Was Pluto Ever REALLY a Planet?

Ever since the infamous 2006 reclassification of Pluto off the list of “official” planets (which had a rather incendiary effect on many of the distant world’s Earthly fans) the term “planet” has been seen by some as a variable one, difficult to define and apparently able to be given and taken away. But was Pluto ever really deserving of the title to begin with?

This fun info-animation by C.G.P. Grey suggests that it wasn’t, and offers a compelling explanation why.


Grey writes on his blog:

“To my constant surprise the issue of Pluto’s planetary status — which I think should be a dry technical issue — really gets people riled. But it’s also been my experience that the people who most want Pluto to be a planet know the least about it and the history of its discovery. So, I hope that this video can help correct that a little bit.”

We still love you, Pluto, no matter what you are!

See more of Grey’s excellent animations on YouTube here.

35 Replies to “Was Pluto Ever REALLY a Planet?”

  1. Honestly- the facts are in… giving in to those who want Pluto restated as a planet is like giving in to those who disavow evolution and believe the earth is only 4000 years old. Science should never give in to the loudest idiots in the crowd!

    1. >”Science should never give in to the loudest idiots in the crowd!”
      In that case science shouldn’t listen to a decision by vote conducted when most of the scientists have left, more specifically only 424 remained out of over 2700 according to a quick check. The very fact that they *voted* about the science should make you doubt whether the facts really are in.

      You also missed the best part of the video, the one where it says that the planet definition is unwieldy. There are even more examples of problems than those stated, like the fact that Pluto/Charon is really a binary (dwarf) planet. Also, I believe we’ve found an exoplanet system with three co-orbiting planets which totally thrashes the current clear the neighbourhood criterion.

      I’ve heard no convincing explanation for why eight planets are more scientific than hundreds/thousands (although I’m listening!) I’m quite convinced the culture and the science have collided here, damaging both. I think the planet definition of the public should be decoupled from the science. That way the public could do what it wants while the science wouldn’t have to have votes on opinion.

      1. Finally someone who not only speaks the truth, but sees that there was NO real science conducted in Pluto’s demotion. To this day, Pluto is still a planet, despite what the general public believes; and until the IAU refines our definition of what a planet is (based on the scientific method), Pluto will continue to be a planet.

        If you disagree, then I challenge you to examine each known planet of our solar system, against current criteria of what constitutes a planet. You should find, that there are less than 8 planets going by current definitions. If you refer back to older definitions, and disregard the present one, then there are more than 9 planets. So which is it? Either way, a problem STILL exist, and it needs to be fixed.

      2. How many times do we need to go over the same points? Honestly, we need a TalkPlanets FAQ akin to the TalkOrigins for creationists.

        See my reply to Alaksandu for the very clear science. (Ref on the IAU pages, IIRC.) There is no problem but an improvement here, but as Tim Griffith notes it would become a problem if listening to crackpots. “Pluto is still a planet”, really?

      3. Wrong argument.

        Clearly, this kind of classification is not inherent in the universe around us, but is instead a human construct.

        From dust particles to almost-suns, it’s a continuum. Nature doesn’t come with obvious classes. On the other hand, we humans want those classes to make it easier to talk about what we see.

        No scientific fact has changed over this decision. All that has changed is the terminology – what names we give things.

        Voting about the actual science would be bad. Voting about terminology is not a problem.

        Science is about what is right. Terminology is about what is useful.

        Suppose they’d voted to change Mars’ name to Fred. Same thing.

      4. Yours is a well thought-out reply. It is true that voting on terminology is completely different than voting on the science, and that it can be useful. However, you probably agree that the poster I was replying to went a bit far by calling this terminology a fact and lumping me in with the creationists.

        But even though it’s votable I see problems. Changing the name of Mars to Fred is a great example: they could do it in theory, perhaps some group would wait until there are only ten people left at the meeting. In the current system people at large would be expected to accept this, although of course they wouldn’t.

