Traveller World Stats

Solar System Generation

The standard Traveller world generation only generates one world per system so what does that mean in game terms.


Solar Systems

The science of solar systems is roughly: a solar system has a star (sometimes more than one) from one of a collection of star types and around the star there are multiple orbits which may contain a planet of some kind. These orbits are divided into three zones: inner zone which is too hot for water; outer zone which is too cold for water and a potentially habitable zone in between. The size of the three zones depends on the heat of the star.

For example imagine two solar systems both with 12 orbits, one star is weak and so has say 2 inner orbits, 2 habitable orbits and 8 outer orbits; the second star is hot and has say 4 inner orbits, 4 habitable orbits and 4 outer orbits.



Gas giants form in the outer zone.


Weak stars whose habitable zone is relatively close to the star are more likely to have planets that are within the star’s 100 diameter jump shadow and more likely to be tide locked


Advanced Generation Systems

There are lots of more advanced world generation systems that create the whole solar system:

GURPS “First In”

MegaTraveller “Would Builder’s Handbook”

“Universe” scifi RPG

“Rogue Trader” from Warhammer 40K (my personal favorite)

however for the most part unless your campaign is very heavily focused on exploration this is going to be much more work than you’ll need.

So in game terms we can say that the Traveller world generation system generates the *best* planet in the system.



A follow on from the idea that the world generated is the most habitable is that a system might have other planets that are not as good.

A quick way to generate this is if a generated planet has good physical stats roll a second time – if the world generated is worse make a note and then roll again until you don’t get a worse one.

Example: generated world has 866 physical stats; roll again and 440; roll again 310; roll again 520 – the 4th is better than the 3rd so drop it leaving three: 866, 440, 310


Another follow on from the idea that the world generated in the standard system is the most habitable planet in the system is you can work backwards from the world to generate the solar system.

Mongoose Traveller: “Scout” has a good system for doing this.



In most cases treat the world generated as the best in the system and don’t worry about the rest of the solar system unless needed and if available use the relatively quick and simple Mongoose “Scout” method of working backwards from the world.

In a campaign centered around exploration, either an early colonization campaign or a “five year mission” type campaign I’d suggest using the Warhammer 40K “Rogue Trader” system as that throws up navigational hazards which can be used as Star Trek like “anomalies” to investigate.


Traveller World Stats

The interpretation of Traveller world stats has three cases.


If you use the standard Traveller world generation stats you can get results that don’t seem to make sense, for example
– a system with no planet that has a population of billions
– a system with deadly atmosphere and a low tech level.

The solution to this problem is history or aliens.



One option is to add an alien species for each system that doesn’t make sense – for example a low tech species that is adapted to the deadly atmosphere of a system (or even one alien species per planet).


The second option is to add history – make up a succession of empires or mysterious ancients etc that can explain away any anomalies – for example a system with no planet and a large population could be the home world of a species destroyed in a past war with the only survivors living in habitats on the other planets in the system with enough industry to maintain the habitats.

It’s this that creates the three cases for world stat interpretation.


The Three Cases

1) Third Imperium Universe:

In the 3I the gist of the history and the distribution of aliens is already known so the canon world stats need to be interpreted within that context.

2) Roll Your Own Universe:

If you randomly roll up a sub-sector then you need to make up your own history and/or aliens so the sub-sector make sense.

For example in the Glisten sub-sector the Glisten system has the highest tech level and a huge population but no planet at all. Ffudn has a very high population also and the second highest tech but on a small planet with only a trace atmosphere.

So how come?

In the context of the 3I this has to be explained using the 3I’s history and aliens however if you imagine you rolled up Glisten sub-sector as a random sub-sector then you can roll your own history and aliens to explain it so for example you could say Glisten and Ffudn evolved separate species and space travel on standard earth-like worlds but in a devastating war both destroyed each other’s home world leaving the surviving Glisten population living in orbital habitats around other planets in the system and the Ffudn population living in a giant tunnel warren on a gas giant moon – both having enough industry in those artificial habitats to maintain those habitats.

However there’s also a third option.

3) History First:

If you want to set up a campaign with an implied history, for example
– an early colonization from a home world by one stellar species
– a re expansion of a once collapsed civilization over lost colonies
then the standard world generation method has to be adapted to suit the history.

The implied history of a re-expansion setting requires the players starting on the edge of the advancing interstellar civilization into a region of space which is emerging from a Dark Age so probably doesn’t have any interstellar societies: no class A star ports and possibly a max tech level and where population size is more limited by planet type and tech level as they would have had to be self-sufficient during the “Dark Age.”

The implied history of an early colonization campaign is initially no nearby interstellar species, only the home world has an A star port and all unexplored systems are Xs with zero population (unless pre-stellar aliens or a population seeded by an earlier star faring civilization). The only stats rolled in this case would be the physical ones.


World Stats: Earth-Like Planet

The standard Traveller world generation only generates one world per system and IMTU i’m taking that to mean the generated world is the closest to an “earth-like” planet in the system.

So what does earth-like mean?

I take it to mean a planet in the habitable zone of a star that is going through the same long process that resulted in earth however it might be at the start of the process, at the end or anywhere in between.

So where the world stats can be fitted into this earth-like sequence (not all can) the stats represent a planet similar to earth at the same stage of the process.


The Earth-Like Sequence


Hadean Era

4.5 billion years ago (1/3 of universe age apparently)


Solar Nebula

Load of gas and stuff swirling around, majority congeals into a star, the rest settles into orbital rings





Planets form by accretion of this swirly stuff into planetisimals

ALMA - HL Tauri

(ALMA – HL Tauri)

The distinction between creating planets or gas giants is gas giants form beyond the snow line where water freezes.


