Sci-Fi: Alien Categorization Codes for Space Opera

Most space opera or soft-sci-fi settings use broad categories of life. "Class M planets" and "Carbon-based lifeforms" are two typical examples.

They are useful shorthands for explaining why everyone looks like a human in a rubber suit and why every planet vaguely resembles California or a quarry in England, but they're not particularly interesting. "Carbon-based" is so vague that it's practically a non-sequitur; it tells you almost nothing about how the creature operates and what environment it requires. Extremophile bacteria, tube worms, spiders, and mushrooms are all carbon-based lifeforms.

I wrote about set-dressing aliens in this post.


  • Infections and cross-contamination are not significant issues. E.g. this fungus from Planet A won't happily colonize human skin, this creature from Planet B doesn't casually emit compounds that are toxic to humans, etc.
  • There is enough overlap that categorization like this is useful. If everyone has to wear miniature spaceships to visit each other, it's not space opera.
  • Species are abundant and have reasons to interact. This guide isn't useful in a setting without thousands of potential intelligent species. Some sort of galactic community is required.
  • Interspecies communication is possible (i.e. it's possible for two species to communicate using the same language.) Sure, one of them might use radio and the other might use gestures, but it's possible to build a grammatical base that means something to multiple species.

Categories are listed from most to least crucial. If you're an alien scooping up a creature or sorting out accommodation and compatibility, you'd work from the top down. They're sort of like taxonomic ranks.


The phase (or phases) a creature requires to live. Not the phases it can safely inhabit (e.g. swim in), but the phases that are required for life.
  • Gas Phase
  • Liquid Phase
  • Solid Phase
  • Plasma Phase (?)
  • Vacuum
Humans are G-, amphibians are GL-, fish are L-. Solid Phase creatures are difficult to imagine, but maybe some sort of distributed high-pressure network of tendrils or self-propagating migratory crystal patterns (like rock-prions). Plasma-phase creatures presumably live in the heliosphere of stars, the upper atmosphere of planets, or other equally odd locations. 

Primary Energy Exchange Element

The stuff a creature needs to live on a minute-by-minute basis. For setting purposes, the atmosphere the creature breathes.

It might be more useful to phrase this in electron donor/acceptor sets (e.g. humans would be hydrocarbon+O2 / CO2+H2O, hydrogen bacteria would be H2+O2 / H2O, etc.) but the list would be enormous and unwieldy for soft sci-fi or space opera games. You should still check out the list of real chemolithotropes. Life is weird.

Energy requirements (light, electricity, neutrons, etc) are best categorized with Optimal Radiation (below).

  • O2
  • CO2
  • H2O
  • H2
  • CH4
Humans, dolphins, and fish are -O2-. Plants are -CO2-. Creatures on Titan might be -CH4-.
lifeslittlemysteries.com (defunct)


In setting, this is probably expressed as a single operator or symbol (some sort of compressed ideal gas law thing)

Temperature is given as a 3-point range: minimum, optimal, and maximum. The value is calibrated around environmental temperature. Clothing is assumed; external heating or cooling sources (fire) are not. For humans, a range of -20/20/40 oC is close enough to accurate.

Pressure is also given as a 3-point range: minimum, optimal, and maximum. For humans, a range of 0.3/1/1.3 will do.

(Pressure is tricky; humans can breathe in a 100% oxygen atmosphere, but atmospheric oxygen is around 20%. At 100% O2, humans can survive around 1 atm of pressure, but at 20%, we can survive up to 2.5 atm of pressure. And, of course, temperature ranges vary with humidity...)


Again, given as a minimum, optimal, and maximum range. Humans can operate in freefall, and the maximum feasible gravity seems to be 3-4g, so 0/1/3g.

I'd expect most creatures that can survive falling or rolling for any length of time to be able to survive in 0g environments. Pitcher plants couldn't.

Optimal Radiation

The assumption is that the creature requires this level of radiation, but should be shielded from all other forms. Deriving a numerical value here starts to cross the line between "back-of-the-napkin RPG math" and "actual physics". I couldn't make it interesting.

Humans need Photons of energy X to energy Y, with a radiant energy density of Z. So -P#-
High-altitude gas giant creatures might need Neutrons of energy A to B with a radiant energy density of C. So -N#-
Creatures which have no specific radiation needs (e.g. prefer darkness) are denoted with an X.

It's interesting to note that the sun's output is not evenly distributed across all wavelengths. Without some sort of intensity curve, the "default" light in a room will never be a typical sunlight white.

Ivan Khomenko


Even if creatures have all the above characteristics in common, building structures that can accommodate a variety of scales is useful. For space opera purposes, size bands based on orders of magnitude (0.1m, 1m, 10m, etc.) are probably acceptable. Or maybe there's some sort of convenient clumping principle at work, and intelligent life tends to occur in narrow bands around "about shrew sized", "about person-sized", "about whale sized", etc.

Cognitive Speed

How quickly the creature can communicate a fixed packet of information to another creature, or perform a standard quantity-rearrangement or pattern matching test. Computers have a value thousands of times higher than a human. This is another order-of-magnitude range; 0.1c, 1c, 10c, where c is an arbitrary anthropocentric value.

For a space opera setting, it's probably easiest to assume everyone sharing an environment has a comparable cognitive speed, but it might be interesting for some creatures to operate in very different ways.

Social Organization

A series of codes for typical groups.
  • Individual Asocial (-IA-)
  • Individual Social (-IS-)
  • Collective of size # to # (-C#-#-)
  • Hive of size # to # (-H#-#-)
  • #-sized Indivisible Unit (-#U-)
Basically, "how many can we put in this tank before they start fighting or die of loneliness?"


