posted 08-24-2006 11:33 AM               

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The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defined as 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.

This means that the Solar System consists of eight "planets" Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called "dwarf planets" was also decided. It was agreed that "planets" and "dwarf planets" are two distinct classes of objects. The first members of the "dwarf planet" category are Ceres, Pluto and 2003 UB313 (temporary name). More "dwarf planets" are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate "dwarf planets" are listed on IAU's "dwarf planet" watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.

The "dwarf planet" Pluto is recognised as an important proto-type of a new class of trans-Neptunian objects. The IAU will set up a process to name these objects.

Below are the planet definition Resolutions that were passed.

RESOLUTIONS

Resolution 5A is the principal definition for the IAU usage of "planet" and related terms.

Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the prototype. The IAU will set up a process to name these objects.

IAU Resolution: Definition of a Planet in the Solar System

Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation 'planets'. The word 'planet' originally described 'wanderers' that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

RESOLUTION 5A

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

1. A 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, 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**, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

3. All other objects*** orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

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

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

*** These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies. The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

RESOLUTION 6A

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.

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posted 08-24-2006 05:14 PM               

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Originally posted by FFrench:
...is a much more nuanced compromise than "not a planet". Of course, that doesn't make for as big of a media story

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Dwarf planets are not planets under the definition, however.

"There will be hundreds of dwarf planets," Brown predicted. He has already found dozens that fit the category.

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posted 08-25-2006 12:01 AM               

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Pluto is now a limited edition.

Text books aside, there are hundreds if not thousands of products that were or are now on the market that include Pluto as the ninth planet of the solar system.

On Thursday, 424 astronomers voted to strip the small body of its planethood. Instead, Pluto is now categorized a "dwarf planet," one of hundreds that might be identified in the coming months and years.

In essence, the International Astronomical Union decided that when it came to planets, eight really was enough.

This may be bad news for Pluto fans, but for collectors, it presents an opportunity.

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posted 08-29-2006 06:44 AM               

No space geek is going to tell this space geek what to call Pluto. Pluto will always be a planet to me (and of course Mickey's dog).

posted 08-29-2006 08:12 AM               

Is a dwarf person no longer a person? Is it politicaly correct to call a person a dwarf? Wouldn't it be better to call them "hieghth challenged"? Or better yet, might it not be more polite to not point out others shortcomings? (pun intended)

I wonder how we would feel if future discovered life forms, in Jupiters upper atmosphere, referred to the inner planets a dwarf planets... declaring to the universe that this was a four planet system.

posted 08-29-2006 08:15 AM               

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Originally posted by mensax:
By calling Pluto a dwarf planet... does that mean it is no longer a planet?

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posted 08-29-2006 11:56 AM               

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The Exploring the Planets gallery will need the most work, not the least of which might be changing the lyrics to The Family of the Sun. Sung by a group of schoolchildren, the song has been playing continuously in the gallery since it opened in 1979.

Museum employees paid tongue-in-cheek tribute to the former planet by draping black crepe paper over the Pluto exhibit label. In addition, they placed the international symbol for "no," a red circle with line through it, over the Pluto sign at the gallery's entrance.

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