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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.

o.m.
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smunji
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    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? – Coquí Sep 03 '23 at 23:23
  • Ah, I see, thank you. I guess I didn't understand what 'moon' meant exactly. – smunji Sep 03 '23 at 23:51
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    Hello @smunji, welcome to [worldbuilding.se]. 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! – JBH Sep 04 '23 at 01:27
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    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. – Tamrak Sep 04 '23 at 00:03
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    @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. – JBH Sep 04 '23 at 01:29
  • It would be possible for a planet to have a moon and a "grand-moon," under a very specific set of circumstances: * The moon must be far enough away that it does not get tidally locked to the planet, like our moon is; we can only see one side of the moon. * The grand-moon also has to be far enough away from the moon and the planet so it does not get tidally locked to either. Other than that, it is possible. Over time, the planet would pull on the grand-moon and either send it crashing into the moon, knocking it off balance, or absorb it into the planet. – sin_of_pi Sep 04 '23 at 08:29
  • See https://astronomy.stackexchange.com/questions/10624/why-does-the-solar-system-have-no-natural-satellites-of-satellites for a discussion of why it's difficult (but possible) and why it couldn't happen with our own Moon. – jcaron Sep 04 '23 at 16:50
  • Why would you doubt that and if you doubted it in real physics, why would you doubt your ability to build a world where it worked? – Robbie Goodwin Sep 04 '23 at 20:46
  • @JBH Whops, I'm new to answering questions didn't realize this would be a better course of action. Sorry mate. – Tamrak Sep 05 '23 at 00:00
  • @Tamrak Cheers. You should have seen some of the whoppers I made when I first started using the site. Thanks for your understanding. – JBH Sep 06 '23 at 06:37

3 Answers3

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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.

Monty Wild
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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.

Daniel Darabos
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0

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.

M. A. Golding
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