Pluto is not a planet but Neptune is?

Filed under: news,physics @ 16:28

Today’s IAU vote reduced the solar system’s planet count to eight. No, Pluto has not been forcibly removed from orbit for bad behavior, it has merely been demoted; Pluto has been designated a “dwarf planet” since it fails to meet the newly defined criteria for planetary status.

A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

It is the third test that Pluto fails; it has not “cleared the neighbourhood around its orbit” (whatever that means) since its orbit and Neptune’s overlap.

Fair enough. Too bad, Pluto.

But just as Pluto’s and Neptune’s respective orbits overlap, so too do Neptune’s and Pluto’s. Neptune, then, has not “cleared the neighbourhood around its orbit” either.

Perhaps I’m missing something in the highly rigorous definitions of “to clear” and “neighbourhood”, but it seems the new rules have been inconsistently applied to yield an ‘eight planet system’.

I should also point out that the new definitions do nothing to define the status of bodies orbiting other stars since all of the language used refers to “the sun” and “the solar system”. Are the IAU resolutions science, then, or just self-important verbiage?

I hope my confusion comes only from my ignorance and that there’s a good answer to all of this.


    paul.za 08.24.2006 @ 17:14

    My un-informed opinion (ie. guess) is that the Neptune issue is avoided by the following argument: Pluto could only have arrived in its current orbit from some sort of run-in with another object — it was once a Kuiper belt object or a moon of Neptune, till it was gravitationally propelled to it current orbit. Therefore Neptune did originally clear its orbit; while Pluto didn’t clear whatever its original orbit was.

    And I presume that the definition must apply to other planetary systems too, surely? Is it possible that our one is the only one being reported because it’s the only one that is facing a re-classification at this stage?

    MDA 08.24.2006 @ 20:28

    So that would mean, according to my reading of the rules, that Neptune was once a planet, but is no longer. Actually, the language is again ambiguous there.

    As for the generalizations, yes. I assume the definition generalizes. But why make a definition that has to be generalized rather than a general definition?

    It’s either a poor definition, or the IAU is poorly reporting it. Neither is good.

    HAA 08.24.2006 @ 21:34

    Actually Michael, what we’re witnessing Neptune’s efforts to “clean up its neighbourhood” and the IAU’s cowardly complicity.

    When Pluto was first discovered its estimated size was similar to that of Earth. More recent observations have confirmed that it is much smaller. How do we explain this discrepency? Obviously “measurement error”, often used by “scientists” to explain why they really weren’t wrong when they were clearly wrong, cannot possibly explain a 100000% decrease in observed mass.

    No, the mass of Pluto was, in fact, stolen by Neptune as part of a deliberate campaign to enforce its own planetary status to the detriment of its neighbors. This is part of a vaster campaign by all of the outer gas giants of the solar system: In 1994 Shoemaker-Levy 9 was targeted by Jupiter for “incorporation” and destroyed in order to clear more room in the Jovian “neighborhood”. And, despite the fact that courageous and responsible jouranalists captured this act of destruction on film and broadcast it to the world, neither the UN nor the IAU has ever formally condemned this act of violence.

    The IAU is complicit in these acts of “planecide”: By enforcing the biased, “might-makes-right” rules quoted above, the IAU legitimizes these heinous acts.

    Write your Congressmen and local news representatives: It’s time to take action against these outrages abuses of non-elected officials! Take a stand against “neighborhood cleansing” by jealous and capricious giant worlds! Remember, today it was Pluto — tommorrow, it could be Earth!

    Adam 08.24.2006 @ 23:53

    So, here’s my interpretation:

    Neptune managed to clear its orbit during the formation of the solar system, thus achieving Planetary Status. Pluto, on the other hand, hit something and got placed in an eccentric orbit, thus it did not “clear” its orbit and did not achieve Planetary Status. Pluto’s eccentric orbit may be clear now, but Pluto was unsuccessful in clearing its original orbit and may not have had much (if anything) to clear in the eccentric orbit.

    As for Pluto crossing Neptune’s orbit, I don’t think that’s a problem because the orbits merely intersect, they aren’t identical. After all, there are plenty of comets and asteroids that intersect the orbits of the other planets.

    Adam 08.24.2006 @ 23:59

    Also, while the IAU might like this definition, I agree with some people who are saying that Pluto should be “grandfathered” in as a planet. After all, how much of a difference does it make whether it’s called a planet or not. It’s still a major body in the solar system, and changing the definition merely makes all of the science books out of date.

    If they really want to stick to this definition, apply it to any future discoveries, or perhaps apply it to *other* solar systems.

    jjk 08.25.2006 @ 09:32

    HAA, that was priceless.

    Mary 08.25.2006 @ 09:34

    How much difference does it all really make? In my mind Pluto will always be a planet as that’s what i know it as and it’s hard to change that. All this really serves to do is cause a panic in people trying to make textbooks/lessons/museums etc up-to-date ASAP! It all sounds ludicrous to me…but then again I’m not a scientist!!!!

    MDA 08.25.2006 @ 10:27

    Paul and Adam, you may be exactly right about the Pluto v. Neptune issue, but I still think the resolution is poorly worded and ill defined. Again, it could simply be that I don’t have the requisite background in the field.

    HAA, wow. I thought the IAU were just slapdash. It turns out they’re bastards too. Thanks for spreading the word.

    Adam and Mary, I think this is a fantastic opportunity for educators to explain some neat concepts. Think about it. Pluto did not change, but astronomer’s definition of a word did. All of the sudden, Pluto is (in some sense) no longer what it used to be. That’s weird. Language, communication and “reality” mix in some pretty interesting ways.

