There are numerous ways in science fiction of classifying planets, though in reality there’s never a clear break point planets of one type or another (or things which aren’t event planets – see Pluto at one end, or brown dwarfs at the other). Classifying them though makes it easier to perform the type of top-down generation that WorldGen uses. What this means is, the type of world is determined first, then the details are worked out.
The classification system used by WorldGen is based on the Planetary Classification List (PCL) that was created by John Dollan and others, which I first encountered and started using with permission back in 2006. Most of the links I had to it (and the Arc Builder universe it was part of) are now vanished (the seemingly dead ArcBuilder forum is all I can kind), but there is still an Eclipse Phase resource that uses it, licensed under CC-BY-SA by James Mephit, and this has been my main reference for the latest version of WorldGen.
The PCL organises planets into Groups (at the top level), then Classes and finally Types. Each type in WorldGen has its own generator for both physical characteristics and surface map, everything higher is mostly just for organisation purposes. The Classes are almost never used except in documentation.
It isn’t perfect, and there are some things I disagree with, but I’ve found it to be a really useful starting point to build around, and there’s enough detail to make me think about the different variety of worlds that are possible (rather than just a simple Gas World, Water World, Garden World, Rock World etc).
What follows below is a summary of the different Groups. A list of all the types can be found at my WorldGen site, or more completely in the document above.
The Belt Group is my own addition to the classification scheme, and isn’t really a type of planet. I needed it though because of the way the WorldGen software works – in the OO / Database schema things are either Stars or Planets. An asteroid belt takes up an orbit around a star, so it is considered to be a Planet, and needs to have a type of some sort to denote it as being an asteroid belt.
Belts are either of the Circumstellar Class, in which case they orbit a star, of the Planetary Ring class, in which case they orbit a planet (and are considered ‘rings’).
There are multiple types of Circumstellar Class Belts, the most common of which are either Asteroid Belts or Ice belts. Dust Discs, Oort Clouds and other types are also possible.
A belt is considered to contain countless (billions, trillions) of individual objects, though the bulk of these will be abstracted away. Certain notable objects (largest, or most interesting) will be treated as individual planetoids which will be Planets in their own right. Another oddity due to how the schema relationships work, these planetoids are internally considered ‘moons’ of the belt.
Mapping of these is currently done in a completely different way to all other objects. Rather than creating a texture and wrapping it around a sphere, they are drawn as a 2D ring. Asteroid belts are randomly generated using a fixed seed to plot the current position of a few thousand major objects (so the appearance of the belt will vary over time, as inner objects orbit faster than outer objects). Dust belts and rings are simply drawn as density rings.
Small Body Group
If a Belt represents an entire field of asteroids, the individual rocks within it are normally of the Small Body Group. There are three classes within this group – Vulcanoidal, Asteroidal and Cometary.
In reality, most objects of this Group will be a few kilometres across, but anything that is actually represented as an individual object in WorldGen will be an exceptional member of the Group, up to several hundred kilometres across.
Members of this Group introduce their own complexity for WorldGen, since they are rarely spherical which makes mapping them hard. What I do here is produce a low resolution texture, which is used as a deformation map on a spherical shape. It’s not perfect, but it gets the idea of their irregularity across.
Again, internally these are considered to be Planets, because everything is either a Planet or a Star.
Dwarf Terrestrial Group
These are the first of the real planets, and includes smaller worlds like Mercury and Mars, but also many of the moons of worlds like Jupiter and Saturn. Anything that is from a thousand kilometres across and spherical can be considered a Dwarf Terrestrial world. There is one exception – Vestian type worlds also use a deformation map like those of the Small Body Group.
This group is also where things start getting varied, and there are five different Classes within it – Proto Thermic, Geo Passive, Geo Thermic, Geo Tidal and Geo Cyclic.
The interesting group is the Geo Cyclic group, which includes Mars-like Arean type worlds. The PCL defines these as worlds going through cycles of wet and dry, but this is something that doesn’t actually make sense to me. Mars started warm and wet, and cooled and dried over time – there’s nothing that I’m aware of that will reverse that process.
Note that worlds like Pluto fall into the Dwarf Terrestrial Group.
The Terrestrial Group covers Earth-like worlds, and includes a wide range of world types, all of which are quite complicated since they either harbour life, or could harbour life.
This is where it’s obvious that the classification scheme is interested in more than just the astronomical properties of a world, since Class and Type can vary based on the world’s ecology, allowing a world to change classification over time.
Indeed, some worlds follow an evolutionary path – EoGaian to MesoGaian to EuGaian to Post-Gelidian as the biology on the world changes over time.
There are multiple classes in this Group, which are briefly described below:
- ProtoActive Class – Terrestrial worlds still in the process of forming, split by physical composition.
- Epistellar Class – Tidally locked to their star.
- Telluric Class – Hot worlds with dense atmospheres (Venus would be a Cytherean type world in this class).
- Arid Class – Very dry worlds with a limited hydrological cycle. They may have limited life.
- Tectonic Class – Enough water for there to be oceans and continents, often with life. Earth is a EuGaian type world in this class.
- Oceanic Class – Worlds completely covered in thick oceans tens of kilometres deep. A simple ‘water world’ with a few kilometres of ocean would be in the Tectonic class.
The Telluric Class alone has over a dozen types and sub-types, but given that these are the most interesting worlds since they are the most Earth-like, this isn’t too surprising.
Large rocky worlds with more than 5 times the mass of the Earth are part of the Helian Group. They tend to have thick atmospheres, and are unlikely to have complex life, though Panthalassic world types, with thick atmospheres, may well have the available energy and environment to allow life to evolve in their ocean.
The Group is divided into the three classes of Geo Helian, Nebulous and Panthalassic.
The Jovian Group comprises what are often called Gas Giants – Jupiter, Saturn, Uranus and Neptune. They are sub-divided into Classes mostly according to their mass – Sub Jovian, Dwarf Jovian, Meso Jovian, and Super Jovian. There is also the Chthonian Class for worlds which have lost their atmosphere.
Such worlds will often have a lot of moons, though WorldGen tends to limit generation to just the largest of these – maybe a dozen at most. They also have rings, considered to be planets of the Planetary Ring class mentioned above under the Belt Group.
The ‘maps’ for these worlds are for the upper cloud layers, not for the surface itself. Since it’s so rare for anyone to go too far beneath the clouds, this is probably good enough, though it does mean the clouds are static.
There is also the Sudarsky classification of Gas Giants, which I will probably draw upon when detailing these worlds.
Worlds within the Planemo Group are classified by what they lack rather than what they have – and in this case what they lack is a parent star. They are rogue planets, and since this is their only defining characteristic, they can have a wide range of physical properties.
If they were within a star system, they would be classed as anything from Dwarf Terrestrial up to Jovian worlds.
In terms of generating them, they will be quite common, though generally hidden from the main map. Most will be uninhabited, though they’d make for useful outlaw bases or remote navy depots. Though it hasn’t been implemented yet, if a system hex is empty then there will be a second check for a system (at the same chance), and if that is successful then there will be a rogue planet. For a standard density sector of space (30% chance of a system per hex), 30% of ’empty’ hexes will have a rogue planet.
The Construct Group is another group that I’ve added myself, to cover all types of artificial worlds, from ring worlds to Dyson swarms. This was originally designed to cover really big objects, though I may decide for it to also cover smaller artificial worlds such as bubble worlds – melted and inflated asteroids maybe dozens of kilometres across, or artificial rings of orbital habitats around a planet.
Currently this Group is simply a placeholder, and there hasn’t been a great deal of thought put into it beyond realising that I needed it. How I would map them, I currently have no idea.