Planetary taxonomy: A diplomatic solution

As we've seen over the last week, the planetary taxonomy adopted by
the International Astronomical Union (IAU) on 24 August 2006 has at
once drawn the world's attention to an exciting new perspective on our
solar system, and highlighted a question continuing to provoke lively
debate: "Is a dwarf planet to be regarded as a planet?"

Happily, there is a taxonomic device, familiar in the biological
sciences, which permits us to grant diplomatic recognition, as it
were, to both sides of the question, each of which has its
attractions.

For an astronomer such as Gibor Basri as well as a layperson like
myself, it seems natural to regard the "dwarf" of "dwarf planet" as an
adjective -- much as with "dwarf star." In this view, the term
"planet" has an encompassing sense including both the eight major
planets and the growing number of known dwarf planets such as Pluto,
Ceres, Charon, and UB313 or Xena. Dwarf planets, as Gibor has
suggested, might also be known informally by the charming name of
"beltway planets" -- in contrast to the dynamically dominant or
orbit-clearing major planets.

However, last week's IAU vote has confirmed the demand also for a
narrower sense of "planet" limited to the orbit-clearing major
planets. In this view, now that the "nine major planets" model dating
back to Pluto's discovery in 1930 is no longer tenable, one should be
able to use the familiar word "planet" for these eight dynamically
dominant bodies without also including what may soon amount to dozens
of known dwarf planets. Supporters of the current IAU definition
evidently treating "planets" and "dwarf planets" as mutually exclusive
categories can thus champion it as a powerful engine for tidying up
the conceptual orbit of solar system astronomy.

Fortunately, astronomy can borrow from the life sciences a device
neatly accommodating both views: the distinction between the use of
the term "planet" _sensu stricto_ or "narrowly speaking" to mean only
the eight major or orbit-clearing planets; or _sensu lato_ or "broadly
speaking" to include dwarf planets also.

Thus I would urge that the recent IAU decision doesn't need to be
"reversed," but only refined or tweaked a bit to recognize both the
stricter and the more relaxed usage as legitimate alternatives.

Of course, revisions for consideration by the IAU in 2009 might
address not only this one point, but also, for example, expanding the
scope of current definitions to include extrasolar planets. However, a
quick draft might illustrate the "diplomatic solution" I'm proposing.

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Here is a first draft for a minimal revision which might serve as a
compromise between the adopted Resolution 5A (evidently treating
"planets" and "dwarf planets" as mutually exclusive categories) and
the defeated Resolution 5B (recognizing "classical planets" and "dwarf
planets" with the implication that both are types of planets).

Curly braces { } are used to show text added to the current
definitions adopted in Resolution 5A. A footnote is added specifying
that in a more relaxed usage the term "planet" includes both major
planets and dwarf planets, so that original note 3 becomes note 4.

Incidentally, I suspect that the word "and" in IAU note 2 should read
"or" ("either dwarf planet and other categories"), but have left this
note unchanged.

* * *

Proposed Revision of Current IAU Position

The IAU therefore resolves that planets and other bodies in our
Solar System be defined into three distinct categories in the
following way:

(1) A {major} planet {or planet _sensu stricto_}[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.

(2) A dwarf planet 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[2], (c) has not cleared the neighbourhood
around its orbit, and (d) is not a satellite.{[3]}

(3) All other objects[4] orbiting the Sun shall be referred to
collectively as "Small Solar System Bodies".
___________________________________

1. The eight {major} planets a Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, and Neptune. {In a stricter or
narrower usage, these are the only solar planets.}

2. An IAU process will be established to assign borderline objects
into either dwarf planet and other categories.

{3. In a broader or more relaxed usage, _sensu lato_, the term
"planet" includes dwarf planets as well as major planets.}

4. These currently include most of the Solar System asteroids,
most Trans-Neptunian Objects (TNOs), comets, and other small
bodies.

* * *

Most appreciatively,

Margo Schulter