Pluto is not a Planet

Take out a sharpie and start crossing out your text book. Our solar system just got a tad smaller (well, sort of). We have eight planets, not nine. The IAU (International Astronomical Union) has decreed that (among other things) Pluto (and Ceres) are no longer defined as planets, but as dwarf planets.

This means that the Solar System consists of eight “planets” Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called “dwarf planets” was also decided. It was agreed that “planets” and “dwarf planets” are two distinct classes of objects. The first members of the “dwarf planet” category are Ceres, Pluto and 2003 UB313 (temporary name). More “dwarf planets” are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate “dwarf planets” are listed on IAU’s “dwarf planet” watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.

What is the universe coming to!?

[Addendum] For loads of more information, please check out the IAU General Assembly Special 2006 at The Jodcast (via the comments at Tom’s Astronomy Blog: Pluto is NOT a Planet).

[Addendum II] I want to make it clear WHY Pluto is not a planet (as I have been getting lots of search referrals here relating to this) - I will be referencing IAU’s resolutions 5 and 6 (PDF): It is because of the nature of Pluto’s orbit (in that it does not “clear its neighborhood” [there are objects in its orbit] and that it crosses with Neptune’s orbit). The following reasons DO NOT have any bearing on the decision: (1) Its size (Resolution 5.1.b and 5.2.b are identical meaning that both a dwarf planet and a planet must have sufficient mass to become round, this does not mean that a dwarf planet must be small in size [although generally they are because they don't have enough mass to comply with resolution 5.1.c, so in that sense, size does play a factor, but only in that sense.]). (2) New scientific information/evidence (there was NO NEW DATA discovered about Pluto to changed how we describe Pluto, it was merely a decision by the astronomical community to change how a planet is defined). (3) A realization that Pluto is not a planet (no, the definition of Planet just changed). (4) Pluto is not part of the Solar System (no, Pluto is part of the Solar System, in the same orbit it has always been in, it is just defined differently). (5) New scientific theory about planets (this isn’t exactly right. It doesn’t have anything to do with scientific theories, it has to do with scientific definitions, the thories about planets and how they work has not changed).

I hope that helps, let me know if you have any questions.

[Addendum III] Also, I wanted to add that the reason this came up was because of the discovery of other objects in out Solar System that didn’t seem to quite be asteroids but also didn’t seem to be planets. In order to keep things consistent, this debate came up. It either meant making the new objects into new planets or creating a new class as a way to define these objects. When that was done, Pluto fit into that new category, the Dwarf Planet. Eris (formerly nicknamed Xena) especially, which is actually larger than Pluto, would have been considered a planet had the decision gone the other way. With the new class, Eris is currently the largest Dwarf Planet.

[Addendum IV] Came across a great article by NASA’s Chief Historian, Steven J. Dick: Pluto, Classification and Exploration. It is a great read.

[Addendum V] As I have said, the reason Pluto is not a planet is because it did not pass the third criterion: a planet must “clear its neighborhood” meaning it must remove other objects in its orbit. While looking at the Eris WikiPedia article, I came across a great image showing all the trans-Neptunian objects. Check it out for a VERY helpful visual as to what “clearing the neighborhood” would mean. All the little red dots are “Plutinos” or objects that share Pluto’s orbit. These objects are what remove Pluto from the planetary lineup. You will also notice that ther other planets also have other objects in their orbital distance, this is where the ambiguity in the language of the new definition comes in. Technically it seems that most planets have not completely cleared their neighborhood. But the visual does a good job of showing the huge difference in number of objects in each planet’s (or dwarf planet’s) path, which I am assuming was a factor.

[Addendum VI] Ok, to be explicit, here are the criteria for being a planet: Resolution 5.1

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

It is this final criterion that Pluto fails to meet as discussed in Addendum V above.

