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Pluto has a high orbital inclination compared to the planets in our solar system. What has caused Pluto to have such a different orbit -- was it always like this, or did something happen to make it so?

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  • $\begingroup$ It's also fascinating that Orcus (en.wikipedia.org/wiki/90482_Orcus) is almost the same - 20 degrees and an almost identical orbit... I wonder if there's a common reason... $\endgroup$
    – Andy
    Commented Jul 23, 2015 at 15:21
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    $\begingroup$ AFAIK, the basic models today say that Pluto formed in a circular orbit in a thin protoplanetary disk, like all planets. Pluto's orbit became eccentric and inclined because of gravitational interaction with migrating giant planets soon after their formation. $\endgroup$
    – LocalFluff
    Commented Jul 23, 2015 at 20:25
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    $\begingroup$ It's actually typical for Kuiper belt objects to have high inclination. $\endgroup$
    – Mithoron
    Commented Jul 23, 2015 at 22:40
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    $\begingroup$ A very interesting paper (I skimmed it last night but the maths will need more study!): staff.on.br/rodney/topicos/1/mal_plut_93.pdf As @LocalFluff says interacton with Neptune as it migrated outwards caused Pluto's resonance, then its enlargement to an elliptical orbit, and finally its orbital inclination. (Also probably explains why Orcus went a similar way.) $\endgroup$
    – Andy
    Commented Jul 24, 2015 at 7:32
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    $\begingroup$ I'm not sure if this justifies an answer, but the Kozai mechanism has sometimes been invoked. $\endgroup$
    – HDE 226868
    Commented Jul 26, 2015 at 15:06

2 Answers 2

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I think the best answer is that there is no particular reason.

In any planetary system, most orbits tend to stay close to a common plane - close, but not exactly there. Even the big planets are slightly out-of-plane with each other, but the differences are tiny.

As you move away from the bulk of the planetary system's mass, smaller bodies tend to be more random. They are more easily deviated from encounters and near misses. Their origins might be different from the rest. If they get close to the outskirts of the system, that's a more unruly place, and orbit-changing events are more likely - especially for small bodies. Finally, the whole distribution of orbital planes has a bit of a stochastic (random) component anyway, and there will always be outliers that are quite different from the bulk.

In theory, even some larger planets could be significantly "out of plane" (much more than they are right now), but that's unlikely and it tends to be smoothed out over large periods of time.

So, Pluto is different because it's smaller and it's not really a planet, but don't attach too much significance to it.

Try to install Universe Sandbox and play with some of the included scenarios. Big systems bound by gravity can be pretty complex, and completely orderly configurations are rare.

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It is true that Pluto's orbit is highly inclined and elliptical and there is a reason for it. The main reason is is due to complex interactions with other celestial bodies in the solar system, especially Neptune. The complex interaction can be attributed to a phenomenon called orbital resonance, specifically 3:2 resonance also called vZLK oscillation. Following are the factors:

  • Gravitational Interaction: Pluto and Neptune are somehow locked in a gravitational "dance". As they orbit the Sun, their gravitational forces interact with each other.
  • Resonance: For every three orbits Pluto completes around the Sun, Neptune completes two. This is why it is called 3:2 resonance and it creates a gravitational relationship between Pluto and Neptune as mentioned in point 1.
  • Tugging Forces: When Pluto approaches closer to Neptune's orbit, Neptune's gravitational pull exerts a force on Pluto which either speed up or slow down Pluto's orbital motion and over time, this tug alter the shape and tilt of Pluto's orbit.

Because of Pluto's unique tilted orbit and its crossing in Neptune's orbit once in a while, its planetary status was revoked by IAU and was classified as "dwarf planet". As more and more objects were discovered beyond Pluto (specially in the region of Kuiper belt), the orbit becomes more eccentric and tilted (as Sun's gravitational force becomes weaker on those bodies).

You can read below articles for an indepth explanation on the resonance mechanism:

  1. https://www.quora.com/How-did-Pluto-get-its-tilted-orbit
  2. https://universemagazine.com/en/scientists-explain-how-the-weird-orbit-of-pluto-formed/
  3. https://www.discovery.com/science/pluto-s-weird-orbit
  4. https://www.space.com/pluto-orbit-influences-from-giant-planets
  5. https://phys.org/news/2022-04-pluto-orbit-surprisingly-unstable.html
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