Should Pluto Be a Planet?

I found this really interesting. I thought “Pluto should still be a planet” arguments were mostly limited to nostalgic laypeople like me, but apparently there’s more expert debate on the issue than I’d realized.

It seems like the NASA administrator’s comments were somewhat tongue-in-cheek, but nevertheless, I hadn’t really considered all the different factors that went into classifying planets (aside from just size).


Physicists: welcome to the problem of defining what a species is…


I understood from the recent PBS documentary that Pluto was now considered ab object of the Kuiper belt. I believe Kuiper belt objects orbit around the sun in the same general plane as the planets. Oort objects are not confined to a disk and are found many times further away. I was surprised to learn that Pluto’s distance from the sun varied as much as 30 to 50 times that of our planet. I knew the orbits of the planets were elliptical but that is pretty oddly elongated. Presumably at Pluto’s nearest distance to the sun it could hit Neptune.

Thanks for the link, @Laura. Got to get ready for company now but look forward to reading it soon.

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The mass of Pluto is less than a number of moons. Another oddity is that Pluto is tidally locked to its own moon, a moon which is closest in distance and size to the planet it orbits. This is two ways in which it is more like a moon than a planet. All moons in the solar system are tidally locked to the planet they orbit and none of the planets are tidally locked – the closest is mercury which is tidally linked to the sun with a 3/2 ratio (That gives mecury a few peculiarities such as a day-length which is twice as long as its year).

So the compromise of “dwarf planet” is a good one, where it keeps Ceres company. Though, Pluto should really be called a double dwarf planet. Notice that if you follow the idea of defining planets by the spherical shape that means Ceres joins the club along with a large number of objects in the Kuiper belt.

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The whole controversy (which I understand is more among lay enthusiasts than among astronomers) is a textbook lesson in how human categories are our artificial conveniences imposed on reality. Astronomers are more interested in finding sets with common characteristics and observing what those characteristics are. Is it a “gas giant” or does it have an accessible terrestrial surface? Is it tidally locked? What “weight” class is it in? Does it experience gravitational self-deformation? And so forth. By asking these questions, bodies of mass can be classified various ways, and those differences are what interest scientists more than trying to rigidly define some concept for all time and artificially sorting everything we find into that “preformed” system.

The harder we look the more disturbing blurs we find around the edges of everything we want kept apart. We’re the sorts who can’t stand the thought of different foods touching each other on the plate. That gas giants might be failed small stars or that some planets might be more like moons reminds us that those hard category divisions belonged only to us and not necessarily to reality itself.


Amen to that. And since planets are such a basic idea we form early on about what’s out there we are all the more reluctant to let go of Pluto being one.

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And Gus’ favorite pickup line.


By the way, one of the more humorous peculiarities of mercury is that there are times and places on mercury where the sun will stop in the sky and reverse its course for a short time. This way you can say that the Biblical description of Joshua 10:13 is not as crazy as it might seem when you consider what happens on other planets.

The reason this happens is because not only do we have this 3/2 sidereal days per year subtract one to get the 1/2 days per year… so the sun moves very slowly in the sky of mercury taking 176 days to go all the way around, but the orbit of mercury is not perfectly circular so there are times when it is closer to the sun and moving a little bit faster and then for that short period of time the motion of the sun in the sky due to the orbit is faster then the motion of the sun in the sky due to the rotation – and thus it goes backwards in the sky for that period of time.


This just seems Goofy. For you youngsters, Pluto is a cartoon dog, and once was a planet.


Pluto’s orbit is in resonance (not random) with respect to Neptune. It orbits twice for every three orbits of Neptune. It means that because Pluto never crosses the line of Neptune’s orbit when Neptune is there, the dwarf planet actually comes closer to Uranus than it ever does to Neptune!

There are tons of asteroids in resonance with Jupiter, and Jupiter’s moons are in resonance with each other, and there are also a lot of other Kuiper belt objects in resonance with Neptune. Pretty interesting stuff!