In my fictional world I have selected all parameters to maximize the Hills Sphere of my planet. The planet has three times Earth's mass and it is located in 2 AU from its star that's 1.4 times more massive than our Sun.

I want to have three moons on a 1:2:4 resonance. I also want:

  • the first one to be as large as our moon to the sky
  • the second 1.5 times larger
  • and the third, half our moon's size.

To achieve this I have assumed:

  • the first moon having 1.1 lunar radius,
  • the second 2.3
  • and the third 1.1.

Their orbits are 18, 36, 72 days.

I don't really care about their density and I assumed that their masses are

  • 1 lunar mass (probably rocky)
  • 3 lunar mass (water or ice world)
  • 0.3 lunar mass (probably ice world)

This is my effort to achieve the stability of the system, what parameters should I reconsider to make it more stable? I am willing to change almost everything even the resonance, but I want them to be visible from my planet in the way moon is to Earth.

So any ideas?

  • $\begingroup$ Wouldn't this be better to ask over at astronomy? astronomy.stackexchange.com $\endgroup$ Jun 20, 2016 at 19:15
  • $\begingroup$ I am new here and i wasn't sure.If its more suited there I can do it. $\endgroup$
    – teorf
    Jun 20, 2016 at 19:17
  • 1
    $\begingroup$ @MartineVotvik Questions about the stability of planetary systems are fine here; we've had loads of them. They can be hard to answer, but in general, they're fine. $\endgroup$
    – HDE 226868
    Jun 20, 2016 at 19:21
  • $\begingroup$ @HDE226868 ah, fair enough. It's a but tricky to learn which questions should be directed elsewhere or not. I can see how it would be directed back here as well. $\endgroup$ Jun 20, 2016 at 19:37
  • 1
    $\begingroup$ Thanks for reminding me about Universe Sandbox. And by "thanks" I mean I won't get to sleep tonight. 4 teraflops CUDA card, 3 monitors, on my "work" computer. $\endgroup$
    – JDługosz
    Jun 21, 2016 at 8:24

1 Answer 1


You propose a fairly simple system.

On the other hand, you said nothing about inclination and eccentricity! Maybe the orbits intersect and will crash on the 3rd lap!

If everything is circular, in the same plane, in the same direction, and indeed you don’t have any other planets, than I suppose it would be stable.

The rather large moon will make the central mass of the planetary system lumpy, but being in perfect resonance will mean that everything repeats each time around, so if it works at all with those times it will be stable.

Still, ask an astrophysicist or use a numeric simulation to check.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .