Why Haven’t Neptune and Pluto Collided?

When I was a kid, something really bothered me about the Solar System. If Pluto’s orbit crosses the orbit of Neptune, why don’t the two planets (Pluto was still considered a planet back then) collide with each other?

According to one of my science teachers, Pluto has just been lucky so far. But sooner or later, my teacher said, a collision will happen.


Regular Planet Pailly readers will know that I sometimes complain about the science education I received as a child. I’ve had to unlearn a lot of the things I’d been told, and this is another example of that.

Odds are there used to be other dwarf planets that crossed Neptune’s orbit. Probably lots of them. They’re gone now, either because they collided with Neptune or (more probably) they got hurled out of the Solar System by Neptune’s gravity.

At least one former dwarf planet was yanked out of its original orbit and became Neptune’s largest moon. That would be our old friend Triton. The one that looks like a cantaloupe.

Pluto survived only because it happened to have an orbital resonance with Neptune. Pluto completes exactly two orbits for every three orbits of Neptune. The math works out such that Pluto and Neptune have never met. They miss each other every single time one crosses the other’s orbit.

Dc07 Crossed Orbits

So I guess my old science teacher was right about one thing. Pluto is lucky. Lucky enough that it has its Neptune dodging orbit.

P.S.: Pluto isn’t the only lucky one. Several other objects have 2:3 orbital resonances with Neptune, allowing them to safely cross Neptune’s orbital path. The most noteworthy is Orcus, which is under consideration to be classified as yet another dwarf planet.

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Today’s post is part of Pluto/Kuiper belt month for the 2015 Mission to the Solar System. Click here to learn more about this series.

6 thoughts on “Why Haven’t Neptune and Pluto Collided?

  1. Thank heavens Earth isn’t locked into a weird orbit. Asteroids probably pose our biggest threat and theres so many in our suns orbit and the Mars and Jupiter belt. Its game over, isn’t it? LOL. why doesn’t the suns gravity pull every asteroid in?

    Liked by 1 person

    1. Asteroids don’t fall into the Sun because they are orbiting the Sun, just like the planets; but the planets (especially Jupiter) push the asteroids around a lot with their gravity, so they sometimes end up in some pretty weird orbits.


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