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Define a bound orbit chain as a list of successively less massive bodies, each in a bound orbit with the bodies preceding it in the list.

Then an example of a bound orbit chain would be: <Sun, Earth, Moon>, since the Earth orbits the Sun, and the Moon orbits the Earth and the Sun (and they are listed from most to least massive). This is a chain of length 3.

Question: What is the longest natural bound orbit chain we've ever observed?

Related: Do moons have moons?

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    $\begingroup$ The question is inherently ambiguous, because the n-body problem is unsolved in general for n>2. So for example it's perfectly possible that the solar system will eject some of its planets at some point in the future. The notion of being bound like this is at best approximate or time-limited or probabilistic. For example, our galaxy is expected on statistical mechanics grounds to evaporate spontaneously on sufficiently long time-scales, and this does not violate conservation of energy as suggested by the word "bound." $\endgroup$
    – user15381
    Apr 1 at 13:39
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    $\begingroup$ related: worldbuilding.stackexchange.com/questions/26773/… $\endgroup$
    – Rick
    Apr 1 at 13:55
  • $\begingroup$ Also the sun orbits the galaxy? $\endgroup$ Apr 1 at 18:23
  • $\begingroup$ @BenCrowell Feel free to post an answer that the longest bound orbit chain can be n=2, since all other orbits are inherently unstable. If you can convince me, I'll pick your answer! $\endgroup$
    – Connor Garcia
    Apr 1 at 22:36
  • $\begingroup$ @Rick Thanks for that link - and for your detailed answer to that question. It was very impressive and I love the way StackExchange encourages these sorts of detailed and well-researched answers. $\endgroup$ Apr 17 at 11:41
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Certainly 4 (perhaps 5):

The longest certainly-known chain is "3": Sun-Earth-Moon.

Rhea (a moon of Saturn) has been observed to have a ring of orbiting material: That gives Sun-Saturn-Rhea-Ring for four levels (including unresolved dust)

Looking further afield:

DH Tauri and DI Tauri are a binary pair of similarly sized T-Tauri stars (both small red dwarfs); they are in a mutual orbit.

DH Tauri has a large sub-stellar companion.

The companion has an accretion disc of orbiting matter.

So we have DI-Tauri > DH-Tauri > DH-Tauri-b > matter in disc, for four levels of orbit.

Even this isn't very good, because DH Tauri is bigger than DI Tauri, so it is a bit of a stretch to say that DH Tauri orbits DI. If anything it is the other way round. There are very few planets found in orbit around the secondary star in a system and none of those have likely moons. Moreover the disc of matter around the sub-stellar object is not known to have actual "moons" embedded in it.

So "4" seems to be the best we can do...

But if we can add in Galaxy - Star - planet -moon - dust/ring? we get 5 levels.

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  • $\begingroup$ It definitely could be 4: Sun-Earth-Moon-space dust. But maybe that's a bit of a stretch, as space dust is small, and they aren't very significant in the terms of the question. $\endgroup$ Mar 31 at 21:45
  • $\begingroup$ @fasterthanlight there isn't a size restriction in the question. I would certainly accept space dust if you can show its been observed to orbit the Moon. $\endgroup$
    – Connor Garcia
    Mar 31 at 21:54
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    $\begingroup$ We can add in Galaxy - Star - planet -moon - dust? .... to get 4 (or perhaps 5) $\endgroup$
    – James K
    Mar 31 at 22:17
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    $\begingroup$ Galaxy is totally allowed by the question! Can we go even bigger (or smaller)? $\endgroup$
    – Connor Garcia
    Mar 31 at 22:24
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    $\begingroup$ Why would the galactic centre not count? Is there some debate that it's a collection of matter orbiting a gigantic mass? $\endgroup$
    – J...
    Apr 1 at 12:13
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Short Answer:

It might be possible to have a chain of as many as twelve objects, but that depends on how many of the objects in that chain actually have stable orbits around other objects in the chain.

Long Answer:

My first guess would be something like:

Virgo Supercluster of galaxies > the gravitational center of the Local Group of Galaxies > The Milky Way Galaxy > The Sun > Saturn > Rhea > particle in dust ring, for seven levels.

A more general example of seven levels would be:

Virgo Supercluster of galaxies > the gravitational center of the Local Group of galaxies > The Milky Way Galaxy > a star > an exoplanet > an exomoon > a moon of an exomoon. Moons of moons are considered to be dynamically improbable but possible, and so there should be many in such a vast galaxy as the Milky Way Galaxy.

I note that the Virgo Supercluster of galaxies is considered to be part of the Pisces-Cetus Supercluster Complex, and so might possibly orbit the gravitational center of the Pisces-Cetus Supercluster Complex. If so, it has probably completed only a fraction of a single orbit during the history of the universe.

I note that the center of the Virgo Supercluster of Galaxies is believed to be in the Virgo Cluster of Galaxies, and the center of the Virgo Cluster of Galaxies is believed to be the great galaxy M87, and the center of the M87 Galaxy is believed to be a supermassive black hole.

I note that stars usually form in open star clusters which dissipate due to the gravity of other objects after a hundred million years or so. Thus many young stars and their planets might be orbiting the gravitational centers of open star clusters, probably completing only a few orbits before the clusters dissipate.

I note that star systems often contain two or more stars, which orbit around their common center of gravity. If one star is much more massive than the other one(s), the less massive star(s) might be considered to orbit the more massive star.

So I hypothetically suggest the following possible chain:

Whatever astronomical object might be in the gravitational center of the Pisces-Cetus Supercluster Complex > the supermassive black hole at the center of M87 > whatever (dark matter?) may be at the gravitational center of the Local Group of Galaxies > the gravitaional center of the Milky Way Galaxy with it's supermassive black hole > Whatever object might be in the gravitational center of a young open star cluster > a massive young star > a much less massive star orbiting it > a brown dwarf orbiting it > a giant exoplanet orbiting it > an exomoon orbiting it > a moon of a moon orbiting it > a dust mote orbiting the moon of a moon.

And that produces what might be the ultimate possible chain ending up with objects in our galaxy. Possibly in distant other parts of the universe there could be h chains with more levels. However, the chain might be much shorter depending on which of the orbits actually would be gravitationally stable.

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    $\begingroup$ Does the Milky Way orbit around anything? Can you defend your first guess with any evidence that our galaxy and larger structures have defined orbits around anything? $\endgroup$
    – J...
    Apr 1 at 12:18
  • $\begingroup$ As per the previous answer to this question, there is dispute as to whether Rhea actually has anything orbiting it. See Wikipedia for more. $\endgroup$ Apr 17 at 12:34

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