Reality Check: Habitable moon around earth-like planet

For a complex world-building scenario I want a habitable moon orbiting a habitable earth-like planet. The habitable moon should allow for flora, fauna and landscape as similar to earth as possible. The same goes for the habitable earth-like planet it is orbiting.

I have checked the various topics concerned with habitable moons:

• What would a habitable moon most probably look like?
• Habitable moon of a gas giant: working out the sizes and distances
• Naturally making a gas giant moon habitable
• How would an earth-like planet with a habitable moon work and how to get there?

Based on what I read there, especially Jim2B's answer here, I have created the following moon with this calculator:

• mass: 0.33 of earth's mass (around 3 times the mass of mars)
• density: 1.3 of earth's density (because I want a higher surface gravity)
• radius: 0.6332 of earth's mass (calculated by the application using mass and density as a given)

This is all calculated by the program:

• Diameter = 8070 km
• Density = 7.176 g/cm³
• Surface Area = 204.5 million square km
• Roche Limit = 1000 km (nearest possible natural satelite)
• Surface Gravity = 0.83 Gs
• Geosynchronous orbital distance = 24820 km, or miles (from surface of planet)
• Geosynchronous orbital velocity = 2.13 km/s , or miles per second

Maximum surface* temperature to hold onto an atmospheric component for billions of years, for each type of gas:

• Carbon Dioxide? 2972 °C
• Oxygen? 2087 °C
• Helium? 22 °C
• Hydrogen? -126 °C

Could such a moon possibly exist? And if not what changes would be needed to make this moon around an earth-like planet possible?

Please consider the following points particularly:

• Density of the moon: I need it as high as possible to have the surface gravity of the moon approximate 1 g. Playing around with the calculator I have settled on 1.3 times the density of earth, which gives a surface gravity of 0.83 Gs. This means a density of 7.176 g/cm³. Is this density achievable by still keeping a similar elemental composition to earth's? If not, could that be achieved by replacing some of the iron with a denser element? What properties would such an element have to have?
• Atmosphere: Could this moon sustain the necessary atmosphere considering the surface temperatures?
• Earth-like planet the moon is orbiting: This planet has the roughly same mass and radius as earth. If a larger mass and/or surface is necessary to have such a moon, that would be ok, as long as the density of the earth-like planet could be lower to keep the surface gravity of said planet at 1 G.
• Distances between planet and moon: No specific requirements. Can be anything to make this planet-moon relationship work.
• Size relations between planet and moon: Apparently accretion disk formations would make such a large moon compared to the planet unlikely, but would it be impossible? Could there be any other scientifically explanation for a planet having such a large moon, for instance a "rogue moon" captured by the planet (Theia captured instead of a collision), the moon being debris from the planet itself or something else? This alternate explanation can be unlikely, as long as it is scientifically sound and possible at all.

posted over 7 years ago

DerGreif‭

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