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Scientists Debate Planet Definition and Agree to Disagree

Sept. 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 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.

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