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Now Its Official: Eight Planets (Pluto got bumped)

  1. Aug 24, 2006 #1


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    Some fascinating resolutions were made at this years'
    IAU (international astronomical union) General Assembly 2006 Meeting

    One that has caught our attention recently: definition of a "planet". Pluto is not amoung them. (instead it is placed in a new category: "dwarf planet" )

    Last edited: Aug 24, 2006
  2. jcsd
  3. Aug 24, 2006 #2
    We already have about 5 threads about this...
  4. Aug 24, 2006 #3
    I'm no astronomer, but I don't like the new system. I think a planet should be anything that is rigid and has enough gravity to be round. This would mean that the sun has five planets:


    It would also mean that the Earth had one planet and Jupiter, Saturn, and Uranus each have a mess of planets. All together the solar system has 20+ planets.

    I'd say if it's really gassy and you can't stand on it, then it's not a planet. This means Jupiter and Saturn should get bumped from the list. I mean, how can we possibly put Mercury and Jupiter in the same category? Jupiter and Saturn are planetary systems just by themselves

    I could also be persuaded of the view that only the gas giants are planets while Earth and many other familiar bodies are asteroids.

    We could just dump the categorization system all together. It will never be perfect.

    The current system bumps planets if they have yet to clear their orbital path, or if they are not in a nice Earth-like orbit. In my opinion, this is the equivalent of saying that an electron is not an electron if it is not bound to a proton. If Saturn's moon Titan was orbiting the sun, it would definitely be called a planet. Titan has no intrinsic property that makes it a moon. It is thus my position that the current system makes no more sense than the previous one.
    Last edited: Aug 25, 2006
  5. Aug 25, 2006 #4


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    Unlike an electron, a planet's or moon's identity is partly about where it is.

    What we understand as a moon is definitely not something that would be orbiting the sun; moons are dependent on planets. Planets, likewise, orbit the Sun.

    So what I'm saying is that, true, Titan has no intrinsic property in a vacuum (that's metaphorically, not literally) that makes it a moon, it's about Titan's context. That's part of how we define them.
    Last edited: Aug 25, 2006
  6. Aug 26, 2006 #5
    Yes, but I would say that Earth is a planet, and that Saturn is more like the Sun than the Earth. Therefore Titan is obiting a star-like body and should be given the designation of planet.

    As far as moons go, what would we do if both objects orbit a point that lies somewhere in between the two objects?
  7. Aug 26, 2006 #6


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    Newton help us all if you ever get on that committee:surprised
  8. Aug 26, 2006 #7


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    http://edition.cnn.com/2006/TECH/space/08/24/pluto.ap/index.html?section=cnn_space&ref=google [Broken]

    Pluto no longer a planet, say astronomers

    Maybe they should have told the world that they were considering changing the designation of Pluto as a planet and given everyone time to adjust to the idea.

    I presume by cleared, one means that the planet has accreted or absorbed smaller masses to form a large unique mass.

    Some more news on the matter and a nice history summary:

    Pluto, the Un-Planet?
    by David Kestenbaum
    Last edited by a moderator: May 2, 2017
  9. Aug 26, 2006 #8


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    Here's a sister thread regarding the fate of Pluto, in one of our more laid-back forums.:smile:
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