Some notes on the make up of Sol
Under the current rules Mars, Luna, Io, Europa, Ganymede, Callisto, Titan, Iapetus, Rhea, Dione and Tethys are all treated as O2 bodies despite a massive spread in size, gravity and atmosphere. Also the asteroid belt by base rules is assumed by its distance to have about 115 asteroid colonies, note that Ceres is 32% of the mass found in the Asteroid Belt and far larger than any other asteroid.
Looking at things in detail just reinforces my sense that low mass O2 worlds like Mars should be treated different than the moons of Gas Giants. A world like Mars because of its higher gravity and better surface temperature is going to be a far more favorable target for colonization. If anything I think a strong case could be made that a high tech civilization would find Mars a better environment to colonize than a Heavy Gravity World.
Michael, there's only so much detail that a p&p game can sustain... at least in its official rules.
Yes, Mars, Luna, Io, etc. are all treated as O2 (which is Type B for Barren in Cosmic) bodies, though I would point out that Mars is a planet, albeit a small one, while the rest are moons.
For Cosmic, my definition of what qualifies as a "moon" is any satellite having a radius of 500 KM or greater. And anything below that gets ignored. Note that surface area increases in relation to the square of the radius, so there's a considerable difference in surface area between the smallest (500 km radius) moon and the largest, Ganymede. Still, for the sake of simplicity, they are currently considered the same... I'm a little wary of adding another step into the sysgen process to start creating moon sizes, even though a few of the largest moons are actually larger than Mercury... which should arguably allow them to have larger pop capacities.
As for the asteroid belt and Ceres, Pallas, Vesta, etc., I actually have suggested that AB's be handled differently. Instead of 2 or 5 OP's per sH, roll 1d10 per AB, with the result being the number of "planetoids", like Ceres, Pallas, etc. And it is
only on those limited number of bodies that you could colonize. Those planetoids would be of the type that is proper for their location. That is, Sol's AB would be Type aB, as it is in the Rocky Zone (though just barely). But an AB in the Ice Zone would be coded aF. Those planetoids would be only be able to support an Outpost, since even the largest planetoid is smaller than the smallest (by Cosmic's definition) moon. (Note that there are two upsides to this planetoid concept. 1. There are many fewer populations to track. 2. You know exactly where each of those 1-10 planetoids are for defensive purposes.)
As for Mars vs gas zone moons, it is worth pointing out that currently, Mars can support a larger Settlement level population, while those moons can only support Colony level populations. I think that that's a significant enough population difference. Having said that, if Rocky Zone Type B planets and Moons were to remain as Type B and mB (for Barren), what would you code (and term) Gas zone moons? (Note that I use the prefix 'm' for all moons, hence the code mB for Barren moons.)
However, there is one point that you've missed here, and that's that it's not really a Mass 1 Mars-like planet that you should be pointing to for improved habitability, but the larger Mass 2 and 3 Type O2's ... and particularly those that are relatively close to the outer border of the liquid water zone. There is arguably a substantial difference.
First of all, M2/3 O2's would have gravities much more similar to T/ST planets. M2/3 O2's would quite possibly have retained their magnetospheres, which would protect their atmospheres from being stripped away from solar winds and help protect them from radiation. Their atmospheres would probably be reasonably dense ... denser than Mars' rather thin atmosphere which has been largely stripped away due to a lack of a magnetosphere. Oh, it's likely that these atmosphere would still not be breathable to T/ST races, as there wouldn't be any plant life to produce and sustain higher levels of oxygen.
Also, IF those M2/3 O2's are relatively close to the outer edge of the liquid water zone, the planetary temperatures may be fairly tolerable ... oh, not in a let's go out and bask in the sunshine sort of way, but maybe tolerable in a throw your breathing mask on and a winter coat, and you might be able to walk out on the surface without wearing a full blown spacesuit. (Just speculation, mind you...)
Also consider that there may be a region of "tolerability" just sunward of the LWZ, where it might be a little too warm to sustain liquid water in the open or produce life, but have an overall "environment" that was at least tolerable for colonists.
Obviously, such a type of world would be vastly different from Venus. Venus starts by being a little too close to the sun to even be in this region of tolerability, but it also has the problem of having no magnetosphere, which has allowed the solar winds to strip off the lighter gases from its atmosphere, leaving only the heavier ones ... such as CO2, which has resulted in its immensely dense (something like 100 times that of Earth) and deadly atmosphere.
It's been theorized that Venus may have had a magnetosphere at some point. But at some point in its distant past, Venus was struck not once but twice by large objects that mostly stopped its rotation ... its "day" is about 240 earth days ... and thus caused its magnetosphere to disappear. Thus, it's possible that perhaps Venus' deadly environment is due to extremely bad luck as much as its location. This isn't to say that if it did rotate more normally and retained an atmosphere that it would be habitable. But if it did rotate normally, have a magsphere, and retain an atmosphere, it might at least be something like a very warm desolate world, or at worst, just a large Type H Mercury-like world.
I've actually been considering adding a new planetary type for Cosmic ... Type M for Marginal. A "Marginal" planet would be a Mass 2 or 3 planet that existed just outside of the LWZ, either sunward or outward of the LWZ. (I actually have a version of the planetary formation zone table that includes these "warm" and "cool" zones where Marginal planets could exist.) Thus, Type M's could be just a little too warm or a little too cool to be truly habitable T/ST worlds. But they would be very tolerable worlds and would be more hospitable to colonization to the traditional Type B (aka O2) planets and moons, and would support up to a Small population, though they would fall into the "non-habitable" category, since they cannot produce life. (And since it's my intention that "non-habitable" worlds cannot grow to simplify bookkeeping, populations on Type M's would also not grow except thru colonization.)
Crucis