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    Pluto Redefinition as a Dwarf Planet
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    Pluto Redefinition as a Dwarf Planet

    See also:
    Analysis of Pluto’s Redefinition
    The Astrology of Pluto, the New "Prototype"
    Scientists decide Pluto’s no longer a planet
    Pluto's redefinition as a "dwarf planet"

    The definition of a planet has comprised many different things over the centuries, often simultaneously. The use of the term was never strict and its meaning has blurred to include or exclude a variety of different objects, from the Sun and the Moon to satellites and asteroids.

    The 2006 definition of "planet" by the International Astronomical Union (IAU) states that, in the solar system, a planet is a celestial body that:

    • is in orbit around the Sun,
    • has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape, and
    • has "cleared the neighborhood" around its orbit.

    A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet" (like Pluto), whilst a non-satellite body fulfilling only the first criterion is termed a "small solar system body" (SSSB). The definition was a controversial one, and has been both criticised and supported by different astronomers.

    According to the IAU's new definition (as passed on 24 August 2006), instead of the traditional nine planet model, there are eight planets and three dwarf planets in the Solar System.

    The eight planets:

    • Mercury, with no confirmed natural satellites
    • Venus, with no confirmed natural satellites
    • Earth, with one confirmed natural satellite, the Moon
    • Mars, with two confirmed natural satellites
    • Jupiter, with sixty-three confirmed natural satellites
    • Saturn, with fifty-six confirmed natural satellites
    • Uranus, with twenty-seven confirmed natural satellites
    • Neptune, with thirteen confirmed natural satellites

    The three dwarf planets:

    • Ceres, with no confirmed natural satellites
    • Pluto, with three confirmed natural satellites
    • Eris, with one confirmed natural satellite
    The definition does not apply outside the solar system, and so does not include provision for extrasolar planets. Exoplanets are covered separately under a 2003 draft guideline for the definition of planets.

    Reasons for the debate

    Since the discovery of Pluto in 1930 astronomers had considered the solar system to have nine planets and assorted other bodies. However, since 2000 the discovery of at least three bodies (Quaoar, Sedna and Eris), all comparable to Pluto in terms of size and orbit, had led to a situation where either the minor bodies would have to be added to the list of officially recognized planets or older ones would need to be removed in order to ensure consistency in definition. There were also concerns surrounding the classification of planets in other solar systems. In 2006 the matter came to a head with the need to categorize and name the recently-discovered trans-Neptunian object Eris, which, being larger than Pluto, was thought to be at least equally deserving of the status of 'planet'.

    Debate within the IAU led Julio Fernández and Gonzalo Tancredi of Uruguay to suggest proposals to redefine the term "planet" so as to include other objects beyond the traditional nine planets that had been historically considered part of the solar system. In its final form the proposal was denoted as Resolutions 5A, 5B, 6A and 6B for GA-XXVI. Members of the IAU's General Assembly voted on the proposal on August 24, 2006 in Prague, Czech Republic, with the vote removing Pluto's status as a planet and reclassifying it as a dwarf planet. Pluto had long been exceptional among the planets, being small, distant, and eccentric in orbit.

    In its original form the redefinition would have kept Pluto as a planet and recognized three new planets: Ceres, Charon, and Eris. It was presumed that, after more observation and discussion, astronomers would accept more objects in the solar system as meeting the new definition. On August 22, however, the original redefinition (which recognized twelve solar system planets, including Pluto), was dealt a fatal blow in two open IAU meetings. Jay Pasachoff of Williams College, who attended both meetings, was quoted as saying, "I think that today can go down as 'the day we lost Pluto' ".

    Final definition

    The final definition, as passed on 24 August 2006 is:

    The IAU...resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:

    (1) A "planet" [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 [2], (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

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


    [1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
    [2] An IAU process will be established to assign borderline objects into "dwarf planet" and other categories.
    [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.



    There continues to be criticism regarding the wording of the final draft of the definition. Notably, the lead scientist on NASA's robotic mission to Pluto, Alan Stern, contends that, like Pluto, Earth, Mars, Jupiter and Neptune have not fully cleared their orbital zones either. Earth orbits with 10,000 near-Earth asteroids. Jupiter, meanwhile, is accompanied by 100,000 Trojan asteroids on its orbital path. "If Neptune had cleared its zone, Pluto wouldn't be there," he added. However, his own earlier work on neighborhood clearing supported the distinction between the largest eight planets and the rest of the solar system. There is a substantial difference in the extent to which the neighborhood has been cleared between Pluto and the eight planets. Also, Pluto's position is due to the gravitational effects of Neptune as they are in orbital resonance.

    The debates have clarified that "clearing its orbit" refers to the process that happened during the formation of the planets. It does not talk about the presence of bodies that later strayed into the orbit after the accretions took place.

    The definition may be difficult to apply outside our solar system. Techniques for identifying extrasolar objects generally cannot determine if an object has "cleared its orbit," except indirectly via Stern and Levison's Λ parameter, and provide limited information about when the objects were formed. The wording of the new definition is heliocentric in its use of the word Sun instead of star or stars, and is thus not applicable to the numerous objects that have been identified in orbit around other stars.


    The final vote has come under criticism because of the relatively small percentage of the 9000-strong membership who participated. Besides the fact that most members do not attend the General Assemblies, this lack was also due to the timing of the vote: the final vote was taken on the last day of the 10-day event, after many participants had left or were preparing to leave. Of over 2,700 astronomers attending the conference, about 800 were present on the day for the significant resolutions on a vote on a subsidiary resolution, the first that required a count, only 424 votes were cast.There is also the issue of the many astronomers who were unable or who chose not to make the trip to Prague and, thus, cast a vote. Astronomer Marla Geha has clarified that not all members of the Union were needed to vote on the classification issue: only those whose work is directly related to planetary studies.


    Some astrologers have chosen not to follow the new definiton.

    Petition for a "better definition"

    Within five days of the new IAU Planet Definition over 300 scientists signed a petition that opposes the new definition. The full text of the petition says: “We, as planetary scientists and astronomers, do not agree with the IAU's definition of a planet, nor will we use it. A better definition is needed.” The petition does not list any specific complaint with the definition, so it is unknown whether all the signatories opposed it for the same reasons.


    It is expected that the decision will have cultural and societal implications. It will affect the “industry of astronomical artifacts and toys”. Educational books need to be revised. The decision was important enough to prompt the editors of the 2007 edition of the World Book Encyclopedia to hold off printing until a final result had been reached. The new designation also has repercussion in the astrological world and finds mixed receptions.

    Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)

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