eventually become a standalone fictional work.

Please remember, I am trying to maintain a semblance of scientific plausibility, at the least.  But this List should never be taken as a true source for any kind of scientific understanding of the many worlds out there.  in the end, it is a fun little side project that meshes in with the ArcBuilder Universe, and I hope, adds just a bit of depth to it.

Planetesimal Group (mass = 0 to 0.00001xEarth) (radius = 0 to 0.03xEarth)  These are the comets and asteroids, small worlds that are often characterized as the "leftovers" of a solar system.  Many are mere rubble piles held together by weak mutual gravitational attraction, while others are denser, solid worlds.  None, however, are large or massive enough to achieve complete hydrostatic equilibrium, and so never form into a true sphere.

  • Metallic Class  Bodies that are composed largely of nickel-iron and other heavy elements, with a smaller portion of stony silicate material.  Also present may be varying portions of hydrate materials, such as ices or water locked within the molecular structure of the body itself.  These are very common within most solar systems, and often times represent the most resource rich bodies for any industrialized interplanetary society.
  • Silicaceous Class  Bodies that are composed largely of stony materials, and which are more common towards the inner regions of their solar systems.  Hydrate materials are relatively uncommon within these bodies, though not necessarily entirely absent.
  • Carbonaceous Class  Bodies that are composed largely of dark carbonaceous materials, thus rich in organics, with the presence of hydrate materials ranging from ices to water locked within the molecular structure of the body itself.  These are the most common types of asteroids, often representing 75% or more of a solar system's asteroidal population.
  • Cometary Class  Bodies that are composed of some silicate materials, but which are primarily icy bodies.  These are the comets, and while absolutely rich in water ices, they may also contain frozen methane, ammonia, and other types of hydrates.  Rich in organic materials, comets are believed to be primary delivery vehicles for primordial surface water and organics on forming terrestrial worlds.

Dwarf Terrestrial Group  (mass = 0.00001 to 0.1xEarth) (radius = 0.03 to 0.7xEarth)  These are worlds that are far less massive than most terrestrial worlds, but which may become massive enough to reach hydrostatic equilibrium.  They are quite compositionally varied, and can be found in independent orbits about the local luminary, as moons of other planets, or in the midst of asteroid or Kuiper belts.

