How stable (or possible) would binary moons be, (Luna-sized or possibly slightly smaller) rotating around an earth-sized planet?

Edit: Thanks for the questions! And thanks for the welcome! My apologies for not being more detailed. I want them both to be the same size approximately, look about the size of our moon, (so if they're smaller, they'd be closer) rotate the earth-sized planet at about the same rate our moon does, with a 28 day cycle from full to full, and they would orbit each other around a central point, but not so that they would eclipse each other; just so that at one time, one would rise before the other, at another time the other would rise first.

I'm hoping that such a situation is plausible.

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    $\begingroup$ Welcome to the site, L. Evans. Are you asking about an Earth-sized planet having two moons with unique orbits (question linked by HDE), or two moons orbiting a central point and that point orbits an Earth-sized planet? $\endgroup$ – Frostfyre Feb 27 '16 at 18:19
  • $\begingroup$ Do you want the moons to be where ours is now? Do you want them to be far away enough to only have the gravitational pull on earth that ours has now? (Nice question, by the way.) $\endgroup$ – Xandar The Zenon Feb 28 '16 at 2:36
  • $\begingroup$ Have a close look at the Pluto system. Pluto is smaller than our moon, yet it has a set of moons of ts own. Charon is larger in proportion to Pluto than our moon is to Earth. The orbital mechanics of so many moons in a small system should give you some ideas of what is possible. $\endgroup$ – pojo-guy Dec 30 '17 at 3:21

They can definitely be stable with respect to one another. In fact, there are several possible configurations you can put two moons in order to gain this sort of stability. Here are a couple of them:

1: Moons in separate orbits

The trivial solution to this is to put your moons in separate orbits far enough apart that they don't interfere with one another. The moons around the gas giants mostly behave like this.

2: Moons form a binary pair

If your moons are significantly closer to one another than they are to your planet, they're likely to form a stable binary pair, orbiting their common center of gravity. These moons would likely be tidally locked with each other, rather than the planet. They'd also likely need to be further from their parent planet than Luna is from Earth, since if they orbited too closely, the gravity of their parent planet would likely throw off their orbital pattern.

3: Moons form a horseshoe orbit relative to one another

It's also possible for similarly sized bodies to stably orbit a planet while sharing an orbit if they form a horseshoe orbit with one another. Epimethius and Janus, two moons of Saturn, are good examples of this sort of system. From the surface of the planet, your moons would appear to vary in distance over time, from a position fairly close to one another in the sky to one in which they were on opposite sides of the sky. They'd oscillate periodically between these extremes without ever touching one another. They'd also vary in size, taking turns appearing to be the larger celestial body. This would no doubt lead to lots of interesting stories in the mythology of your planet dwellers.

4: Moons which sit in each other's Lagrange points

Specifically, we're talking about the $L_4$ and $L_5$ Lagrange points. These points are dynamically stable, so barring any particularly large perturbations, two moons should be able to sit in those points relative to one another. These systems are less stable against perturbations when the bodies are similarly sized to one another, so if there are other planets in the solar system, one of the moons may be ejected from its orbit after a period of time.

  • $\begingroup$ Orbital resonance can also stabilize Moon systems. Io, Europa and Ganymede are a good example of a 1:2:4 orbital resonance. $\endgroup$ – Stephanie Jul 16 '16 at 21:23
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    $\begingroup$ Most of these would not be stable in the scenario given. An Earth like planet and two 1/81th as massive moons is a tricky gravitational system. Our moon is simply too big relative to the Earth to be gravitationally neutral to another moon. Your scenarios are all possible when both moons are tiny compared to the planet but not possible in the scenario given in the question. $\endgroup$ – userLTK Mar 16 '17 at 22:21

It would depend on where your moons are in relation to each other and to the planet. The size also matters: if you got them in the right place and the right size, you might be able to get a planet with a stable orbit.

If this did not happen, the moons were not in the right distance proportional to your planet and each other; in that case, you would have very unstable plant seasons, and tides could change suddenly, perhaps in some cases completely erratically. I might be able to give you more details, but without exact knowledge of your moons' sizes, their distances from each other and from the planet, and the size of the planet, the possible outcomes are just be too numerous to list.

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    $\begingroup$ This is a glorified comment. Please do discuss the possible repercussions of the question's posited situation once it had been clarified. $\endgroup$ – Tamoghna Chowdhury Feb 28 '16 at 15:36
  • $\begingroup$ All of the possible complications you mentioned would be great things to flesh out in an answer. If you were to go into detail about what sorts of distances led to what problems with a system, I'd happily give you an upvote. $\endgroup$ – ckersch Mar 15 '16 at 21:51

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