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.
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:
Could an orphan planet have a moon?
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.
There are quite a few possible reasons for a 'rogue' planet not to have a moon
- 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
- 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.
- 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.
- Few Jupiter mass planet with a half earth mass moon fairly close to us
- 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