Pluto Is Not a Planet and Never Was

Before you read another word, please post a one word comment below answering the question: Do you believe Pluto should be classified as a planet?

Done? OK here we go.

Earlier this year I was fortunate to listen to Mike Brown, the man that ‘killed’ Pluto. How did he do this? With data and reasoning. In his most recent blog post he provides two visuals that help to understand why Pluto is not a traditional planet. Mike’s latest post:

The Kuiper belt with Eris and Pluto and the rest is really there, way off on the right side. Try squinting.

The Kuiper belt with Eris and Pluto and the rest is really there, way off on the right side.

During his presentation at on April 15 at the 2009 Science Lecture at Sarah Lawrence College, available for viewing online, Mike reasoned through the decision. He is very entertaining akin to Carl Sagan.

“How I Killed Pluto and Why it Had it Coming”

A quick tid-bit about Pluto:
Pluto is about half the size of the moon.

Comparison of Eris, Pluto, Makemake, Haumea, Sedna, Orcus, Quaoar, Varuna, and Earth (all to scale).

And now some older info that helps put things into greater perspective…

As of 2008, only five objects in the Solar System, Ceres, Pluto, Eris, Makemake and Haumea, are considered dwarf planets. Wikipedia

That’s right, five dwarf planets.

cnn-poll-plutoThis all got started for me when I saw this article on CNN ( Actually it started when I saw a poll on the front page. If I hadn’t seen the presentation I would have opted not answer the poll because I would not have known how to classify a planet accurately. Most people do not hesitate to voice their opinion. The results astounded me, and so I felt it imperative to write about the death of Pluto and provide resources to help others learn what I learned.

Pluto is not a planet and never was. It is a part of an asteroid belt and shares the size of other planet like object within the Kuiper Belt. Blow you mind and look at the number of objects in the Kuiper Belt on the Wiki page –

It is easy to feel that something has been taken away but in fact something has been more appropriately classified. The definition of a planet is not good. Mike told a brief story during his presentation on how the definition for a planet came to be. Paraphrasing, a definition was needed, it was 3 am,  people were tired, and something was needed by 7am. And so a lawyer like definition came into existence. Not the kind of thing one wants in the world of science.

So, now that you know all/some of the reasoning… Do you now understand why Pluto is not a planet and never was? Be sure to toss out your emotions before chiming in. I grew up with Pluto as a planet and my Animaniacs CD is far from accurate. Things change in science and this is OK.


  1. Mike Brown did NOT kill planet Pluto. Pluto IS a planet and not just a member of an “asteroid belt” because it is large enough for its own gravity to pull itself into a round shape–a state known as hydrostatic equilibrium.

    Brown’s view represents only one side of an ongoing controversy. Only four percent of the IAU voted on the controversial demotion, and most are not planetary scientists. Hundreds of professional astronomers led by New Horizons Principal Investigator Alan Stern immediately rejected their definition in a formal petition.

    Supporters of the IAU demotion like to ascribe the motives of their opponents as “emotional,” but this is a straw man. There is strong scientific reasoning for keeping Pluto a planet. In fact, one could argue that the insistence on keeping the number of planets in our solar system artificially low is motivated by emotions as well.

    Brown’s ego is personally wrapped up in his notion of him having “killed” Pluto. His denial that any opposition to his position exists among astronomers is highly problematic and quite unprofessional.

    The IAU definition makes no sense for two reasons: First, it states dwarf planets are not planets at all. This is like saying a grizzly bear is not a bear and is inconsistent with the use of dwarf in astronomy, where dwarf stars are still stars and dwarf galaxies are still galaxies.

    Second, the IAU definition classifies objects solely by where they are while ignoring what they are. If Earth were in Pluto’s orbit, according to the IAU definition, it would not be a planet either. A definition that takes the same object and makes it a planet in one location and not a planet in another is essentially useless.

    There is an easy way to correct this: amend the IAU resolution to establish dwarf planets as a new subclass of planets–planets because they are in hydrostatic equilibrium but of the dwarf subcategory because they do not gravitationally dominate their orbits.

  2. One word answer? Yes Pluto is a planet.

    The IAU got it very badly wrong.

    I favour an inclusive and broad definition of ‘planet’ & then subdividing planets into major classes. I consider Pluto and the other ice dwarfs to be planets & there to be 3 main classes of ‘planet’ in our solar system, gas giants, rocky terrestrial worlds and ice dwarfs.

    My preferred definition would be that a planet is :

    a) Never self luminous by nuclear fusion thus not a star,

    b) not directly orbiting another planet thus not a moon
    c)large enough to be round or if rapidly spinning oblate spheroidal rounded through its own gravity thus not a comet or asteroid.

    The third IAU criteria thrown up undemocratically and unscientifically at the last minute when the real experts ( like Alan Stern) weren’t there fails basic tests of logic like ‘reductio ad absurdum’, & the KISS principle and is hard to define in itself. Eg. What’s meant by a cleared orbit? How far out? What about binary planets and exoplanets that violate this rule & so on. That there are many more planets in our solar system -including Pluto – is to be celebrated not bemoaned – the more the merrier! Why Mike Brown has come down so strongly and unreasonably on the IAU side of this debate is beyond me. The debate isn’t going anywhere until the IAU sees reason and admits and corrects its mistake.

  3. Of course Pluto is a planet!

    A DWARF planet 😀

    Actually the most important planet to space colonization is going to be poor, neglected Ceres. It is differentiated, that means the iron has sunk to the center, the rock just above that, and the lighter rock above that and –


    Ice means water. None of the other asteroids has that (I would hope, for sentimental reasons, that Ceres will still be called an “asteroid”). There are probably deposits of organic materials. Everything we need.

    Someday, backyard astronomers will focus on Ceres and see a soft, green nebulosity surrounding it. This will be millions of space colonies, ready to move out into the cosmos. It will look like a green version of the Pleaides, with a bright “starlike” object in the center of them. Within the green nebulosity will be hamburger stands, school buildings, rock concerts, homes, gardens, children, dogs, cats, petunias, and all kinds of wonderful things.

    And of course there will be arguments, fights, alcoholism, homeless bums, crime, and punishment.

    And there will be things we can’t even imagine. Could Devonian fish have imagined campfires, hot dogs, and marshmallows? There was no reference.

    Pluto is dwarf planet. It is a Kuiper belt object. It is a transneptunian object, a plutino, a plutoid, and (I believe) a Lagrangian. We really don’t need to fixate on one of these catagories exclusively. It depends on the context.

    Maybe somebody (Laurel, maybe? she’s a good writer) will invent a word for “an astronomical object which has an atmosphere and a mixed liquid/solid surface”. There are only two of that catagory in our solar system, Earth and Saturn’s moon Titan. I think this is what most people mean by the word, “planet”. A place where life can start, grow, diversify, think, invent words and poems and pencils and plans and planes. A place you can become sentimentally attached to. A home.

    We know of only one of those.

    So far.

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