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In this answer, I wrote (on another account) why most moons orbit over their planet's equator.

Any rotating planet has an equatorial bulge that shifts its moons' orbits around its equator over long periods of time; but sometimes the Sun's perturbations win out and the moon ends up orbiting closer to the planet's orbital plane.

As I said in that answer there are 3 large moons that don't orbit over their planet's equator (The Moon, Iapetus, and Triton), and I tried to explain why that is the case for each.

But while writing that answer, I realized that there really is no good reason for Triton not having an equatorial orbit. It orbits closer to Neptune than our own Moon, and being so far away from the Sun the solar perturbations are negligible.

So why, exactly, doesn't Triton have an equatorial orbit? I'm sure having a retrograde orbit and likely being a captured orbit have something to do with it, but still.

It has literally been billions of years. Even if Neptune's equatorial bulge was tiny, it still should've been enough to shift its orbit.

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  • $\begingroup$ The answer you are referring to, tells a story, but gives no article references to support it. Triton is thought to be a capture KBO (unclear which population), which is supported by its surface spectrometry. A moon captured at such an inclination that it can start Kozai-cycles, wouldn't have to have its inclination damped, at least to my understanding. But I only have my memory to support this, you should dig up the papers yourself.. $\endgroup$
    – AtmosphericPrisonEscape
    Commented Sep 21, 2021 at 14:47

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Triton is believed to be a captured Kuiper Belt Object that formed completely independently from Neptune. KBOs regularly have significantly tilted orbits, and as such Triton would have entered the orbit around Neptune at an angle as well.

Though of course, it has to be mentioned here that the highly tilted orbit of Triton is one of the main reasons why it is believed to be a captured object.

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    $\begingroup$ Yes, Triton is expected to have entered orbit at an inclined angle. But shouldn't Neptune's non-symmetrical gravitational field align Triton's orbit over its equator? $\endgroup$
    – user177107
    Commented Sep 23, 2021 at 9:56
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The most likely theory of why Triton has an unusual inclined orbit is because Triton was probably a KBO before it approached towards the solar system and was captured by Neptune. Its retrogate orbit proves that it was not formed from proto-Neptunian system and was captured from elsewhere. Triton's capture around Neptune catastrophically altered the Neptune satellite system. Neptune was orbited by several primordial satellites that may have existed prior to the binary-planet encounter and might have been destroyed in the aftermath of Triton's capture to a large inclined orbit.

Several models were made to proved this hypothesis:

  • Craig Agnor of UCSC and Douglas Hamilton of the University of Maryland describe a new model for the capture of planetary satellites involving a three-body gravitational encounter between a binary and a planet. According to this scenario, Triton was originally a member of a binary pair of objects orbiting the Sun. Gravitational interactions during a close approach to Neptune then pulled Triton away from its binary companion to become a satellite of Neptune.
  • Raluca Rufu (Weizmann Institute of Science, Israel) and Robin Canup (Southwest Research Institute) made a model using N-Body simulation which consists of newly captured Triton and a likely primordial prograde system of moons. It exactly showed how Triton was captured and eventually altered the Neptunian satellite system. They showed that if the moons have a mass ratio similar to that of Uranus system or smaller, Tritons interactions with it have a substantial likelihood of reproducing the current Neptunian satellite system. They even demonstrate that the interactions decrease Tritons initial semimajor axis quickly enough to prevent smaller, outer satellites like Nereid from being kicked out of the system. If they are correct, then it neatly explains why Neptune satellite system looks so unusual compared to Jupiter or Saturn.

There were other theories/hypothesis for this questions but they didn't gave a satisfactory explanation.

References

  1. New capture scenario explains origin of Neptune's oddball moon Triton By Tim Stephens, 15 May 2006
  2. Kohler, S., “Did Triton Destroy Neptune's First Moons?”, AAS Nova Highlights, 2017, (link)
  3. Maltagliati, L. Why so different?. Nat Astron 4, 217 (2020). DOI: 10.1038/s41550-020-1057-8
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