    But I disagree with grandfathering Pluto into the astronomical definition of the word “planet”. There’s nothing wrong with having a scientific definition of a word that doesn’t jive with a “cultural” definition. We’re talking about two different contexts, afterall. But I think scientific definitions should be very precisely defined and rigorously applied. Culturally, go ahead and call Pluto a planet. But if it is not defined to be so in the context of scientific astronomy, don’t call it one in that context.

    As for textbooks, they’re are always out of date. Every scientific field is constantly changing. Additionally new texts are only purchased once every five ten years in most schools (unfounded claim). There’s no need to rush things, just say “Astronomers define a planet as [so on and so forth] and state that there are eight planets in our solar system” in the next edition.

    paul.za 08.25.2006 @ 13:30

    I agree, MDA, that this makes for an interesting opportunity to talk about things like scientific method, definitions, and the progress of science. And that there are interesting things being done all the time — like the discovery of inconvenient new things like asteroids larger than Pluto.

    As regards the definition: I happen to think that the result of the choice of definition (ie. 8 planets, with no more to come for the Solar System) makes sense — the 8 there are are clearly larger and better candidates than others. Pluto has such a weird orbit that it’s definitely from somewhere else, and is small anyway. Problem is, those sorts of arguments are hard to make into a rigorous general definition. Maybe I’m becoming an apologist for the failings (and indeed possible planecide). But there it is.

    paul.za 08.25.2006 @ 13:30

    And I appear to be losing the ability to end sentences without messing it.

    Eric Scott 08.26.2006 @ 14:36

    I agree completely with your premises and conclusions. The IAU decision is so poorly worded that Neptune *has* to be thrown out as a planet. Until the definition is tightened up, I don’t see any other conclusion. The rejection of Pluto wasn’t based on where it came from or how it got there; it’s a nearly-round object orbiting the Sun that hasn’t cleared its orbit. Just like Neptune. What’s good for the goose is good for the gander.

    jim forde 09.01.2006 @ 08:18

    Check out my pluto demotion song.



    Jimmy :-)

    Kattelin 01.25.2007 @ 15:40

    i think it is a horriblelly bad idea for pluto not being a planet i so toatally think it is a dumb idea that pluto is not a planet the scientist people that say it is not a planet need to go through there info again and relized it cost them alot of money because now they have to buy new school text book in science cause it states that pluto is one of the planets!! its stupid!!! scientists get a LIFE

    bob 01.31.2007 @ 18:06

    i think that the people who made the definition just dont like pluto. :-( the way i understand it, if pluto is not a planet neither is neptune. if you exclude pluto from being a planet, you also have to exclude neptune…

    Anonymous 02.27.2007 @ 22:19

    because pluto is not a planet because of the planet pluto is far from the sun so that the pluto is the smallest planet!!!!!!!!!!

    ajksdj 04.28.2007 @ 08:07

    pluto is not no planet becaues it has always been a planet

    skdfhpt 10.27.2007 @ 10:19

    pluto is not a planet!

    ddddd 10.27.2007 @ 10:20

    pluto is a celestial body

    Stu 11.06.2007 @ 13:49

    Here’s my argument against Pluto’s planetiness (mostly in jest):


    My argument does not take any account of the rules set forth by the scientific community because, well, I don’t care about their rules. Who says that if I jump up, I have to come back down? I hereby defy them all!

    Anonymous 02.26.2008 @ 15:26


    Anonymous 07.17.2008 @ 07:10

    Neptune is bigger than everything else that orbits that far from the Sun. Pluto isn’t. So Neptune is a planet but Pluto is not.

    Red Harold 03.06.2009 @ 19:19

    Let’s assume sometime in the next few centuries some large objects fly into our solar system and by sheer chance take up various residences in sharply oval orbits that cross the orbits of Venus, Earth and Mars without actually ever becoming a threat to them in the next hundred thousand years or so. Congratulations, Earth will no longer be considered a planet.

    It’s also entirely possible that as our ability to see further into space improves we spot solar systems with Uranus-sized objects criss-crossing each other’s path 180 degrees apart on differently shaped orbits. Hmm. No planets there.

    Heck, an object has to be spherical AND have a clear orbit to even qualify as a DWARF planet. If we find an object of earth-sized mass that somehow holds the shape of a peanut and it’s orbit isn’t quite clear, what do we call THAT? A very large asteroid?

    insherah 03.18.2009 @ 08:57

    is it a planet i have no idea!!


    […] I think it is a horribly bad idea for Pluto not being a planet. I so totally think it is a dumb idea that Pluto is not a planet. The scientist people that… http://www.blogwaffe.com/2006/08/24/417 […]

    Dillyn Winn 11.14.2010 @ 19:01

    People don’t seem to be understanding this, so I’ll actually give you a good answer. It’s not that Pluto crosses paths with Neptune, it’s that Pluto is in the Kuiper Belt and it’s orbiting path, or “neighborhood”, is riddled with asteroids and comets. Also, yes, this definition was meant only for our solar system. A sun is a star that has orbiting planets, and our sun’s name is Sol. Other solar systems have suns. Also, the Solar System, doesn’t mean OUR solar system unless you’re talking as if you want it to mean that. Other solar systems are the solar systems of their areas.

    Aurogra 11.30.2011 @ 07:02

    To down grade it to a dwarf planet is crazy we all know it is a small planet, maybe just a ball of rock and ice but non the less a planet people!

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