Earth has one very large, spherical moon believed to have been formed by a giant impact. Mars has two small, sorta potato-ical moons believed to have been captured. Suppose an otherwise earthlike planet started with one large, round moon like earth's, and then captured 4 small, potato-y moons like Mars', how would those moons need to be arranged to maintain orbital stability?

I'm interested in:

  1. Whether the large and small moons would all be in orbital resonance with each other, like Jupiter's inner moons
  2. Whether the moons would need to orbit in a certain order (e.g., big moon on the inside, little moons on the outside, or vice versa)
  3. Happy bonus: whether these moons could all be close enough or large enough for a person on the planet's surface to see their phases with the naked eye
  4. Sad bonus: whether this whole notion would actually be impossible and why

1 Answer 1


Keep your distance

Don't count on resonance I would say, this is a many body system. Keep their orbits apart.. far apart. Chaotic systems rely on statistics. The more these moon orbits are apart in altitude, the safer your planet will be.

Put the big one on the outside

I haven't calculated this ("science based") but my guess: the safest constellation will be to have the big moon sitting on the outer orbit, the small ones inside. Whenever the big one would disturb a small one's orbit, it will not start spiraling toward the planet, its orbit would be slightly widened.

Sad bonus: Earth is too small for too many

Gravitational gradient around earth does not reach far, it's not like Jupiter or Saturn, where many moons can exist in wide orbits, safely isolated. Mars has two moons, but these are much smaller than Earth's moon. You want to keep that, and add more moons. Maybe sensible to add only one..

Happy bonus: you can choose inclination angle !

Some moons are caught by a planet. Their orbit could have any angle in respect to the ecliptica of the solar system. Suppose some of your moons have a very large inclination angle, they would rarely be disturbed by the other moons.. and close enough to watch them.

  • 1
    $\begingroup$ Thanks for answering! Can you explain a bit what you mean by "chaotic system"? What makes it chaotic? Do you have any thoughts on what max capacity would be for earth's gravitational gradient? $\endgroup$
    – RLoopy
    May 21, 2022 at 4:02
  • $\begingroup$ Any system containing many (>2, in theory) celestial bodies will behave unpredictable in the long run. When the distances are large and the moons are small, that won't be an issue. The nearer the bodies are, the more risk of collisions. A nice read connecting gravity to chaos is this page: space.com/chaos-theory-explainer-unpredictable-systems.html $\endgroup$
    – Goodies
    May 21, 2022 at 8:16

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