Q: Are there known examples of how a planet can have one large moon, a smaller one, and another further away? I know that we have a 1:1 Mars to Phobos resonance and a 3:2 Jupiter to Callisto resonance (though there is a 4:3 resonance too). But are there other examples of planets with 1 large moon and another further away? I can't find any, but I am just curious because it seems to me that you would need two massive bodies to have a decent resonance with each other, otherwise you could have a big moon and a small moon, neither of which would be big enough to provide the extra gravitational pull to keep the smaller moon in place. Could this happen or have I simply forgotten something? A: It appears that your reasoning is correct, and that would imply an extreme instability, at least for the current planetary architecture. The current orbital arrangement of the outer moons of Jupiter are thought to be the result of resonances, including the Io - Europa 3:2, Europa - Ganymede 2:1, and Callisto - Dione 5:2 resonance. If the Jovian satellite system had started out as a single body of twice the mass of Jupiter and with slightly different initial orbital eccentricity and inclination, the system would have quickly been destabilised due to the eccentricity resonance instability. A: Yes, there are others. Mercury has one large moon (which is not in a resonance with Mercury) and another moon (which is in a 3:2 resonance with Mercury). Saturn has two moons: Rhea (a large moon), and Tethys (a small moon). Jupiter has five moons. Callisto and two larger moons which are in 2:1 resonance with each other, and two smaller moons which are in 3:2 resonance with each other. A: I had the same question about the outer moons of Saturn, but found that there are two ways to construct a double moon system without going through an eccentricity resonance. There is a resonance between the rotation of the two large moons and their orbital precession around Saturn. This makes the size ratio 1:2. As you can see there are two different approaches to the orbit of the outermost moon. The two moons that orbit Saturn are in a 3:2 resonance. Source: https://en.wikipedia.org/wiki/Saturn The two smaller moons are in the size ratio 3:2. Source: https://en.wikipedia.org/wiki/Jupiter#Moons Orbital stability can be achieved by the moon's orbital inclination with respect to the equator and its orbit around the planet. Source: https://en.wikipedia.org/wiki/Himalia#Himalia_%28moon%29 I found another planet (Planet HD-209458b) with one large moon that is in a double moon resonance with a larger moon. See this link for an animation of the moons in motion. Source: https://www.youtube.com/watch?v=KFgBfNZ9Ia4 More information about this planet can be found in this wikipedia article: https://en.wikipedia.org/wiki/HD-209458b The orbital period of this planet is about 6.5 days. For this planet the moons are in a 3:2 mean-motion resonance. This means that the two moons orbit at 3/2 times the distance around the planet. This animation from wikipedia shows the moons with period 4.2 years: The moons have been discovered by the Hubble Space Telescope: According to this youtube video the two moons are in a circular orbit and in a spin-orbit resonance: So the smaller moon is in a orbit with a 4.7 days orbital period and rotates with a 5 days period. The larger moon is in a circular orbit with a period of 8.4 days. This indicates a slower rotation rate than the smaller moon which has a spin period of 9.2 days. This might indicate that the distance of the larger moon is slightly larger than that of the smaller moon, which would make them travel around the parent planet in different distances. See: https://en.wikipedia.org/wiki/Himalia#Himalia_%28moon%29 This is an overview of some of the dynamics of the three moons orbiting this planet. See also: https://en.wikipedia.org/wiki/Jupiter#Moons A: Your intuition is correct, however there is nothing special about a 2:1 resonance. The most important parameter for the stability of such a system is the ratio of the orbital eccentricities of the two moons, which should be less than 0.5. It is possible to construct a stable system with two moons, one much bigger than the other, with a mass ratio as high as 2:1. However, in such cases, the two moons are in resonances with each other and the system will be unstable if left to evolve. For more information, see: https://en.wikipedia.org/wiki/Jupiter%E2%80%93Io_resonance https://en.wikipedia.org/wiki/The_Twelve_Planets_of_Europa https://en.wikipedia.org/wiki/Three_Dos_Juan_Moon https://en.wikipedia.org/wiki/Rhea_(moon) https://en.wikipedia.org/wiki/Rhea https://en.wikipedia.org/wiki/Tethys_(moon) https://en.wikipedia.org/wiki/Io_(planet) https://en.wikipedia.org/wiki/Titan_(moon) https://en.wikipedia.org/wiki/Io https://en.wikipedia.org/wiki/Europa_(moon) https://en.wikipedia.org/wiki/Titan https://en.wikipedia.org/wiki/Metis_(moon)