Date: Wed, 30 Aug 2006 12:13:49 -0500
Subject: Mailing #06-27: Message from the Chair: What is a Planet?...
Greetings, DPS members,
1) Message from the Chair: What is a Planet?
2) DPS Meeting Notice: Sheraton Sold Out
3) Comet Wild 2 Samples Now Available
4) Icarus Annoucement: Late Deep Impact Papers
Letter from the Chair: What is a planet?
On August 24, the IAU passed two resolutions that defined three
categories of bodies in the solar system: planets, dwarf planets, and
small solar system bodies. A majority of the IAU members gathered at
the 2006 General Assembly in Prague voted that a planet is defined
as a celestial body that (a) is in orbit around the Sun, (b) is round
(i.e., is in hydrostatic equilibrium against rigid body forces),
and (c) has cleared the zone around its orbit. (For the full text
of the resolutions, see
Eight planets retain their planethood by this definition, and a new
category of "dwarf planets" is defined: objects that are large enough
to be in hydrostatic equilibrium but which have not cleared the
neighborhood around their orbits. Ceres gets elevated to the status of
"dwarf planet" based on recent published results on its shape.
Pluto is specifically recognized as a dwarf planet, and also as the
prototype for all Pluto-like planetary objects beyond Neptune.
2003 UB313 joins Pluto in this category, with additional dwarf planets
in the "Plutonian" category likely to be announced soon.
Some controversy has arisen over the merit of the definition itself,
and the fairness of the process by which the resolutions were
passed. Opinions have ranged from "the DPS membership should be
encouraged to support the IAU resolutions that were approved
by an overwhelming majority" to "the IAU resolution defining the
word 'planet' is fatally flawed and needs to be replaced by something
better." This discord is not surprising, given the long history
of foundered efforts to reach agreement on just what a planet is and
the unwillingness of nature to be categorized into neat compartments.
Two years ago, the IAU appointed a committee of 19 planetary scientists
(15 of whom were DPS members) to attempt to agree on the definition
of a planet, but they could not reach accord. Then, a few months ago,
the IAU appointed a seven-member panel of scientists, historians,
authors, and educators (three of whom were nominated by the DPS Committee)
to take up the task again, guided by the technical findings of the
original 19-member committee but charged to take a broader view
that accounted for historical, cultural, and educational issues as well.
Just prior to the IAU General Assembly meeting, this panel announced
a resolution proposing that planets were celestial objects, in orbit
around a star, that were massive enough to be round and that were
not satellites. This definition, while subject to dispute, was firmly
based on the physical properties of the objects themselves and was
applicable to planets around other stars. Recognizing the authority of
the IAU to render a decision, as well as the considerable input by
DPS members in the process, the DPS Committee endorsed the
definition, mindful of the fact that the final decision would be made
at the IAU General Assembly after open discussion and debate.
As an eye-witness to the proceedings, I think it would be fair to
describe the scene in Prague as intense, highly charged, and dramatic.
The original proposal was quickly discarded, and after several
iterations, it was replaced by a new set of resolutions (including the
two that were finally approved), based not only on the shape of an object
but also on its orbital zone. This second criterion tipped the
balance against Pluto being classified with the other eight planets, and
thus in effect the final vote was about Pluto's status. Of the
approximately 400 voters present, a significant majority supported the
new definition, leaving eight planets as the only "true" planets while
naming Pluto as a dwarf planet and the prototype for its own new
class of objects.
Proponents argued that the definition was practicable and that an
eight-planet solar system was a sensible one, given what we know of
the Kuiper Belt. Critics countered that changes in the resolutions had
been made at the last minute, that the IAU had not allowed for
sufficient discussion and review of the proposals by the full
scientific community, and that basing a definition in part on
celestial dynamics was not well-founded or clear. Combined with the
loss of familiar Pluto as a planet on equal footing with the other
eight, it is not hard to see why the issue is still contentious and
opinions are polarized.
Could the IAU have been more open and inclusive? Probably. Would it
have resulted in a different result? That is far less certain. Nearly
every possible planet definition has been proposed and rejected many
times, and there is no single right answer. What is definitely true is
that the IAU has the authority to make such working astronomical
definitions for its own purposes, that it established a procedure to
define a planet in that context, and that it followed its own rules.
All possible definitions have a degree of fuzziness that requires
intelligent application: what does "round" really mean? What does it
mean to "control a zone"? These are technical issues to be
addressed by Division III of the IAU, currently chaired by
Ted Bowell, a fellow DPS member. There is still work to be done,
too, in constructing a definition that is generally applicable to
extra-solar planetary systems. These and other changes, radical or
moderate, may well be addressed at the next IAU General Assembly in
Rio de Janeiro in 2009. We hope that the DPS community will be
involved in all stages of this process.
What is a planet? Ultimately, the true test will come in time through
common and scientific usage. In the meantime, let's continue the
scientific study of Pluto and all the other beasts in our
celestial menagerie, planet, dwarf planet, satellite, asteroid,
comet, or mote of dust. We'll surely then be in a better position
to decide what it really takes to be a planet.
Richard G. French, DPS Chair
DPS MEETING NOTICE: SHERATON HOTEL SOLD OUT
Our block of rooms in the Sheraton Hotel is fully booked. It might
be possible to snag a cancellation within our block, but the few rooms
they have remaining outside of our block are mostly premium level rooms
at much higher rates ($200 and over). Rooms are still available in
our reservation block in the Hilton Hotel for $135/night. Although
that exceeds the government per diem limit ($110/night) it is way less
than the "rack rate" or any discount I could find at the Hilton,
Courtyard by Marriott, or Westin (all $200 or over per night). There
are a number of small motels that are less expensive, but most are
a very long walk (1.5 mile) or drive away.
Alan Harris, LOC Chair awharris @ spacescience.org
COMET WILD 2 SAMPLES NOW AVAILABLE
The samples of Comet Wild 2 captured and returned to Earth in January
of 2006 by the Stardust spacecraft are now available for allocation
to qualified investigators world wide. Information on the samples,
and directions for how to request them are now on line at:
At this site you will find the main sample Catalog, which is being
augmented daily, as well as the Stardust Investigator's Handbook,
important forms, and other critical news items.
If you have any questions about the samples, or the sample request
process, that cannot be answered by the information at the website,
please contact the sample Curator at:
<mailto:Michael.e.zolensky @ nasa.gov>
ICARUS ANNOUNCEMENT: LATE PAPERS ON DEEP IMPACT
Because of the considerable interest and a large number of papers
submitted late for the special issue of Icarus in connection with
papers associated with the Deep Impact mission to Comet 9P/Tempel 1,
we are announcing that a late partial collection of papers will be
published as a group in a regular issue a few months after the formal
Deep Impact special issue. Some of the papers already submitted late
for the special issue as well as any papers submitted by
September 15, 2006, will be considered to the late collection.
Those additional papers should be submitted to Icarus at:
http://icarus.cornell.edu/submit/ and authors must add in the Comment
section of the submission form that their submission is intended as a
part of the late group of papers associated with the Deep Impact mission
to Comet 9P/Tempel 1.
Michael F. A'Hearn, Guest Editor
Dept. of Astronomy
University of Maryland
College Park, MD 20742-2421
Tel: 1-(301) 405-6076
Fax: 1-(301) 405-3538
E-mail: ma @ astro.umd.edu
Michael R. Combi, Editor
Dept. of Atmospheric, Oceanic and Space Sciences
University of Michigan
Ann Arbor, MI 48109-2143
Tel: 1-(734) 764-7226
Fax: 1-(734) 647-3083
E-mail: mcombi @ umich.edu