  • Metallic Class
    • Ferrinian Order  These are worlds that are composed of an extremely high level of metals, with generally relatively thin silicate crusts.  Typically forming in close orbits about F type or greater stars, their composition is indicative of the extreme temperatures of both their surroundings, and their formative histories.  These worlds are quite dense, and can have a substantial surface gravity.  Atmospheres are extremely rare, and are typically only present in the form of material captured from the local stellar wind.
    • Hermian Order  These worlds have a metallic composition of up to 70% or more, with silicate crusts and often large metallic cores.  Generally forming in relatively close stellar orbits, evidence of substantial geological activity can be found, though for the most part the worlds have long since become quiescent.  Atmospheres are relatively rare, often being little more than vanishingly thin layers of captured stellar wind materials.
  • Lithic Class
    • Hephaestian Order  Such worlds, while largely silicaceous in composition, have large liquid metallic cores, and possess an over abundance of volcanism and other geological activities.  In the most extreme cases, these worlds may have surfaces that are entirely molten, though semi-stable surface regions marked with vast volcanic complexes are far more common.  Subsurface oceans of magma are common as well, and the atmospheres range from vanishingly thin structures to thick layers, all formed and constantly replenished by geological activity.  The origin of this geology is typically tidal stressing, as these worlds are most often found as one of several large satellites orbiting Jovian worlds.  However, some are initiated and maintained by close solar proximity as well as tidal flexing.
    • Cerean Order  These are generally smaller, less massive members of the Lithic Class, with large rocky inner cores and geologically inert surfaces.  These worlds often contain abundant hydrates, with some maintaining icy mantels and even sub-mantel oceans of highly saline liquid water.  Cerean worlds can be interpreted as "embryos" to larger terrestrial worlds, but which never developed or were incorporated into the further planet building processes. Their internal structure typically is differentiated, and surface signs of extensive primordial geological activity may be present. Atmospheres are exceedingly rare, and when present are typically vanishingly thin and composed of outgassed internal materials.
    • Vestian Order  While the surface of these worlds often appear dark and cratered, the underlying geology is one of a much more active world, with a differentiated interior and a surface that shows ample signs of basaltic flows and resurfacing.  Such worlds are believed to have been forming as protoplanets, much as the Cerean worlds, but which were never modified by later events.  Indeed, they can be seen as a "missing link" between the Cerean and Selenian type worlds.  Hydrate material is common, often molecularly bound to the world's silicate materials, while subsurface ice or even liquid reservoirs may also be present.  Atmospheres, when present, are extremely thin and typically transitory.
    • Selenian Order  These are low metal, silicate-rich worlds with a molten or semi-molten core.  While the worlds will have been quite active during their youth, modern geological activity has all but ceased, and is typically marked by rare and sporadic outgassing events of near surface volatiles, often caused by tidal stressing or meteoric impacts.  Atmospheres are typically absent, though extremely thin ones may form and be replenished by either episodes of internal outgassing, or captured material from the solar wind.
    • Europan Order  These worlds are largely rocky in nature, often with an iron-nickel core, while their surfaces and even upper mantel regions are composed almost exclusively of ices.  However, what sets these worlds apart are their subsurface oceans.  Formed and maintained by both internal and tidal heating, these oceans can range from patchy ice and rock "slush" to vast reservoirs girdling the entire body and reaching hundreds of kilometers deep.  The surfaces of the worlds can be thin crusts of ice, marked with the patterns of oceanic ice flows, or they can be thick and cratered, with little indication of the subsurface seas.  All such worlds will have some form of surface geology, however, driven by both tidal heating and oceanic outgassing.  If the conditions remain suitable during the world's lifetime, native biomes can form, and will range from simple microbial extremophile communities to mature macro-biological ecosystems.
    • Titanian Order  Generally relatively massive worlds for this Class, Titanian worlds possess iron-nickel cores and rocky mantels, both of which make up the bulk of the body's composition.  The upper mantel and crust, however, are water ice, and are at temperatures to where they play the role that lithic materials do on warmer terrestrial worlds.  While the internal structures may vary considerably, most such worlds have subsurface oceans of highly saline liquid water, much like Europans, upon which the icy crust "floats".  Movement over this ocean can be the driving force for surface geology, forming mountains and deep basins.  However, the defining characteristic of these worlds are their thick atmospheres and hydrate cycle of liquid methane and ethane.  While some worlds might only have seasonal liquid bodies of these compounds, others will have globe-spanning oceans and full hydrological cycles based upon them.  Indeed, these worlds can be virtual "cold" analogues to Gaian worlds.  Native biomes may also develop, as the surface is typically rich in prebiotic organic chemistry.  Biomes may also form, as they do on Europan worlds, in the subsurface oceans.  For both regions, the biomes can range from simple microbiological communities to fully mature ecologies.
    • Callistian Order  Internally differentiated worlds, with large amounts of both silicate and icy materials making up their compositions.  The silicates are generally confined to the mantels and core regions, along with any metallic materials, while the crusts are generally exclusively various ices.  These worlds have remained relatively unchanged since the finalization of their formation, and are found beyond the snow line of their solar systems.  The surfaces are often heavily cratered, with occasional indications of ancient geological activity.  However, by and large, they are truly dead worlds, with any atmosphere that they might possess both vanishingly thing and likely transitory.
  • Gelidian Class
    • Dionean Order  These worlds are typically composed of two thirds ices, with the rest being confined to a silicate lower mantel and core.  These are the rockiest of the Gelidian Class, and often times show signs of geological activity, usually dating back to the world's youth.  Residual geological activity is often present, however, largely due to tidal stressing.
    • Tethyian Order  These worlds are geologically inert and composed almost entirely of ices, though often there is a small silicate core.  In their youth most of these worlds possessed a subsurface ocean of liquid water, but as they aged and cooled the oceans solidified.  As a result of the expansion of these buried ices, the surfaces are often marked with large geologically distinct terrain, ranging from massive scarps and chaotic terrain to deep rift valleys.  However, these are relics, and the only modern surface activity is created by meteoric impacts.
    • Kuiperian Order  As the name suggests, these are worlds that lie in the furthest regions of their solar systems, the local Kuiper Belts.  Sizes and masses may vary considerably, as can the geological history and present of the sub surface and surface.  Surprisingly, many such worlds, despite their size and low silicate contents, can have quite active geologies, often spurred on my orbital resonances with other worlds, and tidal stresses with their own satellites.  Some of these worlds will occasionally move into closer solar orbits, where they can transit to other world classifications in relatively short periods of time.

