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All celestial bodies lose atmosphere due to a portion of the gas "near space" exceeding escape velocity. The velocity distribution of an ideal gas can be found using the Maxwell-Boltzmann distribution. So an easy approximation for this problem is to say we only want $10^{-6}$ of the molecules to have escape velocity. Using oxygen at 300K, results in an ...

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If you take something like Neptune and pass it through Earth's orbit perpendicular to the ecliptic so it collides directly with the Earth-Moon system at, lets say, 0.1c you would remove the Earth rather quickly. Get another if you want to disappear the moon as well. Neptune isn't so big that it would disturb everything else, maybe a wobble in Venus or Mars ...

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If one of the rules to be a planet is that it needs to clear ALL objects from their orbit, does this also make Neptune a non-planet? This is a somewhat common misconception of the meaning of the term "clearing the neighborhood". None of the planets could be called "planets" if clearing ALL objects from the vicinity of the orbit was what that term meant. ...

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"To clear an orbit" has a specific meaning which may not entirely intuitive. "Clearing an orbit" specifically does not mean emptying an orbit of all other bodies. It means the planet gravitationally dominates other bodies at approximately the same distance to the sun. Now you can wonder perhaps whether Neptune dominates Pluto or Pluto dominates Neptune. ...

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Neptune actually is the dominant gravitational force in the region of the Kuiper belt in which Pluto resides. In fact, if you look at the image below, the belt is being cleared out by Neptune: In fact, there is a class of objects, suitably named the plutinos, that have been captured by Neptune. Solar system models have actually shown that Neptune was ...

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It depends what you consider a cube or a sphere. Is the Earth a sphere or do the mountains make it something else? As you do seem to understand more massive planets have more gravity, which shapes them into a sphere. This is because matter on the outside of the planet is attracted to the center. On Earth a loose rock is more likely to fall from the top of a ...

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There would be a slight restorative force from the gravitational pull of the atmosphere itself. First, imagine an ocean world with a very small core. If the core was more dense than the ocean it will be "sink" to the center. Similarly, if your "core" was a shell, it would also "sink" to the center. Thus, if you had both a sphere and a shell they would ...

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I think you are missing the positive feedback effects of winds flowing from high to low pressure areas as the shell moves. At best you would have a parachute effect to slow things down.

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Rev 2: Partially finished, but now I have Saturday chores to do. I'll finish filling in this outline later this evening. A number of planetary formation simulations from late in the 20th century to early in the 21st century suggested that Mars should be at most a bit smaller than Venus or Earth. This obviously is not the case. Explaining this is the ...

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