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Could a star system exist where a body orbiting the star has an inhabited planet moon orbiting it instead of the sun? And the inhabited planet moon has its own smaller satellite.

I have very little experience with this kind of world building, so thank you in advance.


Edited for terminology: In a nutshell, if it orbits the star and it is large enough, it is a planet. If it orbits a planet, it is a moon. A planet does not have to be inhabitable and a moon does not have to be uninhabitable.

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    $\begingroup$ This might just be semantic, but isn't a moon an object that orbits a planet? If it just orbited the sun, I think it would just be called a planet or dwarf planet. Are you asking if a larger density object in space can orbit a smaller density one? $\endgroup$
    – Coquí
    Sep 3, 2023 at 23:23
  • $\begingroup$ Ah, I see, thank you. I guess I didn't understand what 'moon' meant exactly. $\endgroup$
    – smunji
    Sep 3, 2023 at 23:51
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    $\begingroup$ By definition a moon is an object orbiting something that is not a star. So what you're calling a moon would actually be a planet. If you're asking whether a planet could have a moon that is much larger than itself, it is possible, but the planet would need to be far denser than the moon, to a point which isn't really feasible in reality, but maybe there's a magical explanation. If you're asking whether moons can have moons, I think it's possible just uncommon since it introduces more instability into the system. And there are some rules that have to be obeyed such as the Roche limit. $\endgroup$
    – Tamrak
    Sep 4, 2023 at 0:03
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    $\begingroup$ Hello @smunji, welcome to Worldbuilding. If you search this site, you'll find many questions about habitable moons (which is what I believe you're asking about) as well as questions about moons that orbit moons that orbit planets (e.g. 1 and 2). If, indeed, you're asking about habitable moons, then this question will be closed as a duplicate. If that's not what you're asking, please edit the post to clarify it. Thanks! $\endgroup$
    – JBH
    Sep 4, 2023 at 1:27
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    $\begingroup$ @Tamrak This was a regrettable answer. The moment you posted it, the OP couldn't delete their question to propose a better one. Any change the OP makes to clarify their question obsoletes your answer, which is frowned upon here. Please ask clarifying questions (which is all you've actually done) as comments and not as questions. Flagged as not an answer because you're admitting the question is missing details. $\endgroup$
    – JBH
    Sep 4, 2023 at 1:29

3 Answers 3

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Yes, it is possible for a star to have an orbiting body (a planet) which has a habitable body as its satellite (a moon), and it is possible for such a habitable body to have its own satellites (the moon's moon). It is even possible for the satellites to have their own satellites, though rather unlikely.

The more satellites of satellites a system has, the more unlikely it is... though consider that technically a star may be the satellite of the super-massive black hole at the centre of its galaxy.

If the star is the largest body we are considering, the body orbiting the star will be the next largest (discounting other bodies in orbit around the star), the habitable body will be smaller again, and the body orbiting the habitable body will be yet smaller still.

If bodies approach the same mass, it can no longer be said that one orbits the other, but rather that they orbit each-other, or more specifically, they orbit a common barycentre. Thus, the difference in mass between primary and satellite must be sufficient that the barycentre is within the primary for it to be considered to be truly orbiting the primary.

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  • $\begingroup$ Yes, this was what I meant, a habitable satellite and not a planet directly orbiting a star, similar to Earth. Thank you for your answer. $\endgroup$
    – smunji
    Sep 5, 2023 at 2:34
  • $\begingroup$ Is the barycentre just the centre of mass? $\endgroup$
    – TKoL
    Sep 6, 2023 at 8:40
  • $\begingroup$ @TKoL Have a look at my edit and the link to the definition of Barycentre. $\endgroup$
    – Monty Wild
    Sep 6, 2023 at 8:43
  • $\begingroup$ So Jupiter is not truly orbiting the sun? $\endgroup$
    – TonyK
    Sep 6, 2023 at 10:51
  • $\begingroup$ @TonyK Apparently not. If it's orbiting a common barycentre, then that's what they're orbiting. $\endgroup$
    – Monty Wild
    Sep 6, 2023 at 11:00
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A moon orbiting a moon is called a submoon. It's possible for them to have a stable orbit, but it's more constrained than for a normal moon. None have been observed so far.

The conditions for stable orbits and observations are discussed in Orbital Stability of Exomoons and Submoons with Applications to Kepler 1625b-I (2020) along with a lot of relevant references.

I don't think any of the conditions for a stable submoon orbit preclude habitability.

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There are many scientific articles discussing the possibility that large enough exomoons orbiting giant exoplanets can be habitable.

Looking up such articles would be desirable.

It is possible for a moon to have a sub moon or a moon of a moon orbiting it. However, there are more dynamical problems and restrictions with a giant habitable moon having its own moon of a moon than there are for a planet having a moon.

So possibly you might want to ask for the help of someone expert in orbital dynamics to help design your fictional star system.

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