[UPDATE] Also, Pluto has a new name: 134340 Pluto! Exciting, eh? Just another common asteroid…

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  1. August 24th, 2006 at 17:04 | #1

    I’ve been reading in the headlines that they expanded the definition to include Pluto, Ceres, and UBlkr32lkr390000 (or whatever). Did they change their minds? Did the discussions take a different direction?

  2. August 24th, 2006 at 18:04 | #2

    Yeah, that was the discussion. There were possibilities that they would include a number of smaller objects in the list of “planets” (including Charon). But this is where they ended the discussion. The final is that Pluto is no longer considered a planet (I think mainly because of its orbit).

  3. August 26th, 2006 at 14:01 | #3

    I had to smile to see these two posts one after the other.

    Don’t you see that the powers of our observation are limited, sin-tainted, and subject to fail?

    “Accepted” scientific thought is constantly changing.

    God’s Word never changes. That’s why I believe it instead the current scientific theory. I don’t deny there may be parts of current science that are true. But I can’t accept any current scientific theory that contradicts God’s Word.

    Today, Pluto is no longer a planet. Tomorrow, we admit our carbon dating system is bogus.

  4. August 26th, 2006 at 14:22 | #4

    You are trying to compare apples to oranges. Pluto not being a planet has not changed any scientific theories, the only thing that changed is how it is defined in the community. It wasn’t some new information that forced us to rethink how we view Pluto, it was a panel of people changing what we call it.

    We could change the name of the color “red” to “belian” but it would still have a frequency range of 630-760 nm.

    That being said, yes, scientific thought does have to change as new evidence is found. But there is such a preponderance of evidence from numerous sources that tells us the universe is 14 billion years old. Maybe the Big Bang isn’t correct, but everything seems to point to something along those lines. Maybe F doesn’t equal m*a, but everything seems to point to it being so.

    Christian need to realized that it is their interpretations of the Bible that will contradict scientific evidence, not the Bible itself.

  5. September 21st, 2006 at 12:52 | #5

    Adam Freese: Actually, the fact that Pluto’s path crosses Neptune’s has to bearing on the decision since the two objects exhibit an orbital resonance. Pluto’s not a planet because it’s not dynamically important enough to clear its orbital neighborhood of planetesimals.

    Thanks for pointing that out. But the fact the Pluto does cross Neptune’s path also negates the third criteria. But yes, not clearing their planetesimals is a crucial factor. This also gets into some of the ambiguity in the IAU’s definition.

  6. merlyn
    September 2nd, 2007 at 06:02 | #6

    pluto is not longer a planet because it has insufficient body mass

  7. September 2nd, 2007 at 11:08 | #7


  8. September 5th, 2007 at 14:55 | #8

    Um, yeah, “God’s Word” DOES in fact change. Just take two different translations of the Bible and compare them.


    You can start here:

    Not to mention there are hundreds of other religions out there all claiming to have “God’s Word” written down in their little books.

  9. September 5th, 2007 at 17:42 | #9

    Eh? Was that a Spam comment? What the heck does that have to do with Pluto?

    As far as your comment goes: Christians don’t say that the Bible is literally the Word of God (as in actually spoken by Him, that would be the Koran). Whey we do say is that it is inspired by God. And the fact that there are different translations doesn’t mean that the original words have changed… Also, just because there are “hundreds” of other religions that claim to have God’s word, doesn’t mean they really do.

  10. January 14th, 2008 at 11:41 | #10

    i like the fact that you have reasons why pluto is not a planet. on most of the web pages i looked at they did not give me any reasons. your website realy helped me with my reaserch i had to do. mabe you could put on more reasons!

  11. sofia
    August 13th, 2008 at 13:55 | #11

    i like this order of the planets

    June 15th, 2009 at 15:38 | #12


  1. August 27th, 2006 at 21:29 | #1
  2. September 1st, 2006 at 01:05 | #2
  3. September 14th, 2006 at 17:21 | #3
  4. September 23rd, 2006 at 15:41 | #4