        The IAU gets its authority from all the excellent science, which is why the public listens to them. For this reason I’d love it if the parts where they make things up for convenience could somehow be separated from the public definitions, which will likely have different criteria for convenience. Of course I realize this is likely never happening, but…

      5. I think the original comment has a valid point, the fact is that the convenient IAU definition holds and attempts of changing the definition by outsiders is crackpot.

        As for voting, most voting bodies has lower attendance limits precisely to avoid hijacking. In this case the vote was fair, square and AFAIU representative of the consensus at the time.

        You misunderstand “the current clear the neighbourhood [sic?] criterion”, which is a reformulation of a dynamic criterion for planet populations. IIRC there is a marked 5 order of difference in dynamic stability between planets and dwarf planets like Eris (or Pluto).

        This where the science comes from btw, planets behaves differently than asteroid populations including tehir dwarf planets. Neither Earth nor these planets nor the initial period before clearing occurs are ‘trashes’ the criteria.

      6. I don’t think anyone will read this since I post so late, but I wish you wouldn’t combine your sound scientific arguing with calling any and all opponents crackpots.

        I think achieved hydrostatic equilibrium is a great planet definition (well combined with the others that are not about clearing the neighbourhood.) This means I agree with an earlier proposal at the relevant IAU meeting.

        So basically: agreeing with one group at the IAU meeting puts you on the level of creationist nutjobs? Really? I’d much prefer it if science was capable of being discussed by anyone who wants to.

        As for trying to affect things as an “outsider”: we cannot have a situation where you’re either in the IAU or you shut up. You’re arguing well based on the science, there’s no need for such an appeal to authority.

      7. People get weirdly emotional about the whole Pluto issue and I’ve never understood why. I guess people will get emotional about anything.

        The bottom line, and I’ve stated this before in other similar debates, is that redefining Pluto as a Dwarf Planet does not impact the science of studying Pluto – so who cares if they redefine it? Let them set their arbitrary lines and lets not get upset about it. Pluto is still a fascinating place and exists with its own characteristics regardless of what we call it.

        Recall that all of the planets have had different names, depending on who was naming them. I wonder if the ancient Greeks were upset when the Romans came along and renamed Phaethon to Jupiter? Names and definitions are arbitrarily set by humans and change over time. Live with it.

  2. Does it really matter how scientists catalogue these worlds? I couldn’t care what scientists define Pluto to be. There’s still a the badass mission New Horizons on the way to recover the first hi-res images. How exciting is that!

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  3. By definition, Pluto has never really been a planet…there, I said it. I don’t care either. Dr. Greasy Tyson and his motley crew can shove it. I will always call Pluto a planet, even though I know it isn’t.

    1. It is your privilege. However, scientists wouldn’t understand you and you wouldn’t understand science talking about the observable world.

      It is better to adapt to the current consensus IMHO. But that is _my_ opinion.

  4. Pluto was a planet, now it’s a dwarf planet. Still a planet, just less of a planet. What we really need is to figure out, to what gram, the difference between a dwarf planet and a planet is, as there are an infinate veriety of sizes and compositions. How big does the ice world have to be to crack the ice?

    1. one easy rule you can directly apply is:
      – Solar system general composition (mostly hydrogen etc)
      – What’s left of the starting composition due to … heating, size etc.

      Using this rule, earth would have been much larger in size at the formation of our Sun, but lost quite some weight due to its lack of gravity and surplus in heat to hold on to its (unbound) lighter elements.

      Switching Earth with Pluto would make Pluto lose quite some weight as well, no doubt leaving only a rocky core 1/3rd its current size.

      So to name Pluto a Dwarf Planet instead of a Kuiperbelt Virgin is actualy generous.

      1. Excellent observation. There is also a very definitive difference between bodies that differentiate and those who don’t.

        Ultimately I think astronomical objects that stems from a protoplanetary disc, excluding the star/stars, can most simply and fully be characterized by the two pathways of population evolution (IAU definition) and minerological evolution (Hazen definition).

