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Pluto: The Case of the Demoted Planet

by Callie McNew, IfA Outreach Assistant

The panel, from left to right, Richard Crowe (UH Hilo and `Imiloa's astronomer in residence), Jay Farihi (Gemini Observatory), William Heacox (UH Hilo), David Jewitt (IfA), Inseok Song (Gemini), Michihiro Takami (Subaru Telescope), and David Tholen (IfA). Behind them is a chart showing the relative sizes of trans-Neptunian objects.

Is Pluto a planet? That was the question the International Astronomical Union (IAU) General Assembly tried to answer at its triennial meeting last August. The announcement of Pluto's demotion to "dwarf planet" received international attention both within the ranks of professional astronomers and among members of the public. To recognize the public's interest in and concern about the status of Pluto, the `Imiloa Astronomy Center of Hawaii presented "Bye Bye Pluto: The Case of the Demoted Planet" on September 29.

The event featured an excerpt from the BBC documentary Bye Bye Pluto and a panel discussion by six Hawaii astronomers specializing in small solar system bodies and extrasolar planets. Bye Bye Pluto, made before the August IAU meeting, tells the story of how IfA astronomer David Jewitt and his colleague Jane Luu discovered the first Kuiper Belt object (KBO, also called trans-Neptunian object) in 1992, and explores the implications of this discovery for the classification of Pluto. Essentially, finding the first KBO was the beginning of the end for Pluto as a planet, because it showed that Pluto is just another member of a vast reservoir of objects beyond Neptune's orbit. At the `Imiloa event, Jewitt gave an inspiring personal account of the discovery of the first KBO and the technology necessary to detect such objects.

IfA's David Tholen offered passionate arguments against Pluto's demotion. He was particularly disturbed by the IAU's voting method in which fewer than 500 of the approximate 9,000 IAU members—only those attending the final day of the August meeting—were eligible to vote. Tholen also objected to the new nomenclature. It is confusing because a dwarf star is a type of star, but a dwarf planet is not a planet, he said. Tholen acknowledges that Pluto is somewhat of an oddball compared with the other planets, but he does not concur with the IAU's new criteria for planets.

According to the new criteria, a planet is a celestial body that orbits the Sun, has sufficient mass to assume a nearly round shape, and "has cleared out the neighborhood around its orbit." It is this last part that disqualifies Pluto, since it is located in the Kuiper Belt, a ring of millions of solar system formation remnants. Tholen would prefer an easier definition, perhaps based on magnitude. If an object is brighter than an absolute magnitude of zero, it could be called a planet. If that were the case, Pluto would still be considered a planet. The other panelists agreed that the term "dwarf planet" is ambiguous, but they still considered it sensible to exclude Pluto from the list of planets.

Both Tholen and Jewitt agreed that it makes little difference to scientists whether Pluto is called a planet or a dwarf planet. They will study Pluto in either case. The definition is really for the public, since it affects the way people view the solar system. It was obvious from their questions and comments that most members of the audience wanted to retain Pluto as a planet. In any case, a change cannot be made until the next IAU General Assembly meets in 2009.

Tholen's Objections

"The problem with the IAU definition is that it will not be possible to apply it to a newly discovered object in the distant reaches of the solar system. Suppose someone discovers a 24th magnitude (very faint) object with a nearly circular orbit 500 times the distance of Earth from the Sun. It may be as large as Earth, but we would have no information on whether it has cleared its zone, so the question as to whether it is a planet could not be answered, which I suspect would be objectionable to the public. My proposal to base the definition on whether an object has an absolute magnitude of less than zero solves the problem, but at the expense of scientific rigor. Because the definition is more for the public than for the scientist, I don't have a problem with a lack of scientific rigor.

"We've had thousands of minor planets (asteroids) in the solar system for decades. A minor planet is still a planet in the same way that a compact car is still a car. Indeed, the original question was whether Pluto was a major planet or a minor planet. The discovery of the Kuiper Belt made the situation analogous to the asteroid belt between Mars and Jupiter, where we now have over 300,000 minor planets. Somehow the question morphed from whether Pluto is really a minor planet into whether Pluto is a planet. The IAU didn't just do away with one planet, but rather hundreds of thousands of planets, though minor ones."