Undergrad Can a planet have multiple large moons?

[Sherwood Botsford]

I thought that I'd posted this here, but perhaps not. I've had it on the OtherWorlds and the Physics stackexchange with no results.

I grew up with countless alien planet covers depicting 2-3 moons all considerably larger (telephoto lens...) than Luna's appearance from earth.

Luna from Earth subtends about 1/2 degree. If it were half the distance it would be 1 degree, and we'd have some ferocious tides. (Tidal forces go up as 1/r³, right, so 1 degree luna = 8 times present tides.)

Could Earth have two moons that subtend more than a degree?

Thoughts: visual size goes up as R² while mass goes up as R³ So a smaller moon orbiting closer results in less orbiting mass.

But with tides as 1/r³ it appears that tides are proportional to the visual angular diameter of the moon. Yes?

Now the other way to make bigger moons would be make them out of pumice. Styrofoam moons. Or really big balloons...

Anyway, I played with a orbital simulator and spent a rainy afternoon trying to find an orbital solution that had two moons, one with 1/80 the mass of the primary, one with 1/320 the mass of the primary, but at half the distance. Usually the little guy gets ejected within a dozen orbits.

What are the conditions for a planet to have multiple moons of significant mass?

 

[Filip Larsen]

I would say, as a first approximation and without having done the calculations myself, that if the moons orbit outside the Roche limit and maintain a stable orbital resonance then you may be able to construct a system with visually big moons. However, I guess that the chance of such a system forming by natural accretion would be (very) low as the system would be more likely to end up with rings and small shepherd moons.