Volcanic Outgassing

So big ball of stuff, volcanic activity pushing gases to the surface creating an atmosphere – no oxygen yet.


(Public Domain Pictures)

Lots of stuff still in the orbital rings so lots of collisions and bombardments.

Big collision with another protoplanet possibly creates moon.


Archaen Era

3.5 billion years ago

no ozone layer -> lots of radiation



fewer collisions, cooling, solid crust, less volcanic.

small tectonic plates.

lots of small continents.

liquid water on the crust.


Microbial Mats

microbial mats around hot water vents underwater.

microbial mats can be flat, blocks, spherical, towers etc


(Donnie Reid)

microbes evolve to feed on whatever chemicals are available – including carbon dioxide to oxygen


Proterozoic Era

2.5 billion years ago



Once photosynthesis gained then oxygen starts to be added.

Simple single celled organisms.


Snowball Earth

Some phases of almost total glaciation: general idea as proportions of various gases goes up and down can get colder until runaway glaciation – reversed eventually most likely by volcanic eruptions gradually adding more greenhouse gases into the atmosphere.


Ozone Layer

Oxygen -> ozone layer -> protection from radiation


Cambrian Explosion

580 million years ago

Oxygen tipping point – multi-celled organisms

lots of complex creatures in the oceans – trilobites and arthropods

continents barren apart from microbe mats

hotter, less oxygen, more CO2, high sea level


plants colonize edges of continents (related to reduced radiation from ozone?)

arthropods gradually colonize continents from edges inward





Max Oxygen

When hit max oxygen -> giant plants and creatures




evolution of trees – reduced CO2

meteorite impacts

generally lots of wobbles are possible as the atmosphere components shake out



There are a lot of different stages the most earth-like orbit in a system goes through and it takes a lot of time and even when it reaches the habitable form it may go through erratic cycles of cooling and heating leading in and out of ice ages etc.


Game Translation

In game terms this sequence can be broken down into six stages.

Stage 1: Solar Nebula / Accretion

Not very suitable for adventuring.

Stage 2: Volcanic Outgassing

This stage would start at hell world, volcanic, surface radiation, bombardment followed by cooling and a transition to continents and seas with extreme surface radiation.

Stage 3: Underwater Microbial

Microbial mats develop underwater around hot water vents along the edges of tectonic plates, carbon dioxide to oxygen – first life, extreme surface radiation.

Stage 4: Photosynthesis

Microbial mats spread, feed deep sea creatures, plankton, photosynthesis adds oxygen to atmosphere, ozone starts to reduce surface radiation -> gradually leads to teeming oceans.

Stage 5: Pre Earth-Like

Starts with teeming oceans, barren continents and low-medium radiation -> microbial life gradually colonizes the edge of the continents, then plants then arthropods -> eventually teeming continents

Stage 6: Earth-Like

Starts with teeming life on both land and sea -> cycle of heating / cooling as gases get out of balance leading to ice ages and global warming also occasional wipe outs from asteroid strikes. If sentient life develops to the point of climate control and orbital defenses then these get negated.


World Stats

What world stats fit a particular stage in the earth-like sequence

  • stage 1: planetary size 0-2 and no atmosphere
  • stage 2: exotic atmosphere, fluid or hell world
  • stage 3: exotic or maybe trace atmosphere
  • stage 4: very thin atmosphere
  • stage 5: thin atmosphere
  • stage 6a: standard atmosphere, taint might mean almost standard
  • stage 6b: dense atmosphere could indicate the gases are out of balance in some way, global warming / ice age

This doesn’t have to be applied strictly however it provides a quick rule of thumb for visualizing worlds particularly with thin or very thin atmospheres as being barren rocks in the process of becoming earth like.



This sequence also relates to the type of terraforming that might be ongoing on a planet.

Stage One and Two: assuming forming or speeding the formation of stars, planets and moons is very high tech this won’t apply.

Stage Three: barren continents, barren seas, radiation protected habitat: orbital, underground, or underwater where terraformers propagate microbial mats under the sea near natural or artificial hot water vents

Stage 4: barren continents, pregnant seas, radiation protected marine habitats propagating photosynthesis, plankton, sea life

Stage 5: barren continents, teeming seas, radiation protected habitat propagating life around the coasts of the continents

Stage 6: last stage either propagating life to the interior of pregnant coastlines *or* the process has completed a runaway atmosphere (too hot / too cold) is being rebalnced


Ancients Terraforming

If you have some kind of very high tech Ancients race there may be evidence of their earlier terraforming, for example

  • artificial ozone layer
  • underwater facilities
  • bio-engineered plants and creatures with high radiation protection
  • bio-engineered terraformers i.e. creatures that naturally terraform


Cthulhu Connection

If your Ancients are connected to the Cthulhu Mythos some of the Cthulhu races could be connected to terraforming.

Deep Ones

For example say the Deep Ones are a terraformer species – either natural or bio-engineered. Each colony has a breeding pair they call Dagon and Mother Hydra with soldiers (Star Spawn) and the Deep Ones themselves. The Deep Ones artificially farm microbial mats near hot water vents for food thus speeding up that process and when it becomes possible they farm plankton and then later coastal seaweed also. Oxygen is a taint to them so as the world becomes more oxygenated their population drops. They can move onland for a short time using a kelp water mask. If a sentient species develops they can sometimes interbreed with them.


Tcho-Tcho could be an inland variation on this theme – live in caves or tunnels underground, high radiation resistance – for the stage where life has spread to the edge of the continents but hasn’t spread to the interior yet. Small, fast maturing and breeding, cannibalistic farming some kind of oxygen producing goop with high radiation resistance, too much oxygen is a taint – originally coastal, pushed up into mountains as world oxygenates to get away from the oxygen basically goblins.