Anniversary Post: Pluto Gets Closer Than Neptune

PlutoOn this day, and for the first time since they were discovered, Pluto slipped inside the orbit of Neptune. It stayed closer to the sun until 11 February 1999. But you might wonder, given that Pluto is such a puny object compared to Neptune (or even the Earth), why the larger planet hasn’t “cleared” it (basically: crashed into it). Well, there are a few reasons for that.

Right now, it would be impossible for Pluto to crash into Neptune because the dwarf planet orbits in a plane that is at quite an angle from that of the other planets — including Neptune. And when it is at the same axial distance to the sun as Neptune, it is way off the plane. And I do mean way off the plane: along the axis of the solar system, it gets as far as 8 times the distance from the Earth to the Sun (AU).

Pluto OrbitBut that wouldn’t save Pluto forever. Orbits precess (this explains part of the Earth’s ice age cycle). So there have been and will be times again when it will be on the plane at the right time. But again, Pluto is not in danger. Obviously, if it were in danger, Neptune would have long ago swallowed it up and we never would have had to have this argument about whether Pluto is a planet or not.

The two planets are in a 2-to-3 orbital resonance. This means that for every two times Neptune goes around the sun, Pluto goes around it three times. And they end up right back where they started. This sounds amazing, but there are lots of examples of this kind of thing in our solar system. Thus, they will not run into each other, because their precessions are locked together.

But Pluto Is Doomed Anyway!

Pluto is so small and so far away from the sun that it is chaotic. I mean that in a strict mathematical sense. Models of its future are highly constrained because very minor perturbations can have huge nonlinear effects on its orbit. But thus far, it really is the little planet that could. It has defied the odds and maintained its independence — to a large extent due to its having a powerful friend in Neptune. But it’s okay that Pluto is doom because so is everything. And it is going to last a whole lot longer than humanity.

24 thoughts on “Anniversary Post: Pluto Gets Closer Than Neptune

  1. OK — explain this to a dummy. Why are all the Sun’s planets on the same axial plane? Shouldn’t they be circling the sun in wildly different orbits, like old drawing of electrons labeled “Your Friend The Atom”?

    Also, I don’t know if I’m supposed to capitalize “sun.” It’s pretty important, I don’t want to make it mad.

    • Don’t say dummy. There are no stupid questions — just stupid unasked questions. The answer is: gravitation and the conservation of angular momentum. Imagine before the Sun. There was just a lot of trash in this part of the galaxy. And so it starts swirling around its center of mass, at whatever angle is dictated by all the trash and its starting location and momentum. Roughly speaking, the starting angular momentum will be the ending angular momentum. Thus, this creates a disc of material and not a sphere of material. The planets and the Sun all come out of this disc. So it should not surprise you that the sun rotates in the same plane. This is also why all the planets orbit in the same direction. And in general, rotate the same direction. Conservation of angular momentum is one of the most important principles in all of physics — and one that students have quite a hard time with! Of course, there is a lot of stuff that isn’t in the plane. Comets tend not to be. Halley’s Comet isn’t inclined too much (18 degrees), but it goes around the sun the “wrong” way. This, I assume, is why comets tell us stuff about the distant past, because they weren’t really involved with the formation of the solar system — just trapped later.

      But there are two things to remember. First, I’m no expert on this stuff. People can and have stumped me. Second, we don’t know how the solar system formed. There are a lot of unanswered questions. The last time I checked, there wasn’t a compelling explanation for why the planets formed. You would think dust spirals would lead to stars and then little bits of garbage. But that isn’t the case for our solar system or many others.

      I always capitalize Sun as the object. It is a name. Also, “sun” is often used instead of “sunlight,” and that would not be capitalized. “The sun on my skin felt good.” Similarly, you dig earth; you live on the planet Earth. Also, the Moon circles the Earth; there are plenty of other moons in the solar system.

      • Ok — so like if you spun around holding a string attached to a ball at the end. It’d go around you at roughly a 90-degree angle. More balls would do the same. Except asshats like Pluto and comets.

        I like thinking of the solar system as Space Junk. That’s really all we are. It’s wonderful. It’s quite profound. You didn’t teach kids this, did you?

  2. It would have been a loss to have had no reason for that Rick and Morty episode about Morty’s dad learning that Pluto is not a planet.

    But not a huge loss so Neptune should steal Pluto for itself.

    • So now you do watch that show! The Pluto one is great (I liked the spacecar: “must protect Summer.” That spacecar does not fuck around.)

      My favorite, though, was easily the one where Morty’s parents go to the couples-therapy planet. “They’re codependent!” The SO who’d never seen the show before was laughing hysterically. Apparently people treating each other terribly in relationships is a really common thing.

    • I’ve tried to find Rick and Morty online. It doesn’t seem to be on Hulu. The only place I saw it was an opening for The Simpson, and I thought it was brilliant! An alcoholic scientist with his nephew. That sounds like my kind of series!

        • That’s probably it. I have this problem paying $8 per month to watch TV shows with commercials. But I’d do it, now that I have a little money. The problem is, I have a little money because I’m working all the time and can’t watch television shows!

          Oh the irony!

      • I dunno if you’d like it. It’s kinda sad. Rick is an atrocious asshole, with occasional pangs of guilt. The joke of the show is he’s so arrogant, he doesn’t know how arrogant he is. Then he gets it, and tries to fix it. And fucks everything up worse.

        Now that I’m describing it — there’s pretty much no way anyone wouldn’t love this show. And that’s why your library has a copy you can watch for free.

        • I’ll admit, what I most liked about The Simpsons opening was after the characters were recreated incorrectly, Bart says, “Man! That’s the last time we use guest artists!” Or something like that. But I would think the little boy would make up for Rick’s horrible personality. That seems to be what would make it work. I’m sure I’ve checked our library and it doesn’t have it.

          • That’s pretty much the recipe. The kid does humanize Rick a little. He’s pretty resistant to changing, though, hence the humor.

            Adult Swim periodically rotates free episodes on its site. You have to watch commercials but at least you don’t pay for the pain.

            • I’m sure I will eventually see it. I have now watched W/ Bob & Dave and will probably write about it if I can remember.

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