        The minerological evolution starts with nucleosynthesis (a few ices, mainly hydrogen) over stellar synthesis and protoplanetary disk aggregation (some tens minerals such as silicates), impact aggregation (some hundred minerals such as hydrates), differentiation (some few hundred minerals such as oxides), habitable zone terrestrials (some thousand minerals, see Mars) to oxygenated biosphere planets (some thousands of minerals by biosphere processes, see Earth).

        Eris, Pluto, Ceres, down to asteroids a few hundred meters place in the differentation minerals category. Well placed protoplanets like Mars places as bona fide terrestrial planets despite being diminutive. But only oxygenated biosphere planets will be “superplanets”.

      2. These classifications are new to me. Do you have a paper that explain this classification systems?

  5. I hope someday, as more and more exoplanets are being discovered, a new better classification of planets will come out — not by its size, but by its appearance or by its distance from its star. I hate calling Pluto a dwarf planet, although it is.

  6. I can’t get over how sad it is that there are people — and evidently Grey believes there are enough such people to merit calling them “you” — who have never heard of asteroids.

    That is just … tragic.

  7. Mr Major, I find it very interesting that it’s those who are not in the same “class” as Clyde Tombaugh or anyone else who discovers something astronomically, find it necessary to diss those who do? Go out and make your own discovery and then sit back and let others tell you the same…I doubt sincerely you would like it very much!

  8. Mr Major, I find it very interesting that it’s those who are not in the same “class” as Clyde Tombaugh or anyone else who discovers something astronomically, find it necessary to diss those who do? Go out and make your own discovery and then sit back and let others tell you the same…I doubt sincerely you would like it very much!

    1. It is my understanding, such as it is, that discussing observational results is lauding the observers, not dissing them.

    2. So you’re dissing Mike Brown, the discoverer or Eris, who says, himself, that Eris is not a planet?=

  9. Even right after its discovery, there were many who said Pluto wasn’t a planet. It was too small, the orbit was too eccentric, and so on. There was also the cautionary tale of the early asteroids, which immediately after discovery were declared to be planets, only to later have their classification changed.

    Changing which folder we put Pluto in doesn’t change what it *is*. Though I think that instead of “dwarf planet” we should declare it to be the first known Tombaugh Object. After the man who discovered it.

    1. Didn’t Kuiper predict them before that, however? Don’t have time to look it up right now, but my (admittedly sometimes poor) memory tells me Kuiper predicted them, which is why it is now known as the Kuiper Belt.

      1. Kuiper’s article was in 1951 where he posited a large number of objects beyond Neptune but expected that Pluto (which many scientist’s then believed to have mass about the size of the Earth) would have scattered these objects out of the solar system. He wasn’t the first to suggest it (Edgeworth 1943) nor the last but it is his name that has stuck, even though he no longer believed it existed.

  10. Great video. Good presentation of the argument. I wish it had been around a few years ago. In 2008 I gave a presentation to my son’s Grade 5 class (11 year olds). I started by asking them how many plantes there were, and received answers varying from 6 up to thousands, so led them on to ask what is a planet. Then I gave each of them a sheet with data from one object from the solar system which at some time or other had been called a planet (including sun, moon, traditional planets, asteroids, Jovian satellites, and TNOs) and we went through each attribute in turn, with those having the attribute (e.g. orbiting the sun) on one side of the room and those not on the other. They really got into it and had a great time discussing with each other whether or not they were a planet. Then at the end I gave each of them their object’s name and explained the IAU’s decision. My son still remembers that class so I guess it worked.

  11. I think the real reason to abandon Pluto from the list of planets is the fact that the discoverer of Pluto Clyde Tombaugh, also witnessed a fleet of UFO’s on 20 aug. 1949 and made clear that UFO’s are observed by astronomers to. As this information is easy to find on Google, the best way to prevent the school kids to read about this important UFO witness is to get rid of him by degrade Pluto. Clever but sad. However, it is prove that science is politicized and not neutral as it should be.

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