Neptune is profoundly anomalous when it comes to moons among the gas giants. Jupiter has its four massive moons, Saturn has a several large spherical bodies and Uranus, a world similar in size to Neptune has five or so major moons.

Yet Neptune, the ice giant which sports one of the largest hill sphere radii in the solar system only has one major moon, of which is likely to be ripped to shreds in the future.

I find it peculiar because Uranus has five major moons yet Neptune only has one major moon. What explains this disparity? Did some disruption happen to earlier moons Neptune may have had? What's known about this?

  • $\begingroup$ It can be a bit dangerous to generalize from so few data points. I don't think anyone can really answer this, but the answer might be as simple as, Neptune just didn't capture or form many moons. $\endgroup$
    – zephyr
    Commented Sep 11, 2017 at 13:16

1 Answer 1


It seems possible that Neptune once had a system of moons similar to other outer planets. However Triton, a Kuiper belt object was captured by Neptune, and this disrupted the existing moons. It would have initially had an eccentric orbit, intersecting with the orbits of other moons, setting them into unstable orbits and ultimately either colliding with each other, hitting the planet or being ejected.

The minor moons formed from the debris of these collisions after Triton had entered a more stable orbit. So the disruption was the capture of Triton.

  • 1
    $\begingroup$ If one or more moons of Neptune had been ejected from orbit in this way, I wonder at the odds that they would have hit some other planet, and the odds that we would have detected them and suspect they are errant moons of Neptune, or not. $\endgroup$
    – Dronz
    Commented Sep 10, 2017 at 2:41
  • 2
    $\begingroup$ @Dronz: You can ask questions on this site, you know … $\endgroup$
    – chirlu
    Commented Sep 10, 2017 at 21:11

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