A band of astronomers at the IAU meeting in Prague are revolting
against the proposed IAU definition.
Astronomers, who are normally mild mannered types, are revolting
against the IAU proposal that would
eventually add hundreds of planets to the solar system. A
counter-proposal has been offered, though it is not clear (at least to
me) how such a counter-proposal officially makes its way to the floor.
In my earlier discussion of the different
options I suggested that there were three potential options available
to astronomers who feel a need to define planets. Two of these options
are strictly scientific and one option is strictly cultural. The two
scientific definitions leave us with either 8 or 53 (currently)
planets, while the cultural definition gives 10, with room to (very
slowly) grow. The IAU proposed the more radical of the scientific
definitions, that anything round should be called a planet, and that
the number of planets in the solar system will very quickly reach the
I have been critical of the IAU proposal for several reasons. Most
importantly, though, I disagree philosophically. There is no doubt that
the concept of roundness due to self gravity -- hydrostatic
equilibrium, to astronomers -- is an important and fundamental property
of the large objects in the solar system and that there is a natural
(though fuzzy) dividing line between large spherically shaped and small
irregularly shaped objects. But there is no good reason that the word
"planet" needs to be hijacked to suddenly become the word which
describes this difference. If astronomers need a word to describe an
important concept and if no word is available they should make up a
word. When Quaoar was discovered in 2002, we used the word "planetoid"
to describe just such an object that is smaller than a planet but still
spherical. I have no particular attachment to this word, and would
happily use any other that seemed reasonable (but, please, not "Pluton"
which is a word already in use by geologists [and I teach intro geology
at Caltech!] to describe something else entirely!).
The new proposal from the revolting astronomers takes the
unpopular but much more justified stance that Pluto really and truly
should be demoted. If this proposal is accepted, people all over, from
school kids to astronomers, will feel like part of their landscape has
been ripped away from them, but that is no reason to not accept the
scientific reasonableness of this proposal. I suspect that any sentient
being arriving into the solar system for the first time would very
quickly classify the four giant planets in one group, the four
terrestrial planets in another group, the asteroids in a third group,
and Kuiper belt objects in a fourth group, and have a few leftovers. It
seems impossible to fathom that the biggest one or two or even 43
objects from the Kuiper belt group would be somehow removed and placed
into any of the other groups. They would quickly come to the conclusion
that there are 8 major bodies orbiting the sun. (Well, maybe they would
only say four: Jupiter, Saturn, Uranus, Neptune. But I'm not yet ready
to lead the fight to demote Mercury, Venus, Earth, and Mars.....).
But what about Pluto? Pluto will always have special status as the
first object discovered in the Kuiper belt, the first Kuiper belt
object visited by a spacecraft, and the brightest and most easily
studied of the Kuiper belt objects. Admitting that Pluto is not, in
fact, a major body of the solar system in no way diminishes its
It's true, though, that admitting that Pluto should not really be
considered a planet does diminish
its cultural importance, and this is where this definition of the word
planet gets into trouble.
Is there a way out? Perhaps. There is one other reasonable option
available to astronomers, and that is to realize that the word "planet"
need not have a scientific definition. Consider it this way: if the
word planet is suddenly
redefined to mean either 8 or 53, how will it affect astronomy? Not one
tiny bit whatsoever. Astronomers like me will continue to go to
telescopes and study these objects to learn where they came from and
what they are made out of whether they are called "planets", "Kuiper
belt objects", or "batholiths." For astronomers, this argument is
purely semantic. Who is affected, then? I would argue that it is the
public, it is our culture, that would be affected, and, in fact, this
is why this is the one astronomical argument, out of the many many many
that are out there, that anyone actually seems to care about. In light
of this realization, perhaps it makes sense to have a cultural
definition of the word planet, rather than a scientific definition.
By "cultural definition" what I mean is "what people mean when they say
the word planet." As far as I can tell, most non-astronomers are
content to keep Pluto as a planet, and, when you discover something new
in the outer solar system, the first question they ask is "Is it bigger
than Pluto?" When you admit that, well, no, it is 1/2 the size of Pluto
(2002 discovery of Quaoar) or maybe 3/4 the size of Pluto (2004
discovery of Sedna) they look a little disappointed and say "well, ok,
so I guess it's not a planet, huh?" But when you say "YES! It IS bigger
than Pluto" (2005 discovery of 2003 UB313) they say "Hurrah! The 10th
planet has been found!" Or something like that. To me that clearly
shows that culture believes that Pluto should remain a planet and that
only objects larger than Pluto should be called planets.
This purely culturally based definition is simple and concise. The
major flaw in the minds of most astronomers is that there is no science
there. Absolutely true. If you feel the need for a scientific
definition even though the definition has no affect on science, the
answer is clearly that there are eight planets (and, indeed, I have
signed on as a supporter of the 8 planet counter-proposal to the IAU).
There are other words describing the landscape around us that are
equally unscientific, however, and work just fine. The word "continent"
is the obvious example. No geologist would ever attempt a scientific
definition of the word, and no one in the public seems to mind.
Astronomers would be wise to imitate their ground-dwelling colleagues
here and not try too hard to rearrange what we call the things that we
think of as being in our back yard.