I've had a look around the internet and the only real classification system for planets seems to be the one from Star Trek, which I don't want to use. The real groups that planets fall under seem to be messy and unordered and I don't like it.

I'm looking for a way of categorizing planets into various groups relevant to colonization and what you'd need to colonize them. Bonus points if you can come up with a classification system with an order that's independent of the symbols used to denote each category. Like how the OBAFGKM system for stars used to be in order before we learned more.

The basic idea so far is a classification with 5 letters each of which can have subscripts. A letter for size, a letter for its average temperature, a letter for atmospheric composition, a letter for lithospheric composition and a letter for miscellaneous dangers and boons (like a particularly heavy moon meaning that you're lighter in march than in January or something else extraneous). Each letter will have subscripts denoting things like temperature fluctuation, the most abundant elements and other slightly less important info.

The civilisation in question does have FTL capability and the distance to the planet doesn't need to be taken into account. My main questions are about what type of classifications are most important and can be broken down into a couple of smaller questions.

  1. Would it be better to have a system of classification that used surface temperature (and its range of fluctuation)[Advantage: More directly useful information for colonists] or would it be better to base it on orbital characteristics (distance from star and eccentricity)[Advantage: The system of classification can be expanded to moons, asteroids, and comets.]?

  2. Would it be more important to define a planet's size in terms of its mass or its volume?

  3. What would be a good way to calculate a "Survivability Score" for the atmosphere and lithosphere based on the elements and proportions of elements? (Note: I reckon I'll be using a separate letter for lithospheric and atmospheric composition)

Rather than ask for the definition of planet types lets work on the creation of a system against which to judge and categorize planets in relation to how well they can be colonized.

  • What are the 5 physical planetary characteristics most important in relation to colonization
  • How can these factors be combined to create classes
  • 7
    $\begingroup$ I think you're asking for a bit much. People might refer you to an existing system that you're not aware of, or give you some hints as to how to classify planets yourself, but doing it all for you is out of scope IMO. $\endgroup$
    – AndreiROM
    Commented Mar 3, 2017 at 13:23
  • 2
    $\begingroup$ I'd use some sort of checklist, each point being something your advanced race needs to be able to survive (temperature, pressure, breathable air, gravity, etc...), and sort the planets in order of viability. Each non-checked point would have to be compensated by some technology, meaning colonization would be more difficult/expensive $\endgroup$
    – Irhala
    Commented Mar 3, 2017 at 13:29
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    $\begingroup$ We also need to know what level of technology is available to the planet namers. If they have cheap FTL or better yet portals and interstellar shipping between worlds is common place, they might classify a world habitable even if it lacks critical attributes for long term independent colony survival. A rocky planet with no farmland might need to import all of its food, but its mineral wealth might justify that expense. Such a planet could be categorized Mining(3), where Mining is what it is good for and 3 is its overall survivability. $\endgroup$ Commented Mar 3, 2017 at 13:54
  • $\begingroup$ Could we not alter this to help Douglas create a system for classification, make it a process question? We don't have to define what each type is, just help define parameters so he can classify planets himself. $\endgroup$
    – James
    Commented Mar 3, 2017 at 15:11
  • $\begingroup$ Sorry, I realise that what I was asking was too much. I was writing the question in a hurry. I should have made it more clear I was asking for help making my own system. $\endgroup$
    – Disgusting
    Commented Mar 3, 2017 at 16:49

3 Answers 3


An absolute habitability scale would not be an effective tool because planets, and the methods by which they could be colonized, vary so drastically. For example:

Mars and Venus would likely have a similar "habitability" score. Mars is rocky, inside the habitable zone, and it's surface is safe for human habitation. Flip-side is, there is no atmosphere, gravity is a bit low for our liking, and there's tons of radiation hitting the surface. On the other hand, Venus' upper atmosphere had good radiation protection, comfy gravity, and an atmosphere that a human can be exposed to (though they can't breath it, and the acid could be problematic). Flip-side is the colony would need to float, as the lower atmosphere would disintegrate you.

On an absolute scale of habitability, these would be close, but a number would ignore the vast differences between them (Venus is likely better for a floating research outpost, while Mars is better for long term colonization/industrialization), and the immediate technology available will change this number greatly.


Use Combinations of Categories

Here are some I put together:

Planet Type: RL: Rocky w/ Liquid Water, R: Rocky, G: Gas Giant, A: Asteroid

Gravity: For this one a simple number is likely best, as all planets exist on a clear scale.

