As far as I know, that kind of list is only found in Star Trek EU works, specifically the Star Charts, which shows a planetary classification from Class A to Class Y with some useful information.

It might be too coarse for you needs (as in "not detailed enough"), but you could use the Universal Planetary Profile from the RPG Traveller. (In the modern version of the game, it's called the Universal World Profile and is far more comprehensive than in the original game that I'm most familiar with.)

The format describes:

Starport (Sp)

Planetary Size (S)

Atmosphere (A)

Hydrosphere {H)

Population (P)

Government (G)

Law Level (L)

Tech Level (TL)

I'd personally as a reader want to be able to differentiate between planets easier than that, and an easier way to do that might be to use a taxonomic system for classification.

Instead of having so much variation like animals, you can have a class A-B-C planet meaning different items, with further breakdowns in classification beneath that. Just like using the word "plant" versus "flower", we can use these indicators to provide ever increasing levels of specificity. It doesn't need to be anything completely complicated, but provide the full 5 bits, putting emphasis on certain pieces over others based on how important someone would use them in conversation.

First letter determines size and type.

• A: Gas planet, strong gravitational pull, tidally locked


• B: Gas planet, normal gravitational pull, tidally locked


• C: Gas planet, weak gravitational pull, tidally locked


• D: Gas planet, strong gravitational pull, tidally unlocked


• E: Gas planet, normal gravitational pull, tidally unlocked


• F: Gas planet, weak gravitational pull, tidally unlocked


• G: Rocky planet, strong gravitational pull, tidally locked


• H: Rocky planet, normal gravitational pull, tidally locked


• I: Rocky planet, weak gravitational pull, tidally locked


• J: Rocky planet, strong gravitational pull, tidally unlocked


• K: Rocky planet, normal gravitational pull, tidally unlocked


• L: Rocky planet, weak gravitational pull, tidally unlocked


• etc.

Then further elaborate into atmosphere type and hostility of location.

• A: Weak atmosphere, close to star, hot


• B: Weak atmosphere, close to star, temperate


• C: Weak atmosphere, close to star, cold


• D: Normal atmosphere, close to star, hot


• E: Normal atmosphere, close to star, temperate


• F: Normal atmosphere, close to star, cold


• G: Strong atmosphere, close to star, hot


• H: Strong atmosphere, close to star, temperate


• I: Strong atmosphere, close to star, cold


• J: Weak atmosphere, Goldilock's Zone, hot


• etc.

After that, you can get into habitability, resource availability, breathing possibility, and population with just numbers.

• 01: Not habitable, scant resources, breathing apparatus necessary, not populated


• 02: Habitable, scant resources, breathing apparatus necessary, not populated


• 03: Not habitable, normal resources, breathing apparatus necessary, not populated


• 04: Habitable, normal resources, breathing apparatus necessary, not populated


• 05: Not habitable, heavy resources, breathing apparatus necessary, not populated


• 06: Habitable, heavy resources, breathing apparatus necessary, not populated


• 07: Not habitable, scant resources, breathing apparatus not necessary, not populated


• 08: Habitable, scant resources, breathing apparatus not necessary, not populated


• etc.

Forming from this, you could classify any type of planet, and instantly know what sort of place you're visiting. For instance, Earth might be classified as a K class planet, but with further information on it bearing more fruit.

Earth: K-P17 planet

Population: 7.2b

Crust Compostion¹:

- oxygen, 46.6%

- silicon, 27.7%

- aluminum, 8.1%

- iron, 5.0%

- calcium, 3.6%

- sodium, 2.8%

- potassium, 2.6%

- magnesium, 2.1%

Intelligent Species Types: Humans, Dolphins, Mice

Prevalent Languages:

- Mandarin Chinese (1.1 billion speakers)

- English (983 million speakers)

- Hindustani (544 million speakers)

With this, you have a complete knowledge of a planet, and can further fill in more information on a longer format, but can refer to Earth as a K class planet, then further specify with more taxonomic indicators.

¹ - "Essentials of Geology" (7th Ed., Prentice Hall, 2000) by Frederick K. Lutgens and Edward J. Tarbuck

Weighted k-medoids clustering

k-medoids clustering is a method of dividing a set of points into clusters of similar points. You can use this to divide your worlds into clusters, and then give each cluster a label.

One algorithm to do this is the PAM algorithm:

1. Give each world a weight (representing how likely it is to come up in conversation). Also define a "dissimilarity" function that measures how dissimilar two worlds are. This should make the set of worlds a metric space.


2. Give each label a "metroid" world. This can either be done arbitrarily, or meaningfully (the meaning of the labels will be loosely based on these initial metroid worlds). The metroid world does not need to have a high weight; it could even have weight $0$.


3. Assign each world the label whose metroid is most similar to it.


4. Calculate the weighted average cost of the labelling, which is defined as the weighted average dissimilarity between a world and the world's label's metroid.


5. For each label $l$ and each non-metroid world $w$, change $l$'s metroid to $w$. Then do steps 3 and 4 again. If the weighted average cost went up, undo the change.


6. Keep doing step 5 until the weighted average cost stops decreasing.


7. The current labelling is the output of this algorithm. You may want to rename the labels if you want them to have meaningful names, as the algorithm will probably have relabelled worlds multiple times until it found a local minima.

Check the wiki for planet types.

There are eight types by size, seventeen by orbital regime, seventeen by composition and three miscellaneous types. That does for 6936 possible combinations, so if you wish for one letter descriptors you may need to use chinese characters.

Otherwise, you can use any fictional classifications such as Star Trek's, as long as you don't do it commercially I guess.

