Traveller: Earth-Like Planet

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.


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