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 | Is Pluto a Planet? |
A Note from David A. Weintraub On August 24, 2006, at the 26th General Assembly of the International Astronomical Union (IAU) in Prague, by a majority vote of only the 424 members present, the IAU (an organization of over 10,000 members) passed a resolution defining "planet" in such a way as to exclude Pluto and established a new class of objects in the solar system to be called "dwarf planets," which was deliberately designed to include Pluto. The exact text of those resolutions can be found below. These resolutions were not carefully reviewed by the international astronomy community and many members of the professional community find them problematic, at best. For example, Pluto crosses Neptune's orbit and Jupiter shares its orbit with objects known as Trojan asteroids. Does this mean that, despite the apparent intent of the IAU, Neptune and Jupiter are not planets (see 1.c in Resolution 5, below)? As for the words used in the IAU resolutions, many astronomers assert that, using the IAU's own resolutions, a dwarf planet is merely one kind of planet. Thus, despite the apparent intent of the IAU to ban Pluto from the family of planets we all know and love, Pluto was in fact defined to be one of those planets. Clearly, the resolutions passed by the IAU in 2006 did not represent a broad consensus and remain controversial and unacceptable to a very large portion of the professional community. Thus, on August 31, 2006, more than 300 astronomers, myself included, submitted a petition to the IAU declaring that "We, as planetary scientists and astronomers, do not agree with the IAU's definition of a planet, nor will we use it. A better definition is needed." By rule, the IAU cannot correct or reaffirm this action until its next General Assembly in 2009. A major, international conference is being planned for 2007, at which as many as one thousand astronomers are expected to gather and work together to search for a broad consensus as to what a planet is and whether Pluto is a planet. Some have described this whole episode as astronomers behaving badly. But that is too simplistic. Astronomers make discoveries that change our perception and our understanding of the universe. Those new discoveries and understandings sometimes wrench us from one suddenly outdated picture of the universe to another more correct but perhaps less comfortable picture. The word "planet" has a played an incredibly important and fascinating role in history. By examining the changes in what we have meant and understood by the word planet, we can trace much of our history of our understanding of the universe and of ourselves. The history - perhaps biography would be better - of the word planet has culminated in this international and very public fight about whether Pluto is or is not a planet. Anyone interested in this debate will want to understand this history, to know about the discoveries in astronomy that have changed and colored our views of the solar system and the universe. Those who understand the full history of what we have meant by the word planet will appreciate the passions and importance to astronomers of the present debate. This history is described in Is Pluto a Planet? Readers will learn that Pluto was not the first object known as the ninth planet; they will also learn why, if Pluto permanently loses its status as a planet, it won't be the first ninth planet to earn that distinction either. IAU Resolution 5: Defining of a planet in the Solar System Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation "planets". The word planet originally described "wanderers" that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information. The IAU resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:
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