Friday, December 2, 2011

Is the IAU Definition of "Planet" A Quality Definition?

In this post we continue to learn lessons from the International Astronomical Union's definition of "planet" in 2006 (  The question tackled here is whether the IAU's definition of "planet" is a quality definition.  After close examination, it seems it is not.

Here is the definition:

'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.

[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.' 

And here is the analysis of the definition:

(a) The definition is not actually of "planet" but of "planets in our Solar System".  This can be mined out of the text of Resolution 5A, which states: 

"RESOLUTION 5A The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way..." 

So the IAU did not define "planet" at all, but merely "planets in our Solar System".  We will need to explore this in a further post, but it is clearly a source of confusion, and hence the definition is of poor quality (definitions are not supposed to cause confusion).

(b) The superordinate genus identified in the definition is "celestial body".  If I look up "celestial body" in Wordnet ( I get "natural objects visible in the sky".  So, celestial bodies must include planets, stars, comets, asteroids, nebulae, galaxies, and so on.  As such, the genus seems too remote for a quality definition - it is little better than "thing".  There seems to be a strong possibility that it could be divided into genera that are superordinate to "planet", but subordinate to "celestial body".  What are they?  That is not my job - I am not an astronomer.  But I can tell you that a more proximate superordinate genus is required for this to be a quality definition.

(c) The definition contains the phrase "is in orbit around the Sun".  This clearly shows that only our Solar System" is being considered, as we saw above.  Guess what - I could tell from the term "planets in our Solar System" (the definiendum
) that the planets would be orbiting the Sun.  That is part of the definition (or description) of "our Solar System".  The term "Sun" should not have been used in the definition.  It is an essential characteristic of "our Solar System", not "planet".  Another point that shows we do not have a quality definition.

(d) If the IAU chooses to define "planet in our Solar System" it is obliged to define "extrasolar planet" (the coordinate species) and "planet" (the proximate superordinate genus).  At the very least these should have been referenced in Resolution 5A.  There is no such reference in 5A.  Again, an indication of a poor quality definition.

(e) What does "(c) has cleared the neighbourhood around its orbit" actually mean?  What is "neighbourhood" in the celestial context?  It is easily understood as a general (human, non-astronomical) term, but that cannot possibly apply here.  What is it?  Suppose I make the presumption that "cleared" means "to have removed matter".  I really do not have any right to do so, but suppose I do.  Well, the Earth has not absorbed the Moon or ejected the Moon from its proximity.  So is the Earth not a planet?  This is a big failure, because definitions are supposed to make things clear.  Again, we have a poor quality definition.

In defense of the IAU, it is quite difficult to produce definitions in natural science.  There is usually no alternative to them being other than descriptive (as opposed to essential or causal).  However, the IAU could have done better.

Thus, we see that we have a poor quality definition of "planet".  Sorry, I meant to say "planet in our Solar System".

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