# Could two moons orbit each other around a planet?

Let's say that there is a planet orbiting a star. Would it be possible for two moons to naturally orbit each other, while orbiting the planet? Also, how would other planets and the host star affect that orbit, and would one of the moons eventually crash into the planet or fly off into space?

• astronomy.stackexchange.com/questions/856/do-moons-have-moons Commented Jul 4 at 14:19
• en.m.wikipedia.org/wiki/Stability_of_the_Solar_System A three+ body problem is hard to solve, and usually one of the planets will eventually get flung out. Mercury being an interesting example: "with 1–2% probability, 3–4 billion years into the future. This could eject it from the Solar System or ..." So yeah, depends on your parameters, and even then we'll usually call it "stable enough" instead of "stable". Commented Jul 5 at 18:28

A binary or double star is two stars which orbit around their common center of gravity.

A multiple star system would have three or more stars orbiting. A four star system might have two binary stars orbiting their common center of gravity.

A tripe star system might have a binary star plus a single star.

If the total mass of the binary is several times the mass of the single star, it could be said that the single star orbits the binary.

If the mass of the single star is several times as great as the mass of the binary, it could be said that the binary orbits the single star.

Now imagine a similar system, with the binary star replaced by a binary planet with much less total mass. In that case the binary planet could be said to orbit around the single star.

Now imagine that the binary planets are much smaller, and that the single star is replaced by a large planet with several times the total mass of the binary planet. In that case the binary planet would be defined as a binary moon.

If a system where a low mass binary star orbits a higher mass single star is possible, then an analogous system where a low mass planetary mass object orbits a higher mass planetary mass object should also be possible.

And how stable the arrangement would be would depend on the distances, orbital speeds, and masses of the three objects, the planet and the binary moons, as well as other objects in the star system, such as one or more stars, other planets, etc.

There are probably websites were you can create an arrangement of astronomical objects and see how stable it would be.

• The Moon is so close in size to the Earth that the Earth-Moon system is sometimes considered a binary system, so this is kind of an example of what you're describing. Commented Jul 4 at 18:09
• @Barmar for the Earth and Luna to be a binary system, I'd expect them to be much closer in mass, with their barycenter far out in space, rather than the Lower Mantle. Commented Jul 5 at 2:06
• @Barmar, Earth and Moon is generally not considered as binary system, but Pluto and its moon Charon is generally consider a binary pair. Commented Jul 5 at 13:37

A small moon, located at a L4 or L5 point formed from a parent planet and larger moon, is in effect an orbiter about both larger objects simultaneously.

The Saturnian moons Tethys and Dione have such companions at both L4 and L5 of the respective moons, making the four small moons Lagrange-point co-orbiters.

Earth's moon does not have a specific trojan moon associated with it, but the Kordylewski clouds were confirmed via polarimetry in 2018.

Yes, it is totally possible, I guess you are referring to the situation called submoon, there are potential candidates, but right now there is no confirmed case.

But if you consider Sun as a central object, thus all the planets orbiting around can be called "moons", thus the moons of the planets can be called "submoons". As long as the mass ratios are small enough and there is no other massive body nearby, they will orbit each other just like how we expect them to orbit. (I thought this may make sense, if it doesn't please delete it)

• No, that is not two submoons, that is a double moon or binary moon. Commented Jul 4 at 13:44
• @M.A.Golding And what's the difference? It's not like they would be identical. There's still larger and smaller. All pairs, like star - planet, or planet - moon, orbit common center of mass. Commented Jul 4 at 14:10
• That wikipedia page seems to talk about the exact same setup OP is asking about. Can you have the 4 levels: star - planet - moon - 'moon moon'? In theory yes, we don't know any real world example, it is considered unlikely but we know fairly little about any satellites outside of the solar system. Commented Jul 4 at 15:29

If you replace the question:

"Would it be possible for two moons to naturally orbit each other, while orbiting the planet?"

...with the equivalent question:

"Would it be possible for two objects to naturally orbit each other, while mutually orbiting a third object?"

...then, yes, we see this happen in all cases where "planet" has one or more moons.

In the case of Luma and Terra, the mass of the former is about 1.2% of the latter, so the barycentre (centre of mass of the two objects) is about 1700 km under Terra's surface. e.g. see here.

Because of the large difference in masses, we visualise Luna orbiting Terra and Terra orbiting Sol, but in fact both Luna and Terra orbit their mutual barycentre. Similarly, if only Sol, Terra and Luna are considered the Terra-Luna system obits their barycentre with Sol which is deep inside Sol at all times.

However, considering the whole Sol-ar system, it's barycentre moves as planets orbit, and it is sometimes outside Sol's surface, but often inside it.