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My house rule for planetary size

A problem with the random generation of worlds in Traveller is that for small worlds (where size < 4) is that these worlds might get an ordinary breathable atmosphere. This is a problem, since the minimum molecular weight retained for a planet shouldn’t allow that. EDG has explained minimum molecular weight retained here, and here is a nice visual tool for checking what gases will be retained depending on size, density and temperature.

This problem leads to silly explanations. Like the one in both the GURPS and Mongoose Sword Worlds modules where a planet (Enos) is made of Osmium. Other explanations might be terraforming by the ancients.

These silly explanations might explain how a few (very unique) worlds got their atmospheres. But it seems unlikely that these explanations can explain every world equal or smaller than size 3 with an atmosphere equal or greater than 2. In an average sector with standard stellar density there are about 80 such worlds. (In the Spinward Marches, there are 73 worlds with this problem.)

I usually ignore this problem, since our group of Traveller players don’t care about details like this. They want to play the game. But if the question is asked, I now have a solution. Instead of explaining every world or changing the UWP, I use this table.

1 4000 6400
2 4500 7200
3 5000 8000
4 5500 8800
5 6000 9600
6 6500 10400
7 7000 11200
8 8000 12800
9 9000 14400
A 10000 16000

As can be seen is this table, the problem is solved by making the smallest planets big enough to retain a breathable atmosphere. (The original table can be seen at Traveller Map.)

Terraformed Mars
Terraformed Mars: File from wikimedia. Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.