I don't think there is a good way to simply classify planets with the granularity you want without creating a barrier between you the creator and the audience you are presenting to.

For your system to work, the user needs to know and remember it. This is hard. You can't just present it in a table (I'm assuming your users are both gamers and readers) in the appendix or index because a user needs to keep flipping back to it. This breaks the flow of your story or game (sort of like tutorial prompts) and creates a disconnect for casual or first time users who might not want to invest in learning everything and just dive straight in (this is one of the reasons why casual and mobile games are such a huge market compared to knowledge intensive RTS games).

A very basic system would be something along the lines of:

• Planet/Asteroid/Star


• Livable/Unlivable

Which can give you the following combinations:

• Livable Planet


• Livable Asteroid


• Livable Star

Which you could then sort into a code that you want e.g. (LP, LA, LS).

But a casual user doesn't understand this, especially if you want to factor in all possible factors of a planet. The only thing you are doing with such a code is making it harder for a user to understand your game.

If you really want to... Unicode has 137,374 mappings which means you could, if you really really wanted to, present 137,374 unique combinations to the reader with a single character (some of them look exactly the same, but they are different). But at that point you might as well be using Webdings.

These seemed like an interesting challenge, and while some simplicity is possible, you cannot fold in the complexity and intelligence of life into it without making it overly complex.

The detailed way of identifying a planet or moon needs too many values:

• mass and surface gravity, surface composition


• biosphere (mass, area, complexity, intelligence)


• atmosphere (thickness, composition, surface pressure)


• temperature (surface mean, range)


• orbital dynamics (zone, eccentricity, length of year)


• rotational dynamics (length of day, axial tilt)


• solar dynamics (mass, sequence, brightness, spectral class, wind output, stability, ttl)


• geomagnetism, geological stability, and more...

This is just too much data. But what if we limited our criteria to ranges of conditions favorable to human life? I used a 3-dimensional 3-deep grid (think rubics cube) to break down a planet to human habitability. The 3 dimensions and their depths:

Surface Temperature (T):

• T1 = below 0C mean, 0C max over 90% of surface, liquid surface water would be mostly frozen, equatorial zone may be ok


• T2 = mean 0C to 40C, good for humans


• T3 = mean above 40C, 40C min over 90% of surface, generally too hot, polar/high altitude regions may be ok

Biosphere Favorability (B):

• B1 = little to no biosphere, or non-oxygen biosphere


• B2 = habitable biosphere, atmosphere mostly friendly to humans, minimal food and resources


• B3 = lush biosphere, edible plant-like and animal-like organisms

Hazards (H):

• H1 = little to no hazard to humans, or minimal protection required (dangerous sections of biosphere, small pockets of radiation, some harsh weather, large temperature ranges, etc)


• H2 = moderate hazard to humans, protection required (gravity, extreme weather, dangerous biosphere, hostile intelligence, high surface or solar radiation, corrosive or dangerous atmosphere or pressure or temp, etc)


• H3 = severe or extreme hazard to humans, substantial protection required, or too dangerous to even land

This gives you 27 combinations. I used a combination of English and Greek letters (with overlap) to format a grid. Greek letters and Y are for H3 worlds, J through Q are most optimal for human habitation or colonization. I skipped letters I O S Z because they look like numbers when written, and also U which looks like V.

• A/Alpha: T1 B1 H3
  
  


• B/Beta: T1 B2 H3
  
  


• Delta: T1 B3 H3
  
  


• C: T1 B1 H2 (The moon)
  
  


• D: T1 B2 H2 (Delta Vega)
  
  


• E: T1 B3 H2
  
  


• G: T1 B1 H1 (Mars)
  
  


• H: T1 B2 H1
  
  


• J: T1 B3 H1
  
  .


• Gamma: T2 B1 H3
  
  


• Lamba: T2 B2 H3
  
  


• Sigma: T2 B3 H3
  
  


• F: T2 B1 H2
  
  


• N: T2 B2 H2 (Vulcan)
  
  


• P: T2 B3 H2 (Pandora, P3X888)
  
  


• K: T2 B1 H1 (Vorash)
  
  


• L: T2 B2 H1
  
  


• M: T2 B3 H1 (Earth without humans)
  
  .


• Y/Upsilon: T3 B1 H3 (Demon class, Venus)
  
  


• Theta: T3 B2 H3
  
  


• Omega: T3 B3 H3
  
  


• V: T3 B1 H2
  
  


• W: T3 B2 H2
  
  


• X: T3 B3 H2
  
  


• R: T3 B1 H1
  
  


• T: T3 B2 H1 (P2X338)
  
  


• Q: T3 B3 H1
  
  

Unfortunately this does not tell you about the complexity or intelligence of life. It would be easy to optionally categorize the most complex lifeform, and if there is intelligent life, its level of technological evolution. It does however do a pretty good job of telling you what to expect, or if you would need additional classification to determine if you should even enter orbit.

Can you use something like vignettes? A vignette is basically a short description of something with a catchy name.
Define a list of the most useful planet types and create a small vignette for each.
For example:

Industrial World

Habitable world with a well developed, space faring infrastructure. bla bla bla

Planetoid

Lifeless world with little athmosphere.

The trick is to write these in a way which allows for modification - e.g. add a "heavy" before industrial world and you get a slightly different meaning.

This list has to be shared with your group, so when they hear "industrial world" they know what to expect and what to ask to further define the planet.

Maybe use Chinese characters. It would keep a creatively thoughtful SciFi mystique it all, much like how Firefly uses Chinese in its world.

There are thousands of them, but, there would only be a limited set that is used most frequently for the most important-to-classify planets, such as habitable, dangerous, or mineral rich worlds.

It would also be clever.