Responding (here) to the points raised in your post on your blog.
“A problem with the random generation of worlds in Traveller is that small worlds (where size < 4) … might get an ordinary breathable atmosphere. This is a problem, since the minimum molecular weight retained for a [small] planet shouldn’t allow that.” Basically, small worlds cannot retain most atmospheres over billions of years: the consitituent gases escape the gravitational pull of the planet and dissipate into space.
True ... BUT ... not every world is in a circumstance of "been here for 4 billion years already" during which nothing much happened (after the Late Bombardment phase). My point being that it is perfectly possible that the CURRENT atmospheric conditions on a small world are a TRANSITORY PHENOMENON, rather than some kind of "steady state" that's going to last for billions of years.
It could be something as simple as in the (recent) geologic past of a planet there was a LOT of volcanic outgassing, which gave the planet "too much atmosphere" for its surface gravity and the atmosphere
is in the process of being stripped away by solar radiation ... but that process hasn't finished yet. One of those "you're 200 million years into a 500 million year atmospheric loss event" kinds of deals, in which the CURRENT atmospheric conditions are an "anomaly" that will change over time scales that are civilization relevant but not current resident relevant.
Sort of like how Sol will continue to increase in luminosity over the next 500 million years to the point where the water oceans of Terra will start to evaporate into the atmosphere until Terra becomes a runaway greenhouse gas hellworld like Venus that boils off the oceans completely, transforming all that water from liquid to gas/vapor. So EVENTUALLY Terra will become "not exactly habitable" long before Sol enters its Red Giant phase of Helium fusion (and expands the photosphere outwards to engulf Terra's orbit completely, really making the planet "a bad place to try and live, long term") ... but RIGHT NOW it's a pretty hospitable place to live, on a sophont civilization timescale, but not on a geologic/stellar evolution timescale.
The UWP doesn't
have to be something that represents the way conditions would need to be for billions of years in an undisturbed equilibrium. The UWP simply has to show what conditions are RIGHT NOW.
Take
Dinom/Lanth that was featured in LBB DA2 Across The Bright Face for example.
Because of how Dinom orbits the star and the rotation axis of the world, the atmosphere FREEZES onto the surface during "summer/winter" and sublimates back into being an actual atmosphere during "spring/autumn" cycles, so there is a seasonal cyclical nature to the atmosphere. Since the orbital period is so long (1600 years) you get about 400 years per season "annually" on Dinom. Point being that the UWP for atmosphere on Dinom is "not a constant" but instead varies, but the variation is on a timescale that is so long that it is "daily lived experience irrelevant" so the UWP simply reports CURRENT conditions, not eternal ones.
I therefore submit for your consideration that the "worlds too small CAN'T have breathable atmospheres" problem is only a problem if you're trying to approach the issue from the standpoint of being a "steady state, simplistic planetary evolution" where nothing particularly notable has happened in all of geologic history.
Mars can be an arid dusty wasteland because radiation from Sol has stripped away the atmosphere from the lower Mars gravity ... but if there was a Major Collision™ in the recent geologic past with a
substantial icy comet type object of some kind, Mars might receive "a lot more atmosphere" as a result (and even have liquid water on its surface!) ... for a while. It wouldn't be a "permanent" condition that would last billions of years, but if it lasts the next "several hundred to a million years" that's good enough for UWP data results!
In other words, by taking the "small worlds with breathable atmosphere" as being a PROBLEM TO CORRECT rather than as an OPPORTUNITY TO EXPLAIN ... you preclude a lot of potential for "world building" in Traveller writ large.
Even something as simple as a "planet captures rubble moon rich in ices" into an orbit that somehow takes the capture down inside the
Roche Limit, causing the captured moon to eventually disintegrate and "rain down onto the surface of the planet" could potentially be the reason why a world that "shouldn't" have a "breathable atmosphere" actually DOES have one ... for a while ... long enough to be relevant to a UWP code when surveyed.
Basically, small worlds cannot retain most atmospheres over billions of years: the consitituent gases escape the gravitational pull of the planet and dissipate into space.
True ... BUT ...
Do we NEED our UWPs to report "eventual steady state conditions" that will exist billions of years from now ... or do we need UWPs to report conditions that prevail RIGHT NOW, which could potentially change in the future?
Spoiler alert: I prefer the latter over the former ... which means that some "anomalous" combinations of UWP codes, such as small worlds with low gravity having relatively thick atmospheres being a "legal" result out of world creation. To me, that just means that the "current" environmental conditions on those worlds is a "temporary" condition, rather than a permanent one.