This has been discussed on this forum before, but I think it needs revisiting.

First thing first - I think current model is way off.

For starters, I don't think one should realistically expect Earth mass or larger planet to have thin or no significant atmosphere.

First of all, in our own solar system we have two earth-size planets - one is, well, Earth itself, with 1 bar atmospheric pressure, the other is Venus which has somewhere around crushing 80 bars. Ok, two planets do not a statistic make, but Pioneer's often large and massive planets with rarely even one bar of atmospheric pressure feel just wrong.
Second, universe is generally made of around 70% hydrogen, 30% helium. The rest is dominated by light elements (of which volatiles are composed). Pioneer just seems to have way too little of them.

Then we have the atmospheres. I don't think the current model listing atmospheres as if they were made of single substance cuts it. Atmospheres are typically mixes of different stuff. Earth's for example is mostly nitrogen (there goes that oxygen atmosphere...), some oxygen and various other gases, Titan, unexpectedly, is mostly nitrogen as well, despite everyone nearly instinctively associating Titan with methane (all 1.4% of it). Venus is nearly pure carbon dioxide. A lot of stuff we're thinking of as primary constituents of atmospheres are just notable additives (like our oxygen) or substances that form planet's weather cycle (whatever makes clouds and precipitation) - water here, sulfuric acid and sulfur oxides on Venus, ammonia and its salts (as well as water!) on Jupiter, same, but with methane cloud layer on top on Uranus, photopolymerized organics on Titan and so on - I think Frontier's model listing primarily weather system was much better in this regard.

Even if you decide to go with current model, it seems to assign atmospheric gasses more or less randomly, which isn't how it works at all.
What atmosphere you have is mostly determined by how massive you are and how hot.
Atmosphere, at least the one you start with, is basically what you can hold onto out of all the stuff flying around in the proplyd. You hold onto stuff using your gravity - if it's flying at more than your escape velocity, you can't have it.
How fast atoms or molecules of gas are flying is in turn determined by temperature, molecular mass and molecular structure.
Basically, AFAIK (we could really use a true physicist here) molecules of different stuff at same temperature will have same average kinetic energy (so heavier molecules are slower) and their kinetic energy will be equally split between different kind of movement (so diatomic molecules that can oscillate and rotate in addition to just flying around will be slower than single atoms of same mass, more complex ones that can rotate in more different ways and wobble around will be even slower and so on) - you can basically determine velocity distribution in gas by its molecular mass (set for any given gas), degrees of freedom per molecule (easily determined from molecular structure, also set) and temperature, then check with planet's escape velocity, although it's a bit complicated by the fact that we're speaking of a distribution of velocities, not single value, and we can allow for some gas to escape, just not too much.

Anyway, in short, hydrogen and helium are hardest to hold onto. Both are very light (meaning high velocities at given temperature), and helium, being monoatomic, doesn't spin nor oscillate (it's twice as heavy as hydrogen, though). Since they are also very abundant, I don't think we'd be dealing with any terrestrial planets with appreciable but thin (completely tenuous atmospheres may be based on different principles, like substance being released in some way and escaping) or even thick atmospheres made of those gases. If you can hold onto any hydrogen or helium, there is enough of it going around that you snowball into a gas giant - it's compunded by the fact that if you're cold (which would help reduce velocities in your gas), you'll also collect a lot of solidified volatiles (which are more abundant than rocks), which will push your mass to the point where you bloat up into a gas giant.
Oxygen is heavy and abundant enough, but awfully reactive - we wouldn't have any here if it wasn't constantly replenished by plants.
Nitrogen is a nice general purpose gas, since it's both abundant, stable and relatively heavy.
Carbon dioxide is excellent, not as stable, but much heavier - but to compound things it seems dependent on geological activity and can be tied up by stuff like liquid water and some minerals (which is why it dominates on both Venus and smallish Mars - Venus lost liquid water while Mars is just about heavy enough to hold CO2 at it's temperature).
Methane and ammonia are nice, but light-ish.
Hydrogen cyanide, carbon monoxide and hydrogen sulfide seem nice enough as well, not sure about abundance - carbon monoxide seems like it would common in atmospheres that would otherwise contain a lot of methane and water vapour, but are hot enough to make them react.
I'm not sure about ever finding appreciable amounts of argon and such.

Now, life. I think the problem with life in Pioneer is that the condition's just don't seem restrictive enough in some aspects. If you're basing life on liquid surface water, then, while the lower temperature boundary is practically set (you'll need at least some water at more than melting point which generally stays at around 0 Celsius), the upper temperature limit will change *wildly* with atmospheric pressure - below 6 mbar you can't have *any* liquid water, at 100mbar it's game over at mere 50 degrees, while at 2 bar you can go to around 170 degrees and still have liquid. For life to exist you'd probably want the range between ice and vapour to be reasonably broad so you won't have worlds with no significant atmosphere and complex ecosystems - you'll need *some* pressure for surface life to exist, instead of just blindly checking for 0-100 range - that should help with the overabundance of life otherwise resulting from too much volatiles.

Of course, you could also have underground bacterial life or oceans teeming with life under icy crusts or even small bodies (provided they get enough tidal heating). Not to mention life based on different biochemistry and solvents other than water.

Finally, I have always thought that "outdoor world" meant planet where you could survive outside without a spacesuit - I mean otherwise all the planets have some outdoors.

Again, I'd appreciate some discussion.

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