[meteorite-list] Scientists Debate Planet Definition and Agree to Disagree

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 19 Sep 2008 15:28:51 -0700 (PDT)
Message-ID: <200809192228.PAA16541_at_zagami.jpl.nasa.gov>


Scientists Debate Planet Definition and Agree to Disagree
Planetary Science Institute Press Release

September 19, 2008 - Two years ago the International Astronomical Union
(IAU) elected to define the term planet, restricting it to the eight
largest bodies orbiting the Sun, and deleting Pluto from the list. The
demotion of Pluto sparked considerable public controversy. Numerous
planetary scientists and astronomers protested the IAU's definition as
not useful, while numerous other planetary scientists and astronomers
supported the outcome.

Recognizing the need for further scientific debate on planet definition,
more than 100 scientists and educators representing a wide range of
viewpoints on the issue converged for three days on the Applied Physics
Laboratory of Johns Hopkins University (APL) for "The Great Planet
Debate: Science as Process" conference <http://gpd.jhuapl.edu/> last
month. The conference was sponsored by NASA, APL, the Planetary Science
Institute, The Planetary Society, and the American Astronautical Society.

Different positions were advocated, ranging from reworking the IAU
definition (but yielding the same outcome of eight planets), replacing
it with a geophysical-based definition (that would increase the number
of planets well beyond eight), and rescinding the definition for planet
altogether and focusing on defining subcategories for serving different
purposes. No consensus was reached.

A sample of the opinions expressed by conference participants follows:

"I was impressed with two things that came out of The Great Planet
Debate meeting: first, that no one liked the IAU's definition of
planethood, and second, that there are strongly divergent scientific
opinions about what a planet is, with those who study orbits and those
who study planets themselves seeing the matter very differently." said
planetary scientist Alan Stern, currently a visiting scholar at the
Lunar and Planetary Institute of Houston, Texas. "My view is that the
dynamically based definitions are deeply flawed because they do not take
into account any physical properties of the body in question, and give
ridiculous results, for example classifying identical large objects in
different orbits differently - so that even Earths are not always
planets, which is crazy," Stern concluded.

"Gravity forces large bodies to be round, whereas small bodies can be
quite oddly shaped. But the proposed 'geophysical' definition of
planethood based upon roundness uses a poor criterion because there is
no good dividing line. Indeed, there are likely to be more intermediate
solar system objects that are in the fuzzy 'roundish' area than there
are objects that are clearly round. In contrast, the eight planets
recognized by the IAU are significantly different from the numerous
small objects that are classified as 'minor planets' (asteroids) in
terms of both physical properties and their effects on bodies orbiting
nearby," said Jack Lissauer, planetary scientist at NASA's Ames Research
Center in Mountain View, California.

"We all have a conceptual image of a planet. Therefore, we need a term
that encompasses all objects that orbit the Sun or other stars," said
Larry Lebofsky, Senior Education Specialist at the Planetary Science
Institute in Tucson, Arizona. "The debate is a great teaching moment.
Whether dwarf planets are grouped together with the classical planets is
not as important as the process by which scientists arrived at their
conclusions. Scientists look at the same information in different ways;
there may be more than one 'answer'. Facts change. What we know now may
not be what we know in two or three years. Learning to think critically
and understanding how scientists organize facts to develop theories are
lessons that will serve students for a lifetime."

"The word 'planet' has a deep cultural context that cannot be decided by
vote of a subset of astronomers meeting in a room somewhere, especially
when that debate is rushed and the vote close", said William McKinnon, a
Professor of Earth and Planetary Sciences at Washington University in
St. Louis, and an IAU member. "The IAU should reopen the issue to
electronic debate by the entire astronomical community. I am sure the
outcome in that case, whatever it turns out to be, or even if it is
concluded that no universal definition is necessary, would be more
satisfactory to all parties," he said.

"I believe the IAU definition correctly recognized the utility of a
dynamical criterion, but that it needs clarification, not abandonment.
In particular, 'clearing' the neighborhood should be replaced by the
concept of 'dynamical dominance'," said Steven Soter of the American
Museum of Natural History in New York.

Jay Pasachoff, from Williams College, who is spending this year at
Caltech studying Pluto's atmosphere, says, "I have long tried, in my
textbooks, to reflect consensus rather than trying to legislate new
terminology. I think that the IAU should have limited their decision to
the administrative assignment of naming responsibility and not tried to
make decisions for the general public. If third-grade students
eventually decide that Eris, Makemake, Haumea, and their successors are
too many to learn about, then a new consensus may emerge. In the
meantime, let's let scientific discovery continue to take its course and
let us hope to excite new generations of students with the new
information that emerges."

"I think the IAU made a mistake getting into the business of defining a
widely used word, 'planet', and sowing confusion thereby.
Scientifically, the useful discussion would be about categories of
planets (e.g., gaseous planets, rocky planets, dwarf planets, icy
planets, free-floating planets, etc., and an individual celestial body
may fall into more than one category). This approach would address the
main practical problem of nomenclature without confusing the public
about 'planet' itself," said Renu Malhotra, a Professor in the
Department of Planetary Sciences of the University of Arizona.

Neil deGrasse Tyson, astrophysicist with the American Museum of Natural
History and panelist for the Great Planet Debate commented, "The word
'planet' has surely outlived its usefulness. The time has come for us to
create a fresh and sensible classification scheme from the ground up --
one that applies to all objects of our own solar system, yet is flexible
enough to embrace newly discovered objects elsewhere in the galaxy.
Other fields, such as biology, and even subfields of astrophysics that
study stars and galaxies, have strong needs to classify objects and have
solved this problem long ago. It's time for the community of planetary
scientists to do the same. We should not 'agree to disagree, we should
'agree to converge'."

"It was a mistake for the IAU to dictate a definition when there is no
consensus among planetary scientists. It is also counter-productive to
focus only on the planets in our solar system, ignoring some 300
exoplanets," said David Morrison of NASA Ames Research Center. "The IAU
definition of planet should be withdrawn or ignored."

"Historically, 'planets' are just objects that orbit the Sun. Even
asteroids are called 'minor planets' By the IAU. The controversy caused
by the IAU officially declaring the term to be restricted to eight
objects in our solar system was unnecessary, but a natural consequence
of one group of people trying to impose their views on everyone else,"
said Mark Sykes, Director of the Planetary Science Institute, in Tucson,
Arizona. "Ultimately, over the years, the process of science is not
guided by imprimatur and ensures that the most generally useful
perspective will prevail."

The debate continues.


The Planetary Science Institute is a private, nonprofit corporation
founded in 1972 and dedicated to solar system exploration. It is
headquartered in Tucson, Arizona.

PSI scientists are involved in numerous NASA and international missions,
the study of Mars and other planets, the Moon, asteroids, comets,
interplanetary dust, impact physics, the origin of the solar system,
extra-solar planet formation, dynamics, the rise of life, and other
areas of research. They conduct fieldwork in North America, Australia
and Africa. They also are actively involved in science education and
public outreach through school programs, children's books, popular
science books and art.

The Institute's researchers are based in 15 states, the United Kingdom,
Russia, Switzerland and Australia.
Received on Fri 19 Sep 2008 06:28:51 PM PDT

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