Getting Moon(s) to orbit a set of binary planets orbiting a single star.

[Pixelfck]

I succeeded in getting a set of binary planets in a stable orbit around a k-type star (thanks to Omnigeek6).

However, I now again feel quite stupid because I get any moon to orbit the pair of planets.

It is my intention to make at least two moons orbit around the two planets. Ideally, one of the moons would be in circumbinary orbit and the other one (the smaller of the two) should orbit the heavier of the two planets.

Whatever I try though, the moons just won't stick around, they either take of in various directions or settle in an orbit that is close to the orbit of the binary planets.

Any help is appreciated,

Pixelfck

[Omnigeek6]

Don't bother with trying to make a moon orbit one of the planets in a binary. Unless your planets are very far apart and very far from their star, they will tidally lock each other. This means that any moon orbiting one of the planets will be orbiting faster than its parent rotates, and tidal interactions will cause it to spiral inward and collide with its parent.

As for the circumbinary moon:

1. After you add a barycenter for the planets, place the moon.
2. Select the moon and the barycenter.
Click "make binary orbit" followed by "add barycenter."
3. Select the new barycenter (planets + moon) and the star, then click "make binary orbit."

If the planets stay in orbit but the moon is ejected, it was most likely placed in an unstable orbit. If it's too far in, perturbations from the planets will chaos its orbit to vary chaotically until it collides with one of the planets or is ejected. If it's too far out, perturbations from the star will again cause chaotic variation in its orbit, eventually raising its apoapsis outside the planets' hill sphere and allowing it to escape.

For a circumbinary moon, I would use the same rule of thumb as for a circumbinary planet; the minimum separation of the moon and barycenter should be at least five times the maximum separation of the planets. The apopasis of the moon should also be less than half the hill sphere radius of the planets (and even half the hill sphere radius can lead to wonky orbits).

[Pixelfck]

Thanks for the clear answer!

You also seem to understand the physics behind the problem. I've been reading about barycenters, hill spheres, roch limit, orbital lock radi, etc. But I've not found any (online) resource that gave me any insight in the math. Would you maybe know any resource that could give me a kickstart?

Cheers,
Pixelfck