[trackpick (OP)]

I know about planets orbiting stars, and moons orbiting planets. My question is if it's possible to go further than that: are there stable planetary systems that have large objects orbiting around moons? Or is the number of levels of orbit restricted to two? Thanks!

[Origin]

When the Apollo command module was in orbit around the moon we had such a scenario so we know that's possible.

[trackpick (OP)]

True. However, I was thinking of something of a more significant mass. If I've got my calculations right, the mass ratios are as follows:
earth/sun = 3.0x10^-6
moon/earth = 1.2x10^-2
apollo/moon = 1.6x10^-19

Could we have something orbit the moon that is 1/(1.2x10^2) the mass of the moon?

[evan_au]

Could we have something orbit the moon that is 1/(1.2x10^2) the mass of the moon?

The Moon/Earth system is very "similar" in size: The Moon is 1/80 the mass of the Earth. Some have called it a "twin planet"
- Probably due to the method of formation: The current popular theory is a Mars-sized body smashing into the Earth.
- But other ratios in our Solar system are more extreme:
- Mars has the tiny Phobos & Deimos
- Jupiter & Saturn have significant sized moons, but the planet is huge.

When we come to smaller objects:
- Pluto & Charon are fairly similar in size
- There are a number of double asteroids
- Some comets look like they were originally two objects which stuck together, with descriptions like "duck" and "snowman"

[Halc]

The ratio can be far less than that.  Charon orbits Pluto, and its mass is a whopping 12% of the primary.

Mathematically, there seems to be no limit to the number of levels for at least reasonable stability. True stability cannot be achieved in any 3+-body system.

One can add levels to the other side as well, say by having our sun orbit some larger star well away from our local solar system, but it needs to be quite far away to not disturb the orbits of the more distant things. So best way to get a lot of levels is to not have a lot of separate things (planets) orbiting any one thing.

[Halc]

I suppose a lot depends on one's definition of orbiting something else.
If two identical mass objects orbit each other, is that an orbit on your list, or does one (Earth) have to be larger than its satellite? If so, what is the smallest ratio of primary to secondary?

[Bored chemist]

It depends how long you want them to last.

[alancalverd]

Big fleas have little fleas
Upon their backs to bite 'em
And little fleas have lesser fleas
Et sic ad infinitum