IAU draft definition of "planet" and "plutons

[Rick]

The world's astronomers, under the auspices of the International Astronomical Union (IAU), have concluded two years of work defining the difference between "planets" and the smaller "solar system bodies" such as comets and asteroids. If the definition is approved by the astronomers gathered 14-25 August 2006 at the IAU General Assembly in Prague, our Solar System will include 12 planets, with more to come: eight classical planets that dominate the system, three planets in a new and growing category of "plutons" - Pluto-like objects - and Ceres. Pluto remains a planet and is the prototype for the new category of "plutons."
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The part of "IAU Resolution 5 for GA-XXVI" that describes the planet definition, states "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet." Member of the Planet Definition Committee, Richard Binzel says: "Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet."

The draft resolution in full: http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_resolution.html
More: http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.html

[Miriam]

Having formed a committee of one, journeyed to the end of the sofa and deliberated for the best part of an hour, I suggest a planet should be defined as having a mass greater than 10 to the power of 50 neutrons.

This introduces a universal constant, maintains Mercury at twice the threshold mass, and relegates everything else to moons, plutons, asteroids or anything else you may wish to call them.

However, as none of us in the society were invited to Prague, my thoughts will remain within this correspondence.

Jeff

[Rick]

Heh! Is Gilbert there?

[Rick]

There's now (or maybe there was before, but I didn't notice (Thanks, DMM)) an extensive (repetitive, even) FAQ file here: http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_Q_A.html

[Rick]

Q&A New planets proposal [The BBC weighs in...]

Astronomers have drawn up a new way to define the word planet. This would mean adding three new planets to the Solar System, boosting the current tally of nine to 12.

The draft proposal by the International Astronomical Union (IAU) will be put to a vote at a meeting underway in the Czech capital Prague.

Did we really have no definition for a planet before this?

More: http://news.bbc.co.uk/2/hi/science/nature/4798205.stm

[Rick]

mass greater than 10 to the power of 50 neutrons.

But that's so arbitrarily decimal! Universal constants are pretty much always weird long (and maybe even irrational) strings of digits.  

[Rick]

Planet status looking shaky for Pluto

Just when things were looking up for Pluto, astronomers have come up with a new definition of a planet that would leave the poor little mite out in the cold.

After last week's proposed definition of a planet, Pluto, along with the asteroid Ceres and two other large Kuiper belt objects, looked set to be officially classed as a planet. Or at least a pluton*, a sort of minor planetish thing, but still a planet.

However, scientists being scientists, a counter definition was almost immediately put forward at the International Astronomers Union in Prague, according to New Scientist.

Following a bit of a scuffle behind closed doors, accusations of ignoring the democratic process, and much bad language (we're just guessing on that last bit) a compromise definition has been written.

More: http://www.theregister.co.uk/2006/08/23/planet_status/