# Is it possible for a planet to have three moons?

If you have ever played an Elder Scrolls game and observed the night sky, you'd notice that there are two moons (most notable in Oblivion and Skyrim). They both orbit Nirn, the planet the game is based in. At a closer inspection, it is revealed that the smaller moon revolves around the large moon.

Which brings me to my question: Would it be possible for there to be one large moon with two small moons in close proximity to each other orbiting said large moon that orbits the planet? Or, would there be catastrophic outcomes like erupting volcanoes or tsunamis?

Let's say the planet is Earth but is a 1/3 the size, and the largest moon is a 1/3 the size of Earth's moon and then smaller still (because I've read Earth's moon is abnormally large).

• What sort of planet are we talking about, and how massive would these moons be? The gas giants have quite a few moons, for instance, and even Mars has two (with no ill effects). – HDE 226868 Feb 8 '18 at 1:03
• @HDE226868 Of course, Phobos and Deimos are teeny tiny … – can-ned_food Feb 8 '18 at 1:35
• Are you asking about moons with moons, or about moons with synchronized orbits so they appear to stay together? The question makes me think the former, but you accepted an answer that addresses the latter. – Monica Cellio Feb 8 '18 at 23:14
• The outer planets say yes. – Loren Pechtel Feb 9 '18 at 3:26

The simple answer is yes, it is possible for a planet to have three moons. Many planets in the Sol system have more than three moons. Jupiter has 69 known moons, for example. Neptune has 14.

Of the rocky planets, Mars has two moons, and depending on your specific definition of moon, Pluto has as many as five.

I can't speak to whether or not two smaller moons could be orbiting a larger moon. A system like that is theoretically possible, but probably highly unlikely to occur naturally.

With multiple moons, consider setting up some kind of orbital resonance. A good example of this are three of Jupiter's Galilean moons:

As long as your moons aren't too large compared to your planet, their tidal forces aren't going to cause catastrophe.

• Interesting! Because of your comment, I've concluded that I will have the three moons orbit the planet similar to Jupiter's Galilean moons. Would it matter which moon was closest to the planet if I had three different sized moons: a small, medium, and large? Preferably, I'd like the smallest closest to the planet, and the largest furthest away as I want there to be an extremely rare occurrence where all three moons and the sun are aligned. – Will Robinson Feb 8 '18 at 1:35
• So interestingly enough the mass of the orbiting body (moon in this case) is cancelled out. The formula is T^2 / R^3 = (4 * pi^2) / (G * M) where T=orbital period, R=distance between moon/planet, G=gravitational constant, M=mass of the planet. This means you can arrange your moons in any order of mass. If you want to create a laplace resonance like the one above, figure out what your orbital periods are for each of your moons, and then calculate the distance they would need to be from the planet from that. – Pasqueflower Feb 10 '18 at 1:03

The problem is a small moon in orbit of a larger moon.

Anything orbiting the large moon will be perturbed by the mass of the nearby larger planet. Rather quickly (in geologic time) the perturbations will crash the small moon into the large moon, into the planet, or fling the small moon out of the planet-moon system entirely.

You can place a space station or asteroid in orbit around the large moon. Just don't expect them to stay there.

Such perturbations are similary caused upon the orbit of the large moon by the Sun, too...but gravity's inverse-square relationship with distance means that solar perturbations are minor, even across epochs.

• That would actually look pretty cool. One day a large asteroid appears in the sky and looks like it will hit the planet, only to veer off and start circling the largest moon. For a few centuries the somewhat erratic orbit of the captured asteroid becomes normal and expected. Then just as suddenly as it appears it either crashes into the moon in a marvelous explosive crash visible from the planet, or shoots off into outer space never to be seen again. Lots of religious implications for it. – Dan Clarke Feb 8 '18 at 4:34
• Yes, totally different scenario with a moon orbiting a moon. – Kilisi Feb 8 '18 at 6:10
• True, although =this is probably why Venus and Mercury do not have moons, they would be unstable over billion-year timescales because of the proximity of the sun. – Andrew Dodds Feb 8 '18 at 8:32

The region in which the orbit of a satellite around a larger body is stable is defined by its Hill sphere. Just as you can have a planet with a moon orbiting a star (the planet is in the Hill sphere of the star and the moon is in the Hill sphere of the planet) you could have a planet with a moon where the moon itself has its own satellite.

The formula for the Hill radius is

rH approx. = a(1-e) * cube root (m/3M)

The Hill Sphere actually defines the region in which the body dominates the attraction of satellites, but is only an approximation: the true region of stability confines stable orbits to within 1/2 to 1/3 of the Hill radius.

You can play with some masses for your Nirn, Jone and Jode and obtain some radii that would work. The formula does assume the satellite is much smaller though so as not to introduce additional complications through its own gravitational pull, so although the smaller one may transit in front of the larger one they will not appear to eclipse each other in the way that Jone and Jode are sometimes depicted as doing.

If you're considering binary moons that are close to equal in mass then the Hill radius formula no longer applies and you would have a 3 body problem which would likely require simulation to test an orbital arrangement for stability.