Atmosphere Composition: B: Breathable, E: Exposure Possible, H: Hazardous, N: None/Negligible

Hazards: (pick as many as relevant) P: Pressure, R: Radiation, Tc/Th/Tch: Extreme temperatures (cold/hot), A: Acid, B: Biological (you get creative and have fun with the rest of these)

Life: AI: Advanced Intelligent Life, PI: Primitive Intelligent Life (pre-space travel), L: Non-Intelligent Life, N: No Life

I'm not sure about classifying mineral wealth, as you run into a lot of problems. Either you have way too much data (breakdowns of common minerals and ease of mining them), or too little to be useful (a fixed number classifying the "value" of minerals on a planet, that would change constantly due to market fluctuations and ease of mining). Also, most planets have a variety of minerals available, so it's hard to say if one is "rich" or "poor". Still, perhaps a few general categories determining mineral wealth/composition could be possible in necessary for the universe.

An example of this system in action: Mercury: R-.38-N-(RTch)-N, Venus: R-.9-E-(PThA)-N, Earth: RL-1-()-AL, Mars: R-.37-N-(RTc)-N, Jupiter: G-2.5-H-(RP)-N, Eurpoa: RL-.13-N-(RTc)-N, etc


Start with numerical scores

If you're capable of colonization, why not abstract the difficulty of colonizing it into a numerical score?

E=mc² so energy is mass. You're going to need some of both to effectively colonize, but for mere abstract purposes you can reduce your shopping list to either one. Then all you need is to pick a suitably large unit value and you can easily rank colonizable planets.

This nearby planet that's a lot like your home? That's a 5. Terajoules? Exagrams? Doesn't really matter.

That icy rock floating lightcenturies away? 500.

Everything is taken into account to reach your score. Energy in order to get there, energy to warm it up if it's too cold, mass if you need to import food, less mass if you can grow food but need to import soil, etc.

The point is, there are so many factors that abstracting it all is the only way to reach a simple system. And unless you're the guy packing the colonization ship, do you really care if you need those 5 terajoules of energy just to get there, or if you need them to heat up the planet? No, you just need to find those joules. Maybe you have fusion engines and those joules are better stored as mass, but again, that's a practical concern.

Some additional notes:

  • You will likely end up with a double number, ie. one for initial colonization and one for long-term living. Some colonies could take a lot of effort to start up but they might have everything to become self-sufficient.

  • Orders of magnitude will probably become much more important than the actual values. 5 or 8? Who cares? But 5 compared to 500 is a big deal. This also allows explorers to eyeball things. Barren asteroid? 10k, more or less.

  • This could easily lead to a secondary (maybe exponential) scale used in comman parlance. Class A is anything with an initial cost in 2 digits and an upkeep cost of 1 digit.

  • $\begingroup$ Why not make it monetary investment? That way, you can take the classification directly from the last page of the business case for colonizing a planet. Seriously though, this is an excellent way to do it, as it makes sense regardless of the exact technological, political and economic situation of the aliens. $\endgroup$
    – Cyrus
    Commented Mar 3, 2017 at 15:26
  • $\begingroup$ This wouldn't be a great system to start with. Its hard to judge the energy needed to colonize our first planet. $\endgroup$
    – kingledion
    Commented Mar 3, 2017 at 20:11
  • $\begingroup$ It wouldn't be there first. They've colonised most of the milky way and private companies have made progress in Andromeda. Anyway, the energy to get there at this point in the future is negligible. Also, whilst a fantastic idea that I want to use and will do but as a secondary one to the more complex five letter thing I want. I'd just like as much info as possible to be IN the code so I can use it to partially randomly generate planets. $\endgroup$
    – Disgusting
    Commented Mar 4, 2017 at 0:23
  • $\begingroup$ Watt is power, joule is energy. But you would need both (energy to get there, terraform, import soil, and power to maintain comfortable temperature, fight animals...) $\endgroup$
    – b.Lorenz
    Commented Mar 4, 2017 at 17:29
  • $\begingroup$ The problem with numerical scores is that different planets are hard to Colonize in different ways and for different reasons. Venus and Mars would have kinda simmilar scores, for example, yet their characteristics are so wildly different. Plus, this system could be used post colonisation, at which point viability scores are much less useful. $\endgroup$ Commented Mar 7, 2017 at 3:43

The classification system, based on a planet's habitability status would have to incorporate several features, resulting in a nested structure.

Basic Planetary Groups

There would be 3 primary groups of planets: habitable, potentially habitable and uninhabitable. Note that the habitability of a planet would not only be confined to its position wrt goldillocks zone around its star, but also whether its atmosphere is breathable, its magnetic field can keep off the solar wind etc.

