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Debates over definition of planet continue and inspire

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Planetary science is in the midst of a revolution. As recently as the early 1990s, “the planets” consisted of just nine famous objects in our solar system that every school kid learned to recognize by name and appearance. But then, advances in astronomical technology unleashed an explosion of new planetary discoveries on two fronts.

One of these fronts involved a bewildering variety of planets discovered around other stars. In rapid succession, we learned about extrasolar pulsar planets, hot Jupiters, superEarths and more. And there is now a widespread scientific consensus that the 300-plus planetary discoveries made so far around other stars only hint at the true variety.

The other front formed in a revolution much closer to home, with the discovery of dozens of Pluto-scale (2,400-kilometer diameter class) “dwarf planets” in our solar system. Dwarf planets, which are expected to be common around other stars (but are as yet beyond the reach of most observational techniques), are likely to prove the most dominant type of planet in the galaxy.

As with other revolutions, change consternates some people and organizations. And just such consternation led a few hundred mostly astronomers (mind you, I did not say mostly planetary scientists) at an International Astronomical Union meeting in mid-2006 to vote that dwarf planets are not planets. Ignoring the simple fact that voting is not a valid process to vet scientific principles (what if the IAU voted the sky was green?), many press reports incorrectly described the IAU reclassification of Pluto and other dwarf planets as a fiat.

However, in the two years since that widely publicized vote and its widespread reporting in the press, it has become clear that the IAU’s action has neither been widely applauded nor very well accepted. Consider: Within a week of the IAU’s vote, several hundred planetary scientists—considerably more planetary scientists than were in the room when the IAU voted—signed a petition refusing to use the IAU’s definition owing to its flaws and biases. Then in 2007 and 2008, both the European Geophysical Union and the American Association for the Advancement of Science held special meetings to debate planet definition. Would there have been any need for such meetings if the IAU’s planet definition had been satisfactory? And just this past August, over a hundred scientists, educators and others gathered in Maryland for a three-day discussion called “The Great Planet Debate.” The outcome: There remain sharp debates on what planetary practitioners consider a planet to be.

I myself am a partisan in this debate. I embrace the diversity of new planet types, including the dwarf planets, because they have no fundamentally different characteristics than their larger cousins, other than that they are somewhat smaller—just as Chihuahuas have no fundamentally distinguishing characteristics from other canines, except that they are smaller, and just as dwarf stars like our sun have no fundamentally different characteristics from giant stars a hundred times the sun’s diameter.

Like many of my planetary science colleagues, I consider a planet to simply be any natural object in space that is large enough to be rounded by self-gravity. It therefore behaves as a body whose shape is dominated by self-gravity, rather than by the body’s own mechanical strength, as with rocks and asteroids. If this means there are too many planets to name—like stars and galaxies, that planets are not special or that planet taxonomy will be complicated, so be it. It is not up to scientists to saddle taxonomy with preconceived biases. Instead, it is up to science and scientists to adjust to new data and revise paradigms accordingly, as when new data demonstrated that the Earth is not the center of the solar system, and again when new data demonstrated that the Milky Way is not the only galaxy but one of countless billions.

Planetary scientists must care about planet definition because without knowing what a planet is, a planetary scientist cannot describe what the field’s central objects of study are.

But why should the public care? First, for the sheer inspiration and excitement that comes with revolutionary discoveries involving the newly found diversity of planetary sizes and taxa. Equally, because if planetary types different from our own dominate the galaxy’s population of planets, then we are inspired to revel in the fact that the Copernican revolution continues even today. And finally, because educators have in this classification revolution an utterly inspiring, teachable moment emanating from the debate. And what is that? It is that science adapts to new data—such as the prevalence of dwarf planets—rather than running from it, and finds its way to better paradigms, not by fiat or voting but by reasoned debate that converges to consensus.

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