[meteorite-list] Mike Brown: Astronomers Are Revolting!

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Aug 23 13:36:53 2006
Message-ID: <200608231734.KAA14302_at_zagami.jpl.nasa.gov>

http://www.gps.caltech.edu/~mbrown/whatsaplanet/revolt.html

A band of astronomers at the IAU meeting in Prague are revolting
against the proposed IAU definition.

Mike Brown

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 hundreds.

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 scientific importance.

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.
Received on Wed 23 Aug 2006 01:34:08 PM PDT