Detailed Grouping

Considering that all intelligent life forms that we know of, can only evolve on a terrestrial planet, so gas giants and ice planets are definitely off the list for habitability.

Within terrestrial planets, we would want to classify planets on the basis of 5 factors.

a- Size/mass of the planet

This is the most important factor. You can (with much advanced technology levels) change the atmosphere and maybe even alter the magnetic field of a planet, but you cannot change its mass without making gigantic changing to a planet.

Surface gravity of the home planet would be the standard here. The gravitational habitability value of a planet would be a measure of how similar (in %) the target planet's surface gravity is, to home planet. For example, for us humans, Mars (with a surface gravity of 3.711 m/s²) would be given a value of -37.87% as its gravity is lesser than Earth (hence negative) and the difference between Earthly and Martian surface gravity is 37.87%.

b- Atmosphere

Once again, we would want to classify the atmospheric composition and pressure according to resemblance with the conditions on our home planet. For the sake of brevity, we would want to incorporate both values as one, split with a x (multiplication) sign. Once again, taking the example of Mars, we would get a value of 0.1x0.6 meaning that the composition of Martian atmosphere is only 0.1% similar to Earth's atmosphere while the atmospheric pressure is only 0.6% that of Earth's.

c- Water vs Land Surface

Here, you would not want to classify the habitability of the planet based on how similar its water distribution is to home planet (since it would be incredibly difficult and complex) but simply go with a percentage value of water wrt the total surface area of the planet. You would also want to include the suitability of the oceans for life. This would be simply marked as a letter from a to z, a meaning extremely unlikely for marine life (as on home planet) while z meaning completely suitable for marine life of home planet. A target planet might get a value of 37r, meaning that 37% of the total surface is covered by water and it is suitable to support 69.23% of marine life forms on Earth (r/z x 100, r=18th letter, while z is 26th).

d- Orbital Statistics

The most important orbital statistics wrt habitability of the planet would include its orbital eccentricity and its angle of inclination/axial tilt. It would also include whether the planet is tidally locked to its parent star or not. For example, Mars would get a value of -0.093x25. - means that it is not tidally locked with its parent star. 0.093 is the orbital eccentricity while 25° is the axial tilt.

e- Miscellaneous

The last parameter would include codes for a list of variables which are a hindrance in it being habitable. For example, Mars might get a code of 2B. These would have to be checked from a list of all hindrances. 2 might mean that the soil is toxic. B would mean that there is an increased risk of asteroid impact.


Note that I have not included the mean surface temperature of the planet in this list, because that would already be included in the goldillocks belt part. If a planet cannot maintain liquid water on its surface, it would already belong to the uninhabitable planets list and there would be no need to categorize it further, based on these 5 subgroups.

  • $\begingroup$ For both surfaces and atmospheres, you should probably just say phase state. In our own solar system, there are 'planets' with liquid surfaces other than water (Titan), planets without any liquid or solid surface (the Gas Giants), 'planets' with a surface of 'water' that is frozen (Europa, Titan too, mostly, Callisto, others), planets with some solid surface that sublimes to gas during some season (Mars), etc. Lots of complications. $\endgroup$
    – kingledion
    Commented Mar 3, 2017 at 20:10
  • $\begingroup$ Thanks for the input. I didn't understand at all what you are suggesting. What is the phase state you are mentioning and how elaborate would it be for the sake of classification of planets by their habitability score? :S @kingledion $\endgroup$ Commented Mar 4, 2017 at 13:09
  • $\begingroup$ I meant that phase state would be solid-liquid-gas. You said 'land-water' division, but on Titan the 'water' is the land, and methane is the liquid part. Also, if cloud habitats work on Venus, they could conceivably work on gas giants, so you wouldn't need solids or liquids to colonize a planet. So I was suggesting a more valuable classification is whether there is a solid surface, and if there is a liquid on it $\endgroup$
    – kingledion
    Commented Mar 5, 2017 at 0:04
  • $\begingroup$ Oh, that! Hmm. I only included the planets where you could live on the surface. That's the only permanent residence plan on a planet. You cannot perpetually live above the cloud layer on artificial balloon colonies etc. Someday, after hundreds of thousands of years, some catastrophe will happen (a meteor shower or something) and the whole hell would break lose if you are living artificially. That's why didn't include that option. @kingledion $\endgroup$ Commented Mar 5, 2017 at 13:41

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