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International Astronomical Union 2006 General Assembly


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It is official: The 26th General Assembly for the International Astronomical Union was an astounding success! More than 2500 astronomers participated in six Symposia, 17 Joint Discussions, seven Special Sessions and four Special Sessions.

 

lefthttp://hypography.com/gallery/files/9/9/8/iau0603a_thumb.jpg[/img]New science results were vigorously discussed, new international collaborations were initiated, plans for future facilities put forward and much more.

 

In addition to all the exciting astronomy discussed at the General Assembly, six IAU Resolutions were also passed at the Closing Ceremony of the General Assembly:

1. Resolution 1 for GA-XXVI : "Precession Theory and Definition of the Ecliptic"

2. Resolution 2 for GA-XXVI: "Supplement to the IAU 2000 Resolutions on reference systems"

3. Resolution 3 for GA-XXVI: "Re-definition of Barycentric Dynamical Time, TDB"

4. Resolution 4 for GA-XXVI: "Endorsement of the Washington Charter for Communicating Astronomy with the Public"

5. Resolution 5A: "Definition of 'planet' "

6. Resolution 6A: "Definition of Pluto-class objects"

 

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 © 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"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 © 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 , © 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.

 

IAU Resolution: Pluto

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.1

 

Source: IAU

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While I agree with the idea that Pluto does not fit my definition of a planet (largely because of the inclination and eccentricty of its orbit) I don't agree with the definition as is set forth above. I think they copped out. I don't think this definition is complex enough to describe physical characteristics of each body, but I think it is overly complex when it comes to (or rather lacking in definition which makes it complex to determine whether or not) the restriction of clearing the orbit.

 

I don't know if I would put in the sufficient mass to overcome rigidity. I think that are enough other things you could say that would make it so that if it met the other parts then it would have to meet the spherical shape, and if it wasn't spherical, it would still be able to be called a planet.

 

I also don't like that they limit the definition of the word planet to objects revolving around our sun.

 

Hope everyone understood that, maybe my writing here is too rushed.

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I am suprised only that the orbital plane of the objects is not considered in its definition. The remaining 8 planets share an orbital plane, and this is to me very significant in defining them and their relationship to the solar system. Maybe at the next conference there will be Primary Planets that share a common plane and origin, and Secondary Planets (like Pluto) that are not on the same plane, and have a different origin.

 

Bill

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....but I think it is overly complex when it comes to (or rather lacking in definition which makes it complex to determine whether or not) the restriction of clearing the orbit.

I guess one could technically say that since Pluto's orbit overlaps Neptune's orbit that Neptune hasn't cleared it's own orbit huh? Does that mean Neptune's not a planet either? :)

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I guess one could technically say that since Pluto's orbit overlaps Neptune's orbit that Neptune hasn't cleared it's own orbit huh? Does that mean Neptune's not a planet either? :)

This was actually brought up by one of the scientists present at the assembly. The answer was, "See footnote 1," which basically lists the 8 planets and nothing more. Then, another scientist yelled out, "why not do away with the complicated definition then and just use the footnote?" To which the entire audience burst out laughing. :)

 

 

 

http://www.npr.org/templates/story/story.php?storyId=5705254

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I guess one could technically say that since Pluto's orbit overlaps Neptune's orbit that Neptune hasn't cleared it's own orbit huh? Does that mean Neptune's not a planet either? :)

Yah, that was my point exactly. If it overlaps then Neptune has not cleared it's orbital path.

Or in saying this, are they referring to any two objects that share an orbit but which do not orbit around a common planet with a barycenter within the mass of the planet.

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And now for some serious levity.

 

According to the Washington Post today, 8-25-06.

 

In a move that surprised many legal observers, one of Pluto's leading public-interest groups yesterday sued the International Astronomical Union after the IAU declassified Pluto as one of the nine "classical" planets that circles the Sun. It is not yet known where venue for the case ultimately will be placed or precisely which judge eventually will hear the case.
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Yah, that was my point exactly. If it overlaps then Neptune has not cleared it's orbital path.

Or in saying this, are they referring to any two objects that share an orbit but which do not orbit around a common planet with a barycenter within the mass of the planet.

Neptune will eventually clear Pluto off the path. It we roll the orbits forward in time far enough we can find the date that it will happen.

 

Bill

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Neptune will eventually clear Pluto off the path. It we roll the orbits forward in time far enough we can find the date that it will happen.

 

Bill

That sounds like you know that for a fact. Is it possible that Pluto will pass by Neptune close enough at some point and slingshot into another orbit? Just a thought....

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I’m personally pleased and satisfied with the resolution – I think the assembly managed, after much committee fumbling, to reach a workable consensus astronomers will be able to live with for a while.

I guess one could technically say that since Pluto's orbit overlaps Neptune's orbit that Neptune hasn't cleared it's own orbit huh? Does that mean Neptune's not a planet either?
I think this question points out that resolution 5A’s phrase “has not cleared the neighbourhood around its orbit” needs to be more explicitly defined.

 

The idea is that “planets” orbit in “clean” space relatively empty of dust from the original solar nebula, while asteroids, comets, and Pluto-like bodies don’t. Putting even approximate numbers to this idea is a challenging, since measurement of the interplanetary medium is to date largely indirect and uncertain, but fortunately our solar system doesn’t appear to have any difficult borderline planetary candidates – only the 8 planets are predicted to “clear their neighborhoods”.

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i think i enjoy most how science can change fact. what an odd thought.

 

I don't know. Posed this way you point at an interesting thing. "Fact" is not "reality". When we say "Pluto is a planet" or "Pluto is not a planet", neither of those have any real meaning. Pluto is still out there, in the same orbit, just as it has been for a long time, and will be for long time still.

 

The definition of what a planet is does not change the *fact* that we have celestial bodies orbiting a star. What has changed is how some astronomers want to classify them, to save themselves some work when we discover thousands more "dwarf planets" in the coming years.

 

So I think this will be a cool experience. It has been a *false* fact that there are nine planets in the solar system, according to the new definition. How will teachers deal with this philosophical problem - that fact is not the same as truth?

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How do you manage to draw a galaxy map and even include the Sun in it, let alone Pluto? :shocked:

 

um.. i should be flamed for this, i suppose i should rather have related to our solar system in specific rather than making some diagram of an entire galaxy. one might imagine that to be quite a tricky feat.. atleast for a elementary children.

 

apologies, should have caught myself before i leaped.

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