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Eight Planets!

August 24, 2006 pm31 11:44 pm

Stop press on this one. The International Astronomical Union (IAU) at their General Assembly in Praha reversed course.

Instead of keeping Pluto and adding Charon, Ceres, and Xena (?) to our list of planets, as widely reported, including here by me, they removed Pluto and left off the other three.

Pluto is no longer a planet

The sticking point? They altered the definition of a planet to include the requirement that it “clear its orbit” which Pluto clearly (ha ha!) does not do.


Planets on top, Dwarf Planets below. Et tu, Pluto?

This definition is Resolution 5A. Click the link and scroll down, or click to read it on the next page ——>

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 (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.

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5 Comments leave one →
  1. August 26, 2006 am31 12:45 am 12:45 am

    Jonathan:

    Interestingly the reversal and vote occured after the majority of scientists left Prague. In fact only 450 votes were recorded out of 2,500 participants at the two week conference. I thought Pluto should have been grandfathered and I believe many scentists would have voted for this.

    It remains to be seen if this is not reversed at the next conference in three years.

  2. August 26, 2006 am31 2:22 am 2:22 am

    It remains to be seen if this is not reversed at the next conference in three years.

    But Neptune is the furthest from the Sun of the EIGHT planets in our solar system. And Pluto used to be a planet.

    Tongue back out of cheek, astronomers seem to be sharply divided on this one. It seems to me that the best we can say is that the new definition of planet is less bad than the old one, but we still have a way to go.

    They could bat this one back and forth for years.

    Oh, and Ceres was a planet when first discovered. They didn’t grandfather that one.

  3. August 26, 2006 pm31 9:08 pm 9:08 pm

    Jonathan:

    Ceres were surrounded by other astroids and none of them were revolving around it. Pluto, besides being larger and having an atomosphere, has two moons revolving around it, three if Charon’s orbit is found to revolve around Pluto.

    By the way your blog is slow to upload.

  4. August 27, 2006 am31 5:42 am 5:42 am

    Instead of starting with a definition of “planet” and then applying it, most of the arguments on both sides seem to begin with how the arguer wishes to categorize Pluto, and develops from there.

    I wonder, should there even be a class of object called a “planet”? Are we just trying to measure these bodies against Earth, seeking some arbitrary level of similarity?

    In the book I read in July about the evolution of theories of human evolution, The Fossil Trail by Ian Tattersall (of the American Museum of Natural History), a thread running through the discussions has been which fossils represented members of genus homo? In fact, this question, and fixations on similarities to us (lots about brain size here) seem to have impeded progress in this area, and more than would have occured studying much more distantly related creatures.

    So that’s my point. When the discussion becomes “how human-like?” or “how Earth-like?” we get wrapped up in it a different way. We become involved. I think that is part of what has happened with the definition of planet.

    Thanks for letting me know about the blog loading slow. I uploaded the pictures in the Salonica and Van Cortlandt Park South posts in a bad way. I will avoid making the same mistake.

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