I'm currently working on a sci-fi story, which largely revolves around a single world (which the current name for is Thea, you'll see why when I describe it...). Okay, I understand I've got some hard questions, so I've cut them into 5 labeled sections. Any help is vastly appreciated! :) #1. - Planetary formation and dynamics This Thea is the moon of a gas giant, Salacia (Well, I say gas giant, though it has a significant rocky core)... I had to stretch some numbers to get a desired effect, but I want to know if this is too unbelievable... Parent Body Mass: 6 Earth mass Thea mass: 0.295 Earth mass Thea Orbital Semi-major axis: 2,187,982 km Orbital period: 152 days I realize it's a large orbit, for a very large moon. It's around Tau Ceti, a very old star system, so I wonder if an Earth-Moon sort of relationship wouldn't lead to this, except the body that hit Salacia to form Thea hit in such a way that the moon is in a higher orbit? Could this, coupled with the fact that it's retrograde orbit would slowly expand it's orbital altitude, plausibly lead to this extreme orbital altitude? #2. - Orbital stability Also, the Salacia-Thea system needs to orbit Tau Ceti at the outer range of the habitable zone. I'll do all the hard work of calculating gravitational strengths at different distances, but at what point is the moon orbiting so far out that the sun would make the orbit unstable? (i.e., if 90% of the gravitational force a moon experiences is from the planet it orbits, it will most likely be stable. But if 99% of the gravitational force it feels comes from the star, wouldn't that de-stabalize it's orbit around the planet? If so, at what % does it's orbit start becoming unstable?) #3. - relationship of pressure and density under compaction from gravity This is probably the most interesting one: the relationship in-between atmospheric density and pressure. So, IIRC, I think human tolerance for atmospheric pressure is ~6 atm max... Thea's gravity is 74.5 % G. So if the atmosphere has 6x as much density at the surface, would it only have ~4.47x as much pressure? (Because pressure comes from gravity compacting the air, is the relationship proportional or exponential?) #4. - gasses under compaction of gravity. And now orbital altitudes... On Earth, ~140 km is the lower end of a safe orbit. So... I need to know what the lower end of a safe orbit would be over Thea, and my question is; are my assumptions correct, close to correct, or just plain wrong?: So, in order to get 6x as much atmospheric density at the surface, would I need 36x as much atmospheric mass, because air pressure trying to expand into space would resist the compacting force of gravity? And so then, would the atmosphere stretch 6x higher because of the increased mass(Since 6 is the square root of 36)? (setting the low orbit for Thea at 840 km) #5. - aerodynamics... Say, an aircraft has a stall speed of 212 mph on Earth (stall speed of shuttle), what would it's stall speed be on Thea, which has 6x as much atmospheric density, 0.745G, and 4.47x atmospheric pressure? I'm thinking... Would it be: 212 * 0.745 / (0.5 * (6 + 4.47)) mph? That is, I'm multiplying gravity in G's, and dividing the average of atmospheric pressure and density. Big thanks to anyone that replies! It's quiet an undertaking! And I'm sorry I couldn't explain why I'm making things certain ways, I think the idea may possibly be unique, so I don't want to give up trade secrets :P Once again, many thanks to anyone who takes up the challenge!