A strange code, but I suspect creatures with similar symmetry plans tend to have broadly compatible views of time, design choices, etc.
  • Radial #
  • Bilateral
  • BiRadial #
  • Icosahedral
  • Asymetric

Dominic Qwek


I've had to eyeball a few values. Nobody, as far as I know, has ever put a whale into freefall.
  • Humans are G - O2 - -20/20/40oC - 0.3/1/1.3atm - 0/1/3g - P# - 1m - 1c - IS - B
  • Blue Whales are L - O2 - 4/12/20oC - 1.5/3/10atm - 0/1/1.5g - P# - 10m - 1c - IS - B
  • Ants are G - O2 - 10/30/40oC - 0.2/1/3atm - 0/1/3g - P# - 0.01m - 1c(?) - H100-2000 - B
  • Titan Spiders are G - CH4 -  -190/-180/-120oC - 0.7/1.5/4atm - 0/0.1/0.6g - X - 1m - 10c - IS - R8
  • Hydrogen Rays are G - H2 - -80/200/500oC - 0/0.01/0.1atm - 0/2/6g - N# - 100m - 0.1c - IA - B


In a setting like this, I'd expect compatible species to form parallel galactic civilizations. The oxygen-breathing approximately 1g people might have their own wars, only vaguely aware of the politics of the methane-breathing 0.1g ice-worlders. Intermediate or interpreter species are probably valuable. Collaborative space stations probably feature a wing for the most common or influential types, plus a neutral zero-g zero-atmosphere area.

This guide also helps colony building. A survey team can scan a world, generate the correct codes, and then figure out what species might be interested in visiting. Jupiter could be a tropical vacation paradise. Humans can't use it, but could trade access or ownership rights. It's much easier than terraforming. Instead of fixing Venus, sell it to someone who likes it as-is, or just needs to make a few tweaks.

Humans have an incredibly tiny livable range on every axis. We're used to taking oxygen, pressure, and temperature for granted because that's how space opera works. Push any value outside the habitable range and everything falls apart.

Mass and special requirements are a real pain. If dolphins built the Saturn 5 and wanted to land on the moon. Assuming they ate the same amount of food, consumed the same amount of oxygen, and took up the same space as the 3 human astronauts, they'd still need to haul an extra 36,000lbs of water into space, effectively doubling or tripling the payload. And because of the tyranny of the rocket equation, that means a much bigger rocket. Still plausible, but inconvenient. Then again, vacuum-adapted creatures probably laugh at humans hauling all that neutral gas around.

It's best to pack your own food (or food additives). Gene-tweaked humans can probably get by with a small pack of pills and requests for water and glucose, but it's not exactly high cuisine.

Each species needs to have its own experts. The life science technology of one species, no matter how advanced, might do absolutely nothing for the next species. First aid and emergency triage (e.g. basic medieval medicine) is probably cross-trainable, but everyone should carry their own first aid kit, ideally with instructions. Aliens can sell us a better microscope or fab up a few thousand litres of morphine, but they're probably baffled by white blood cells and drug interactions.


  1. Oh this is brilliant. I'd always assumed such a thing existed in my space opera setting but never thought to actual try and make it you genius madman! I've always called mine the Interspecies Compatibility Index.

  2. *cracks knuckles*

    The tools have arrived, the work begins.

    One note though is that 0-G probably shouldn't be the low scale on gravity, considering how awful it is on the human body. Maybe a nice 0.25 or 0.3? (A shame and a frustration, we don't have any good long-term observations of humans in low gravity, at least as far as I know)

  3. -20 Celsius as a minimum? *laughs in Canadian*

    ... seriously parts of Canada are seasonally barely suited to human life.

    This is a great post. Diversity in aliens is good, but since we are doing an RPG (imagination, better than any CGI!), we can actually explore what this means fully. Bravo.

    1. -20 for sustained periods, with only personal imperfect insulation (i.e. warm clothing that isn't fully sealed), and relying only on personal metabolic heating (i.e. no fires, no heaters, etc.), while still being able to function, seems like a decent compromise value.

  4. I've been working on a solar system generator for semi-hard space opera myself and life there has two base values: base chemistry and what they breathe (which can be vacuum). you have to fill in the blanks in that case as the thing spits out things like "ruins, probably created by ancient silicone-based co2-breathers".

    As for your dolphin-example: Would cetaceans _really_ need to take water into a zero-g-situation?

  5. But this categories face the challenge of " If everyone has to wear miniature spaceships to visit each other, it's not space opera. "

    If we can't survive in the same atmosphere the kicks one get out of aliens are much minor. As a player I would have less fun due to limited interaction with aliens. It makes barriers and making interesting NPCs is already hard. Worse if they have to be PCs. Size as well, anything more then 1 order of magnitude different then a human becomes a McGuffin/Boss monster and not something you routinely interact with.

  6. Have you read any of James White's Sector General stories? They include a four-letter classification system for sentient species:


    And from a Fan-Wiki (https://sectorgeneral.fandom.com/wiki/Physiological_classification):

    "For example, the first letters A,B, or C signified water-breathers, while D, E and F signified oxygen breathers. O and P were chlorine-breathers; the prefix V represented a species with mental or extrasensory capabilities sufficient to make physical qualities irrelevant."

    Interestingly, it's not particularly "humano-centric"--we get the classification code DBDG.

    (Sorry this is so late--I came here by following links from your post on optimistic SF settings.)