[meteorite-list] The IAU Draft Definition of 'Planet' and 'Plutons'

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Aug 16 11:50:23 2006
Message-ID: <200608161547.IAA17855_at_zagami.jpl.nasa.gov>

http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.html

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

Notes for editors

The IAU is the international astronomical organisation 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.

Science contacts (the persons below can all be reached via the IAU GA
Press Office, see bottom)

Owen Gingerich
IAU Planet Definition Committee Chair
Tel: via the Press Room +420-261-177-075

Iwan Williams
President, IAU Division III Planetary Systems Sciences
Tel: via the Press Room +420-261-177-075

Ron Ekers
IAU President
Tel: via the Press Room +420-261-177-075

Catherine Cesarsky
IAU President-Elect and member of the Planet Definition Committee
Tel: via the Press Room +420-261-177-075

Richard Binzel
Member of the Planet Definition Committee
Tel: via the Press Room +420-261-177-075

Dava Sobel
Author and historian, member of the Planet Definition Committee
via the IAU Press Office
Tel: +420-261-177-075

Mike Brown
Discoverer of 2003 UB313 and a host of other planet candidates
Via Caltech Media Relations
Tel: +1-626-395-3226
E-mail: mbrown_at_caltech.edu

PIO contact
Lars Lindberg Christensen
IAU Press Officer
IAU GA 2006 Press office, Meeting Room 3.2
Prague Congress Center
Tel: +420-261-177-075
Cellular: +49-173-3872-621
E-mail: lars_at_eso.org

Links

Additional material in this release package may be found in:
http://www.iau2006.org/

    * Draft Resolution
    * Question and Answer sheet
    * General Assembly Newspaper articles
    * Committee composition
Received on Wed 16 Aug 2006 11:47:40 AM PDT