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.

SIZE DIAMETER miles DIAMETER km
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.

17 thoughts on “My house rule for planetary size”

  1. This is the classic example that the simple answer is always the best. I wish people reworking the rules the next time can think this way.

  2. So, instead of changing the UWP, you changed what the UWP means. I prefer changing the UWPs: if Atm 1/A/B/C, Size 3+; if Atm 2/3, Size 4+, and if Atm 4-9, Size 5+. It accomplishes the same thing… That Enos explanation pains me every time I read it, and there’s no reason to blame the Ancients for a rule problem.

  3. Changing UWP was the way I use to do. But by changing what the UWP means, I don’t have to change the UWPs for about 80 worlds in each sector. I can still use the same unmodified database as everyone else for my games.

    Someone writing for the OTU shouldn’t use this house rule since it breaks canon, but I would suggest that writers stop explaining small worlds and leave that to the referee.

  4. Thanks for these links – what’s the problem with Enos, though? If I enter the values into the plot tool, it is quite nicely clustered with Earth and Venus. I’m probably missing something.

  5. Thank you fore trying to fix it! But it wasn’t fixed enough. It is still too small.

    The radius is now 2400 km. In the tool it is close to Mars and Mercury and cannot really retain oxygen.

  6. What tool? And we couldn’t change it much, since there was this canon explanation for the oddity. If they had NOT tried to explain it, it would have been easier to fix. But since they explained it, we needed to preserve the explanation ๐Ÿ™

  7. I would actually agree on the explanation, and it is my hope that a future writer for the Marches bulldozes it and makes Enos a size 5+ world. But I cannot invalidate all canon on my personal whim. That’s a Marc superpower. He jumps over tall buildings in a single bound; I require a good running start to hop over short fire hydrants.

  8. Don, How did you decide what to do with the other 72 problematic worlds in the Spinward Marches too small to retain oxygen?

    I see that these are a lot better now. ๐Ÿ™‚

  9. Link to Spinward Marches at the Traveller Map.

    There are lots of good changes. But some other that I don’t understand. ๐Ÿ˜•

    Dinom changed to a size 3 world (from a size 1), and that one didn’t have any atmosphere, so that shouldn’t have been any problem. Since there is a published adventure for Dinom, and it is important that it is a small world, I don’t understand why that was changed.

    Anyway, I am happy for the work you have done. The Spinward Marches is a lot better now. ๐Ÿ™‚

  10. I am very curious about how you knew there were 72 problem worlds in the Marches… did you have a tool or analysis you used on the data?

    As to Dinom — that’s a world that has an atmosphere that comes out every 800 years, right — so it needed a size that would actually support that atmosphere when it does appear. Size 1 doesn’t cut it.

  11. Then I understand why the size of Dinom was changed. Looking at the conflicting problems, I think you made the right choice.

    I knew it was 72 worlds + Enos that had size/atm-problems because I counted them.

  12. The reason I ask that is that I tried to collect all the ways people identified problems in UWPs in the past, so we went through that list during the search.

    Starting with the Marches was useful for building a common set of rules to apply to the rest of the UWPs.

    If anything, the stellar data is worse, which is why I still consider it open for fixes.

    Don’t ask about W values though.

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