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Meaning of Planet? Just 8 planets?

  1. Jul 18, 2004 #1


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    This has been bothering me for some time, no one has a good answer to it. I read a while ago some information about the meaning of "planet" and it said that Pluto isn't a planet, that there are just 8 planets in our solar system. Why? I read that the meaning of "planet" is: A solitary body, for example the Earth, there are no other bodies close ti it with the same characteristics (size especially) so it is a solitary body. Ceres used to be considered a planet long ago but when people found out that there were other asteroids about the same size as Ceres and close to it they re-clasified Ceres as an asteroid (The Asteroid Belt between Mars and Jupiter). Same with Pluto, there are bodies about the same size of Pluto near him (the Kupier Belt) so according to the meaning I read Pluto wasn't a planet. Same with Sedna, they don't want to consider it a planet because if someone in a near future finds that it belongs to a population of similar bodies then they will have to re-clasify Sedna.

    Can anyone explain this to me? English is my second language and it's hard for me to explain myself in English so if you don't get my point, please let me know so I can post the site in which I found the information.
  2. jcsd
  3. Jul 19, 2004 #2
    I can only speak for myself when I say "who cares" A planet is whatever astronomers decide it will be, if mangos are an admissible planet then there are many near-earth planets. Its just a classification scheme, just a reasonable, hopefully consistent, method of labelling this thing and that thing. Don't worry about what they're called, learn how they're made instead.

  4. Jul 19, 2004 #3


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    From http://www.gps.caltech.edu/~mbrown/sedna/

    - Warren
  5. Jul 19, 2004 #4


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    Welcome to Physics Forums, woz!

    Chroot's explanation describes it well. If Pluto were to be discovered today, then it might have been called a Kuiper Belt Object instead of a planet. But, there are good arguments for and against calling Pluto a planet.

    The bottom line for now is that the International Astronomical Union gets to make the decision and they are still calling Pluto a planet.
  6. Jul 23, 2004 #5

    What is the idea around giving the name of this Kuiper Belt Object the name Sedna? The goddess of water, is that suppose to have some meaning at temperatures approaching absolute 0?

    I was going to ask, how many more of these KBO,s have been discovered?
    http://www.ifa.hawaii.edu/faculty/jewitt/kb.html [Broken]
    Last edited by a moderator: May 1, 2017
  7. Jul 29, 2005 #6
    So, what happened to Sedna?

    Is it "alive" or not? :)
    I mean is it accepted as a planet or not?

    Any nice link about recent researches (or news) in Astronomy is appreciated.
    Thank you!
    Last edited: Jul 29, 2005
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