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Is there some reason why an orphan or rogue planet wouldn't have a moon? Let's say it started out in a normal system and then just got flung out by an unstable orbit.

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    $\begingroup$ If it didn’t have a moon in the beginning, of course! $\endgroup$ – fartgeek Mar 7 at 0:40
  • $\begingroup$ It depends on the (bigger) problem that you are trying to solve. If you want an inhabited moon, then having it rotate and orbit a planet may be a useful way to generate thermal energy through tidal forces creating internal friction. What's the underlying world-building issue that you're trying to solve here? Else fasterthanlight's suggestion might be worth following. $\endgroup$ – Draft 85 Mar 7 at 0:46
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    $\begingroup$ The planet most definitely can have a moon. After all, if a passing star messes with its orbit and it ends up flying out into space, it could retain the moon the same way as it does orbiting a single star. There is a limit (Hill sphere, 1.5 million km versus 0.38 million km for the Moon) so getting too close to a star could take away the moon. It may be harder to plot out a solution for ejecting the planet without such a close encounter. $\endgroup$ – Mike Serfas Mar 7 at 1:16
  • $\begingroup$ Is this a duplicate of astronomy.stackexchange.com/questions/1236/… ? $\endgroup$ – David Cary Mar 8 at 1:23
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Could an orphan planet have a moon?

Yes.

s there some reason why an orphan or rogue planet wouldn't have a moon?

Yes, this answer lists some possibilities.

What you didn't ask was:

Is there some reason why no orphan nor rogue planet could ever have a moon?

and based on the previous two, No.

Any non-contact gravitational acceleration that could pull a planet away from its star has some chance of accelerating its moons sufficiently to stay with the planet.

If the perturber were a few times the Moon's orbit farther from the planet, staying with the planet might even be likely.

Looking at this obliquely from a delta-v perspective, Jupiter orbits the Sun at 13.1 km/s, and its inner Galilean moon Io orbits Jupiter at 17.3 km/s.

Without doing the math, I think it's possible that you could give Jupiter a 13.1 km/s kick impulsively in an optimal direction and Io would stay bound to it, but now in a much more elliptical orbit.

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    $\begingroup$ Note that the Jupiter example uses a very theoretical scenario: A sudden acceleration of Jupiter which doesn't also affect any of its moons. A more realistic scenario would be a very massive body passing by and affecting Jupiter with its gravity. That would accelerate Jupiter's moons in the same way as it would affect Jupiter itself. $\endgroup$ – Philipp Mar 8 at 11:54
  • $\begingroup$ @Philipp I propose that the perturber pass within "few times the Moon's orbit farther from the planet" so while it may affect both there will be some difference between the two. I'm making a hand-waving argument that Io will remain bound by looking at the problem "obliquely from a delta-v perspective" but I didn't go so far as to say the perturbation had to be an impulse. I think I'll just go model it in Python and compare to any other models that might arise. $\endgroup$ – uhoh Mar 8 at 12:08
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There are quite a few possible reasons for a 'rogue' planet not to have a moon

  1. In the formation of it's original solar system, the planet never had a moon form. This could likely occur if the planet is a rocky planet close to its sun, where such planets are less likely to form moons when the system is young
  2. It never captured a moon during its tenure in its original solar system. Evidence suggests that many moons are 'captured', perhaps being early planets or asteroids that have irregular orbits and get caught in a planets gravity field.
  3. It did have a moon/moons, but when the planet was 'pulled' from its original orbit (presumably by something large and close) it was flung off into space and the moons were destroyed/pulled apart, absorbed in either the large body or planet, or flung off themselves into a different trajectory.

However, keep in mind that a rogue planet would need to have its orbit distorted so much as to 'escape' the gravitational field of not just its sun, but its sun's other planets. Meaning for this to occur the pull of gravity to fling it out must be large indeed - perhaps a Jupiter-type planet pulling a small Mercury-type planet, enough to severely change direction but not enough that tidal forces pull it apart.

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    $\begingroup$ You can pretty much ignore the effects of the other planets. Their mass is typically insignificant compared to that of the host star, and each pair of planets tends to have relative velocities far in excess of either planet's escape velocity at the mutual distance. $\endgroup$ – notovny Mar 7 at 15:11
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Certainly an orphan planet could retain its moon.

One doesn't need to apply impulse to a planet in order to make it go rogue. One could just as easily apply impulse to the star it orbits instead (though the impulse would have to be orders of magnitude greater). As soon as the difference in velocities between a planet and star exceed escape velocity, the planet has gone rogue.

If a star had a close pass to our Sun, the acceleration introduced to the Sun could cause the differential velocity of the Sun and Jupiter to exceed escape velocity, while leaving the local orbital system of Jupiter relatively intact.

I used Universe Sandbox to create a simulation demonstrating this possibility and showing a moon can be retained on a rogue planet. I uploaded the result to youtube:

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    $\begingroup$ +n! n-factorial upvotes for the answer as simulation as video! Is it possible to mention the name of the software used? $\endgroup$ – uhoh Mar 7 at 23:05
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    $\begingroup$ Also, I'm still curious if my prediction that the pair can be ejected by something (probably much smaller than Sirius, like a 2x Jupiter perhaps) passed close to them rather than the Sun, leaving Io perturbed but still bound to Juipiter. I guess it would have to be fairly fast and moving in roughly the same direction as Io was (relative to Jupiter) at closest approach perhaps as well as the same direction that Jupiter was moving relative to the Sun. I guess that would have to be another rogue Jupiter, a less likely event. $\endgroup$ – uhoh Mar 7 at 23:12
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    $\begingroup$ @uhoh The software used is a game/simulator called Universe Sandbox. I purchased my copy from the Steam Store:store.steampowered.com/app/230290/Universe_Sandbox $\endgroup$ – Connor Garcia Mar 7 at 23:56
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    $\begingroup$ I think I can take out Sirius and add another Jovian to my simulation pretty easily. I will see if I can get an ejection with a close Jupiter pass that retains Io. $\endgroup$ – Connor Garcia Mar 7 at 23:59
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    $\begingroup$ Great! :-) Oh, it's right there in your answer, I'm sorry I missed that. you may be interested in adding answers to Is a General Relativity approximation available in Universe Sandbox (1 or 2)? and Universe Sandbox 2 accurate representation of gravity and Is Universe Sandbox 2 realistic? in fact I'm adding a universe-sandbox tag to many questions now so they are more searchable. $\endgroup$ – uhoh Mar 8 at 0:00
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A possible case of a rogue planet with a moon has been found. See this paper and the general synopsis from Nature. In this case there are 2 possible solutions when modeling the observations:

  1. Few Jupiter mass planet with a half earth mass moon fairly close to us
  2. A low mass star (0.12 Msun) with a super earth (very uncertain mass) far away.

Unfortunately there is currently no way to distinguish between these 2 possiblities. Detecting the faint star may be possible with future optical telescopes. This was an answer forStackExchange astronomy

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