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Pluto Demoted – And Then There Were Eight

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In an historic vote, the International Astronomical Union has finally agreed upon the definition of a “Planet”.

The International Astronomical Union (IAU), the internationally recognized organisation for naming and defining all things celestial, has closed their triennial General Assembly conference in Prague by deciding that there are only eight planets currently present in our solar system. Pluto has been demoted to “dwarf planet” status – albeit a new family of objects within its own right. The eight planets are (in order from the sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The first “dwarf planets” to make the grade, consist of Ceres, Pluto, and 2003 UB313 (commonly referred to as “Xena”), with more to be added in the near future. The concluding vote took place on Thursday, August 24th, 2006.

Pluto and CharonIt’s not all bad news for Pluto though, as it becomes the prototype for a yet-to-be named new class of objects that exist in the trans-Neptunian region. The IAU intends to set up a dedicated process for naming these bodies in the near future, which typically consists of large bodies within the Kuiper-belt region.  Having visited all eight planets in the solar system, NASA's "New Horizons" spacecraft is currently enroute to Pluto and is expected to reach the dwarf planet in July of 2015.  In June of 2007, NASA plans to launch off on the "Dawn" mission to fellow dwarf planet, Ceres.  Though the chance to send your name to Pluto has passed, the opportunity is now available to be part of the Dawn mission by doing the same.

The passed IAU resolution reads:

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

The eight planets(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) shape2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects3 except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".

1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
3These 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

The decision is exciting news for astronomers but it was not an easy task for the IAU to reach an agreement on the final definition. The furor began when it was found that Kuiper-belt object 2003 UB313UB313, discovered in 2003 by astronomers Mike Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University), was slightly larger than Pluto, at approximately 2400km in diameter. This new discovery required a new planet (or several) be added to the list of “Planets” or a demotion of Pluto from the rank. Many were not happy with this idea, submitting a plethora of reasons, including historical relevance, astrological usage, text-book changes, and the confusion of children learning about our solar system. Then came about the problem of deciding what defining features actually made a planet a planet, without allowing other celestial objects to fall easily into this category either. A number of proposals were submitted and debated, settling on the final definition with more than 2500 astronomers voting on the resolution.

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  • Are you really saying the New Horizons spacecraft has itself visited all 8 classical planets, or is that just an accident of sentence structure?

    Leaving that minor quibble aside, thanks for the excellent explanation of the issues contributing to the IAU’s decision.

    Now it’ll be interesting to see whether the new definition will stick, both among the astronomers and in the larger cultural context.

  • Oops …. accident of sentence structure … sorry about that! The craft will encounter Jupiter for gravity assist in 2007 and will pass closer to the red giant than the Cassini mission, but I don’t believe it is scheduled to meet with any other planets before it’s arrival at Pluto.


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