Is Pluto a Planet?

Computer-generated map of Pluto from Hubble images, synthesized true color

When picking up The Daughter, I was horrified to see a montage of the solar system on the wall of an adjacent classroom with nine planets.

NINE PLANETS?!?! That’s so 2005.

It’s not that I have anything against Pluto. The problem is that it was mislabeled as a planet when it was discovered because of some bad observations of Neptune. I read Planets X and Pluto a few weeks ago so I had still had a bunch of history and science in my head.

By coincidence, March 13 is the day that the discovery of Pluto was announced. So I put together a post on GeekDad: Happy Pluto Discovery Day.

If still think there are nine planets, you definitely need to read Happy Pluto Discovery Day.

Some of my recent GeekDad posts:

  1. Assembling LEGO Creator Super Speedster
  2. Assembling the LEGO Atlantis Neptune Carrier
  3. 20 Geeky Images from Space
  4. Assembling the LEGO Atlantis Turbo Typhoon Sub
  5. Own Your World With Location-Based Mobile Games
  6. Catalog Your Books Online


  1. Pluto’s original classification as a planet was NOT a mistake. Pluto is still a planet, as are Ceres, Haumea, Makemake, and Eris. Please do not blindly accept the controversial demotion of Pluto, which was done by only four percent of the International Astronomical Union, most of whom are not planetary scientists. Their decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto. Stern and like-minded scientists favor a broader planet definition that includes any non-self-luminous spheroidal body in orbit around a star. The spherical part is important because objects become spherical when they attain a state known as hydrostatic equilibrium, meaning they are large enough for their own gravity to pull them into a round shape. This is a characteristic of planets and not of shapeless asteroids and Kuiper Belt Objects. Pluto meets this criterion and is therefore a planet. Using this broader definition gives our solar system 13 planets and counting: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris. At the very least, you should note that there is an ongoing debate rather than portraying one side as fact when it is only one interpretation of fact.

    1. Laurel –

      I discuss more of the debate over the GeekDad post. The commenters there come down on both sides.

      In my view, the four terrestrial planets are similar and the four gas giants are similar. The others are different.

      I don’t like the IAU definitions. We are likely to find objects that are in an empty orbit so the dominate their neighborhood just because there is nothing else there. We are also likely to find objects that are just on the cusp of hydrostatic equilibrium that will leave us arguing whether the object is massive enough or round enough.

      One thing we can agree on is that there are not nine planets in our solar system.

    2. I’m very much sorry. Your answer is not right. I hate to be mean and disrubitive, I just want you to know that Pluto is too small, to be a planet, it is a star.
      Thanks much. Please; contact me at (507) (995) (4636). I am a science teacher at the school of Harvard University. I would be honored to help all of you with science.
      Professor Dustin Maass.

  2. Thank you for the response. Yes, the four terrestrial planets are similar to one another, and the four gas giants are similar to one another. But that does not mean there are only two types of planets. Dwarf planets should be added as a third subclass of planets for small objects that are large enough to be in hydrostatic equilibrium but not large enough to gravitationally dominate their orbits. Also, with the rapid discovery of exoplanets, the chances are good that we will have find objects that fit none of these three classes and may have to create additional planet categories.

    1. Miss. Jozzye, and friend.
      You two are absolutly right. Pluto is very much not a planet.
      Thank you for your outstanding comments.
      Professor Maass.

  3. hey jozzye you still on im downstairs in Zenk

  4. im 11 years old and i already have my own theory. i think pluto IS a planet. well it has its own orbit and i know it has a very strange orbit well i think that the kuiper belt exploded and a big chunk of it crashed into pluto and that might be why it has a strange orbit. its moon is very simaler to its size i think karen is also a piece of the kuiper belt that got pulled by pluto’s gravitationol pull and instead of orbitting the sun it orbits pluto. all of this i thought of myself

      1. Im also 11 yrs old and i totally agree with u hes crazy lets just agree that until another planet is discovered than theres 8 planets so yea just wait until and object defined by the IAE meets the criteria but for now theres 8 planets

  5. no im not it could be true

  6. why do u think im crazy? it could happen how could it not? i want some reasons and i wanna see your theories. it wasnt very nice to say im crazy cause i worked realy realy hard i googled and did wikipiedia for HOURS and then i came up with those theories so there!!!

  7. oh and professer mass how could pluto be a star i mean it doesnt make sense cuz a star is a big ball of gas and pluto is a terestial planet… right?

  8. Sorry for my bad grammer.

  9. Please write back.

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