Hello, friends.

For years, astronomers have been debating the definition of a
planet. Today, August 16, the International Astronomical Union's
Planet Definition Committee announced their proposal for a new,
official definition of "planet." If the proposal is approved by a
vote of IAU astronomers on August 24th, the number of planets in the
Solar System would swell from nine to twelve.

The International Astronomical Union (IAU) is an organization that
brings together distinguished astronomers from all nations of the
world. IAU's mission is to promote and safeguard the science of
astronomy in all its aspects through international cooperation.
Founded in 1919, the IAU is the world's largest professional body
for astronomers. The IAU General Assembly is held every three years
and is one of the largest and most diverse meetings in the
astronomical community's calendar. (URL: http://www.iau.org)

All of the IAU material on this announcement, including text, images
and animation, is available at the following URL:

http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.htm
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Below is the text from today's press release:

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The IAU draft definition of "planet" and "plutons"
16. August 2006, Prague

The world's astronomers, under the auspices of the International
Astronomical Union (IAU), have concluded two years of work defining
the difference between "planets" and the smaller "solar system
bodies" such as comets and asteroids. If the definition is approved
by the astronomers gathered 14-25 August 2006 at the IAU General
Assembly in Prague, our Solar System will include 12 planets, with
more to come: eight classical planets that dominate the system,
three planets in a new and growing category of "plutons" - Pluto-
like objects - and Ceres. Pluto remains a planet and is the
prototype for the new category of "plutons."

With the advent of powerful new telescopes on the ground and in
space, planetary astronomy has gone though an exciting development
over the past decade. For thousands of years very little was known
about the planets other than they were objects that moved in the sky
with respect to the background of fixed stars. In fact the
word "planet" comes from the Greek word for "wanderer". But today
hosts of newly discovered large objects in the outer regions of our
Solar System present a challenge to our historically based
definition of a "planet".

At first glance one should think that it is easy to define what a
planet is - a large and round body. On second thought difficulties
arise, as one could ask "where is the lower limit?" - how large, and
how round should an asteroid be before it becomes a planet - as well
as "where is the upper limit?" - how large can a planet be before it
becomes a brown dwarf or a star?

IAU President Ron Ekers explains the rational behind a planet
definition: "Modern science provides much more knowledge than the
simple fact that objects orbiting the Sun appear to move with
respect to the background of fixed stars. For example, recent new
discoveries have been made of objects in the outer regions of our
Solar System that have sizes comparable to and larger than Pluto.
These discoveries have rightfully called into question whether or
not they should be considered as new 'planets.' "

The International Astronomical Union has been the arbiter of
planetary and satellite nomenclature since its inception in 1919.
The world's astronomers, under the auspices of the IAU, have had
official deliberations on a new definition for the word "planet" for
nearly two years. IAU's top, the so-called Executive Committee, led
by Ekers, formed a Planet Definition Committee (PDC) comprised by
seven persons who were astronomers, writers, and historians with
broad international representation. This group of seven convened in
Paris in late June and early July 2006. They culminated the two year
process by reaching a unanimous consensus for a proposed new
definition of the word "planet."

Owen Gingerich, the Chair of the Planet Definition Committee
says: "In July we had vigorous discussions of both the scientific
and the cultural/historical issues, and on the second morning
several members admitted that they had not slept well, worrying that
we would not be able to reach a consensus. But by the end of a long
day, the miracle had happened: we had reached a unanimous agreement."
The part of "IAU Resolution 5 for GA-XXVI" that describes the planet
definition, states "A planet is a celestial body that (a) has
sufficient mass for its self-gravity to overcome rigid body forces
so that it assumes a hydrostatic equilibrium (nearly round) shape,
and (b) is in orbit around a star, and is neither a star nor a
satellite of a planet." Member of the Planet Definition Committee,
Richard Binzel says: "Our goal was to find a scientific basis for a
new definition of planet and we chose gravity as the determining
factor. Nature decides whether or not an object is a planet."

According to the new draft definition, two conditions must be
satisfied for an object to be called a "planet." First, the object
must be in orbit around a star, while not being itself a star.
Second, the object must be large enough (or more technically
correct, massive enough) for its own gravity to pull it into a
nearly spherical shape. The shape of objects with mass above 5 x
1020 kg and diameter greater than 800 km would normally be
determined by self-gravity, but all borderline cases would have to
be established by observation.

If the proposed Resolution is passed, the 12 planets in our Solar
System will be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn,
Uranus, Neptune, Pluto, Charon and 2003 UB313. The name 2003 UB313
is provisional, as a "real" name has not yet been assigned to this
object. A decision and announcement of a new name are likely not to
be made during the IAU General Assembly in Prague, but at a later
time. The naming procedures depend on the outcome of the Resolution
vote. There will most likely be more planets announced by the IAU in
the future. Currently a dozen "candidate planets" are listed on
IAU's "watchlist" which keeps changing as new objects are found and
the physics of the existing candidates becomes better known.

The IAU draft Resolution also defines a new category of planet for
official use: "pluton". Plutons are distinguished from classical
planets in that they reside in orbits around the Sun that take
longer than 200 years to complete (i.e. they orbit beyond Neptune).
Plutons typically have orbits that are highly tilted with respect to
the classical planets (technically referred to as a large orbital
inclination). Plutons also typically have orbits that are far from
being perfectly circular (technically referred to as having a large
orbital eccentricity). All of these distinguishing characteristics
for plutons are scientifically interesting in that they suggest a
different origin from the classical planets.

The draft "Planet Definition" Resolution will be discussed and
refined during the General Assembly and then it (plus four other
Resolutions) will be presented for voting at the 2nd session of the
GA 24 August between 14:00 and 17:30 CEST.

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Clear Skies,
Jimmy Thomas