Why is Pluto not a planet?

7 Answers

  • Otis F
    Lv 7
    1 decade ago
    Favorite Answer

    The International Astronomical Union decided that Pluto was no longer a planet. The reason is that while Pluto is round, orbits the sun, and has three moons, it has not cleared (via gravity) its own orbit of debris. Instead, they decided to classify it as a "dwarf planet".

    See the details below.


    The IAU therefore resolves that "planets" and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

    (1) A "planet" [footnote 1] is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

    (2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [footnote 2] , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

    (3) All other objects [footnote 3] except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".

    Footnote 1: The eight "planets" are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

    Footnote 2: An IAU process will be established to assign borderline objects into either dwarf planet and other categories.

    Footnote 3: These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.


    The IAU further resolves:

    Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

  • 1 decade ago



    on 24 august 2006, the international astronomical union reclassified pluto.

    pluto does orbit the sun, is ball-shaped and is not a satellite, but it does not have an isolated orbit (a bunch of other similar bodies have similar orbits.) so it is not a planet.

    this was the right thing to do, believe me. this does not change anything about pluto or the solar system. this just corrects the mistake of classifying pluto as a planet initially.

    i have been waiting for this since i was about ten when i learned that pluto didn't fit the pattern set by the major bodies in the solar system so it was an anomaly. it just felt "out of place". now that astronomers have found hundreds of other bodies with similar orbits, classifying "134340 pluto" as a planet is even more irrational. i feel somewhat satisfied, but i don't know how long this will drag on tho. many planetary astronomers are satisfied that the definition is rigorous enuf. i can accept that the definition is flawed, but i can not accept that "134340 pluto" is a planet.

    this same thing happened has happened before. in 1800, an astronomer found a body orbiting the sun between the orbits of mars and jupiter and thought it was a planet. astronomers finally stopped classifying them as planets after they found several other bodies with similar orbits, and no one thinks ceres, pallas, juno, and vesta are planets today.

    many astronomers consider pluto and charon to be a binary system, but two small bodies orbit that system. they are called nix and hydra.

    incidentally, "134340 pluto" was never a moon of neptune. neptune did capture triton. this is why triton has a retrograde orbit.

  • 1 decade ago

    After a long debate the final decisions is that pluto is an asteroid.This is because the other planets move around the sun without crossing the paths of the fello planets.While as pluto crosses the path of the other planets.Also pluto is very small.Its too small for it to be considered as a planet.

  • Bob
    Lv 7
    1 decade ago

    Because it's simply one of a number of similar objects out there, the Kuiper Belt Objects. We found them years after we found Pluto.

    It's like Ceres. When it was discovered it was called a planet. As many other similar asteroids were discovered, Ceres was demoted too.

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  • Anonymous
    1 decade ago

    They say that mainly because of its size. But it has a moon and is round and it orbits the sun! If that is not a planet then mabe the Earth should be called an Asteroid!

  • 1 decade ago

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  • 1 decade ago

    because its mass is not large enough and it is not a perfect planetary sphere

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