Dot Physics

Pluto. Planet, or not?

Still moving stuff over from my last server. Here is one that keeps coming up.

Surprisingly, I STILL get comments and questions from students and teachers about Pluto. Questions such as:

  • “Why do scientists hate Pluto?”
  • “How did they discover that Pluto was not a planet”
  • “What will happen to our planet songs without Pluto?”
  • “Why does Goofey wear clothes, but Pluto doesn’t even though they are BOTH dogs?”

Here is the explanation I like to give. I like to start with the following question:

Suppose you were outside and saw this (image from wikimedia): i-90e3bc5df8eeee107f42460b0ec6da27-leafy.jpg

What would you call it? Some might call it a tree, others might call it a bush. Does the name change what it is? What if scientist came up a classification for determining if something was a bush or a tree. According to wikipedia, there is not complete agreement on the definition of a tree. This is ok, because the name of something isn’t really that important.

That is the point, Pluto is like a green leafy plant that some would say is a bush and some would say is a tree. To make things clear, scientist have created the following definition of a planet. A planet is an object that meets the following requirements:

  • Orbits the sun (this is ok for Pluto)
  • Large enough to be spherical (this is also ok for Pluto)
  • The most massive object in its neighborhood of the solar system (this is where Pluto fails).

Pluto fails the last criteria because it is in the area of Neptune (sometimes it is even closer to the Sun than Neptune). Thus, alas, Pluto is not classified as a planet. It shall be called a dwarf planet.


  1. #1 Adam Solomon
    August 4, 2009

    Wait, what? It was all true until the very last sentence 🙂 The problem with Pluto isn’t that it crosses Neptune’s orbit 1/10 of the time. It’s that it’s a member of the Kuiper Belt which is full of similarly-sized objects. That’s why it doesn’t meet criterion #3.

  2. #2 Diane Loh
    September 23, 2009

    Very interresting! I will lead all my friends! I actually am researching about PLuto. Need alot of information! Next week monday, I need to present my presentation. I have a skit too! Very funny!!! NoW I dont know what to even say, dude…

  3. #3 Hellstromm
    September 28, 2009

    Adam is correct, there are a few errors near the end, although it is not merely that Pluto is a member of the Kuiper Belt — #3 is incorrectly stated. It is not “most massive,” it is “cleared the neighborhood,” which means it has obtained gravitational & mass dominance in its respective neighborhood.

    Pluto is the largest object in the Kuiper Belt (Neptune is not part of the Kuiper Belt), but it constitutes only 0.07% of all the mass in the neighborhood (Kuiper Belt) and is sufficiently influenced by other objects in the Kuiper Belt so as to undermine its qualification for #1 (orbiting the Sun). Still, the ultimate disqualification is #3, in that it does not dominate within it’s neighborhood, within the Kuiper Belt.

  4. #4 Hellstromm
    September 28, 2009

    Correcting myself, Pluto constitutes only 7% of all the mass in its neighborhood.

  5. #5 tryphena
    November 2, 2009

    pluto for me is a planet it was first a planet then no more now just 8 planets

  6. #6 Christina
    December 15, 2009

    Actually, Pluto’s not even the largest known Kuiper Belt object, that honor goes to Eris. And given how little we know of the Kuiper Belt, it’s possible that there may a few others larger than Pluto that we haven’t found yet.

  7. #7 Thoufeeq
    December 22, 2009

    It is true because the universe also runs to some gravity power , but the pluto was some changes to other planets.

  8. #8 Laurel Kornfeld
    March 24, 2010

    That third requirement stating an object must “clear its orbit” to be considered a planet was contrived for the specific purpose of excluding Pluto and should be thrown out. 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.

  9. #9 Tristan
    February 12, 2011

    Pluto should be a planet!!!, here is a funny joke I saw about why pluto should be a planet,

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