Terrestrial Group (mass = 0.1 to 2.0xEarth) (radius = 0.5 to 1.9xEarth)

  • Iron Class
  • Silicate Class
    • Arean Order  Perhaps closest to the Gaian worlds, these are nonetheless bodies that have actively hostile surfaces, barren of any liquid water, although surface and subsurface ice deposits might be present.  Geological activity is either present or at a much diminished state, and often the atmosphere has become a shadow of its formal self.  The driving force behind the inhospitability of these worlds is the lack of an active magnetic field, without which the atmosphere can be eroded by the stellar primary, to the point where all surface liquid will vanish.  biological activity is still often present, however, although typically this is restricted to microbiological extremophiles in subsurface environments.
    • Cytherean Order  While geologically active, with active cores of semi-solid nickel iron, the lack of a hydrosphere as a conducting medium lends itself to creating a world truly hellish in nature.  The atmosphere is often crushingly thick, the result of constant volcanic outgassing, but with no tectonics or other medium with which to reabsorb atmospheric materials back into the crust.  They are dominated by massive runaway greenhouse effects, and can possess surface morphologies and compositional materials quite unlike anything else.  Biological activity on such worlds is almost always limited to upper atmospheric microbial forms, when present at all.
    • Gaian Order  These are, as far as Humanity is concerned, the jewels of the planetary family.  These planets are unique in that they are able to support fully mature biomes, while at the same time these biomes help shape the planets themselves.  The greatest defining marker for Gaian worlds is the presence of liquid water on the surface, be it in the form of small seas or globe-spanning oceans.  A perfect combination of geological activity and formative processes leads to the presence of free standing water, within which life forms will swiftly develop.  And yet these worlds are transitory, having a full habitability rating for less than two billion years, on average.
      • EuGaian Type  These are the archetypical Gaian worlds, with a watery environment that covers up to 80% of the surface.  Fully mature biomes are the norm, and while the surface conditions may vary according to continental configuration, in general the surface temperatures are quite warm.
      • PelagiGaian Type  These are Gaian worlds with relatively small continental plates, and a globe-spanning ocean.  Terrestrial life forms may be somewhat restricted, especially if the continental plates, small as they are, are often flooded or subducted.  however, oceanic biomes are fully mature and incredibly varied.  The atmosphere is quite dynamic, and the average surface temperatures are quite warm due to the effective heat convection provided by the oceans.
      • BathyGaian Type  Like their Pelagic cousins, these Gaian worlds are covered by a global ocean.  However, there is a lack of tectonic plates, and the only terrestrial regions are transient volcanic islands.  Oceanic biomes are very extensive, and some forms may adapt to take advantage of the short lived volcanic islands, but there will not be any advanced true terrestrial forms.
      • ChloroGaian Type  These are exceedingly rare worlds with a high percentage of hydrogen chloride and liquid water.  The resulting biosphere has come to utilize chlorine as a base component in their metabolisms.  Chlorine, being quite unstable, can only exist when the worlds in question are specific distances from a stable sun, allowing the biosphere to form around the compounds over evolutionary time scales.  While the atmospheres are still predominantly nitrogen and oxygen, chlorine content can still exceed 5%, making the planets quite unsuitable for Humans.
      • AmmuGaian Type  Ammonia based Gaian worlds, where the global solvent is a rich ammonia-water mix.  Such worlds are located at further distances from the central luminary, where ammonia will neither freeze nor sublimate.  However, the hydrological setting of these worlds is much more viscous than it is on standard Gaian worlds, and because of the lower temperatures much of the biology is often just as lethargic.
  • Carbon  Class
    • Carbide Order  These worlds form in solar systems with high amounts of carbon and low amounts of oxygen.  The interior is differentiated, with an iron-nickel core, but it may also contain layers of molten silicon carbides and titanium carbides.  Above that is a layer of graphite, with a possible layer of kilometers thick diamond strata, if the pressure is high enough.  The surface, which is typically dark with organic carbon compounds, is composed of frozen or liquid hydrocarbons.  Weather systems are dynamic on these worlds, although the nature of these will vary depending on the thickness of the atmospheres.  Water in any form, however, is entirely absent.  There are possibilities of biological activity on these worlds, although as with any planet the occurrence and complexity of these forms depends on an entire host of circumstances.
  • Ice Class

Super Terrestrial Group (mass = 2.0 to 10xEarth) (radius = 1.3 to 3.3xEarth)

SubJovian Group (mass = 10 to 50xEarth) (radius = 2.1 to 5.7xEarth)

EuJovian Group (mass = 50 to 50,000xEarth) (radius = 3.5 to 27xEarth)


The ArcBuilder Universe is a science fiction project established, authored, and copyrighted ©
by John M. Dollan 2002-2010
This page first uploaded in July 27, 2004
Most recent update for this page January 17, 2016