The Observant Astronomer

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

Wednesday, August 16, 2006

Defining "Planet"

The 26th Triennial General Assembly of the International Astronomical Union has opened in Prague, and the first press release deals with the proposed definition of the word planet. The Globe and Mail has a reasonably good popular level article. (CNN's is appallingly bad and manages to not only suggest that this is the only reason for having a GA, but also confuses the solar system with the galaxy. Of this sentence:
Far outside the realm of science, astrologers accustomed to making predictions based on the classic nine might have to tweak their formulas.
there is little that can be said.)

In short, the proposal is that planets are objects which orbit a star and are in hydrostatic equilibrium. (I.e. that the gravitational force inwards is balanced by pressure outwards. "Small Solar System Bodies", the excluded category, are supported by chemical and mechanical forces, i.e. the strength of their constituent rocks.)

The novelty is the distinction between the eight classical planets---Mercury through Neptune---and a new class called "plutons", which are planets in highly eccentric or inclined orbits. The prototype is Pluto, which, with its moon Charon, are to be considered a double planet. Newly discovered 2003 UB313 would also be a pluton, with possibly many more waiting in the wings. The currently minor planet (a term to be deleted from the lexicon under the proposal) (1) Ceres would regain its planetary status. All of these would be called "dwarf planets". There are potentially another dozen dwarf planets (all but three plutons) waiting in the wings.

There will be several session to discuss the issue over the next two weeks, with a vote likely on 24 August.


Blogger PsychoToddler said...

I thought it was interesting that they decided to call Charon a planet and not a moon.

Why is Mercury a planet and not our moon, then?

1:53 PM, August 17, 2006  
Blogger The Observer said...

Mercury is a planet because it orbits the Sun on it own. The difference between the cases of the moons of the Earth and Pluto illustrates an interesting sub-clause of the proposed definition.

Both objects are in hydrostatic equilibrium, so, on their own, they would be planets. The difference lies in the location of the center of mass of the system including both the primary and secondary. Imagine a cheerleader's baton with one ball much larger that the other. The place where you would put your finger to get it to balance is the center of mass. In our case, gravity provides the attachment.

For the Earth-Moon system, the center-of-mass is just below the surface of the Earth. For Pluto-Charon, it is in the space between the two. So, the Moon is a moon and Pluto and Charon form a double planet under the new definition.

10:26 AM, August 18, 2006  

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