The Observant Astronomer

The passing scene as observed by an observant Jew, who daylights as an astronomer.

Friday, August 18, 2006

Defining "Planet" II

Further to my previous post, here is an interesting article from the US Naval Observatory discussing the time when (1) Ceres was last a planet, and how it lost that status.

And this is a new technical paper (in PDF format) by Steven Soter at AMNH that discusses an alternative physically-motivated definition of planet. (The middle bit is somewhat technical, but Section 1 gives a good historical summary, and the rest can be skimmed with profit.) This is an eight-planet solution.

After a couple of day's reflection, I think that, if the proposed definition passes, and especially if the number of planets under this definition becomes unreasonable (e.g. 53 planets as claimed by Brown), then the end result will be the same as Soter's. As written, the proposal creates a primary distinction: classical planets (Mercury through Neptune) vs. plutons ( Pluto, Charon, 2003 UB313, and probably more). All of the latter are also included in the category of dwarf planets, in addition to Ceres, and possibly 3 other asteroids. Now, when the number of plutons becomes excessive, and the category of dwarf planets overloaded, it will be convenient to forget that plutons are planets and both "pluton" and "dwarf planet" will take on the connotations now attached to the soon-to-be-discarded "minor planet". The upshot will be a sensible eight-planet solar system, but with time to soften the emotional blow a sudden, full-scale demotion of Pluto would entail.


Blogger Steg (dos iz nit der ┼íteg) said...


What will happen to the brilliant mnemonic My Very Excellent Mother Just Showed Us Nine Planets? Will it have to become "My Very Excellent Mother Just Showed Us Nothing"?

Couldn't they "grandfather" Pluto in, or something? ;-)

12:32 p.m., August 23, 2006  
Blogger The Observer said...

That's what Brown would refer to as a cultural definition of "planet".

As to the mnemonic, the addition of Ceres and Charon will already cause a problem, not to mention the as-yet-unnamed 2003 UB313.

How about: My very excellent, caring, mother just showed us numerous planets counting UB 313?

1:24 p.m., August 23, 2006  
Blogger Steg (dos iz nit der ┼íteg) said...

LOL, brilliant save there :-)

10:47 a.m., August 24, 2006  
Blogger The Observer said...

But, as I've just posted, the new definition leaves us Plutoless. So your excellent mother just showed you Neptune, and left it at that.

11:46 a.m., August 24, 2006  

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