| Written by David Boyce, on 03-01-2008 15:50 |
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 By Laurel Kornfeld
Laurel is a writer and astronomy enthusiast that works out of New Jersey, USA. She has become well known in the blogosphere for championing the fight to save Pluto's planet status
The decision by the International Astronomical Union (IAU) last summer to demote Pluto to the status of dwarf planet was in part due to anti-American political sentiment around the world and is likely not the last word on the subject because of the confusing definition of the word “planet” the organization adopted, Brother Guy Consolmagno, an astronomer at the Vatican Observatory, told about 300 people at a lecture sponsored by the Rutgers Geology Museum as part of its annual Open House. Although Consolmagno is American by birth, he lives in a Vatican apartment one floor above the pope and therefore was selected by the IAU as a European representative in the early 1990s, when the group sought to broaden the membership of its committees to include non-space faring nations.
Consolmagno provided a brief history of the IAU, which was founded in 1919 after World War I to establish international agreement on astronomical issues. At that time, every nation had its own observatory, resulting in newly discovered celestial objects being given different names in each nation and many nations having their own prime meridians. At that time, the Vatican established its own observatory, partly to strengthen its efforts at being recognized as an independent nation and partly “to apologize for Galileo,” Consolmagno said. He began his presentation by outlining three questions. What objects are currently being found beyond the orbit of Neptune, and are they planets; what is a planet and who decides this; and why do the first two questions matter at all.
Understanding the nature of an object helps determine what questions to ask and what tools to use when encountering that object, Consolmagno explained. “The tools to study an asteroid are different from those to study a planet.”Scientists’ motivations in searching for new discoveries are “a mix of noble and crass,” he said. Finding a new object puts discoverers’ names into textbooks and can play a major role in their obtaining funding and tenure. For this reason, there is less motivation to find objects that are considered minor and will not gain scientists a place in posterity. Initially, the IAU was interested only in stars, not planets, Consolmagno said. When Clyde Tombaugh discovered Pluto in 1930, the director of the Lowell Observatory, where he made the discovery, announced the object’s name. In contrast, today the IAU not only names newly discovered comets, asteroids, and planets; it also has separate committees that name specific types of objects. The IAU is run by an executive committee, which then appoints divisions which are further subdivided into commissions and working groups. For example, Division 3 deals with Planetary System Sciences and has two commissions under it, Commission 15 for dealing with Minor Bodies (comets and asteroids) and Commission 16 for dealing with Planets and Moons.
Guy Consolmagno
Consolmagno served as president of Commission 16 from 2003-2006. In the early 1990s, through the use of better detectors on telescopes, scientists began discovering objects beyond the orbit of Neptune in a region known as the Kuiper Belt, Consolmagno noted. The first Kuiper Belt Object, or KBO, was discovered fifteen years ago, and as many as 1100 have now been identified, he said. These discoveries created a dilemma for the IAU because the organization could not determine which categories they belong to and therefore had no way to decide who would name them, Consolmagno said. As more KBOs were discovered, scientists recognized that Pluto is very similar to the largest of them in that it has an odd orbit tilted 17 degrees from the plane of the other eight planets, is roughly the same distance from the sun, and is in a 3 to 2 orbital resonance with Neptune, meaning it and the other KBOs complete two revolutions around the sun for every three revolutions completed by Neptune.
These similarities between Pluto and the new KBOs led scientists to begin questioning whether Pluto is a planet or just another KBO, Consolmagno said. The situation came to a head when a KBO slightly larger than Pluto, originally known as 2003 UB313 and eventually named Eris, was discovered by Michael Brown at Caltech in 2005. Eris reflects light in the same way Pluto does, and it and other large KBOs have similar orbits and are tilted in odd ways, Consomagno said. Eris is tilted 45 degrees from the plane of the first eight planets. Pluto has a diameter of 1,430 miles while Eris has a diameter of 1,500 miles. “If Pluto is a planet, all these should be too,” Consolmagno said, describing the debate among scientists. Some also proposed that the moons of planets be considered planets themselves.
To address these issues, the IAU in 2005 established a 19-person Planet Definition Committee of which Consolmagno was a member. However, its members could not reach any consensus on the matter. A second committee that included non-scientists such as writer Dava Sobel came up with a definition stating a planet must be in orbit around a star, must not be a star itself, and must be large enough that its gravity pulls it into a round shape, Consolmagno reported. That definition expanded the solar system to 12 planets, including Ceres, the largest asteroid between Mars and Jupiter, Eris, and Pluto’s moon Charon, which was added because the barycenter, or orbital point around which Pluto and Charon revolve, is located between the two objects, essentially making it a binary or double planet. That definition was presented to the IAU at its General Assembly last August.
The definition raised objections from astronomers known as dynamicists, who differ from planetary scientists in that they study the dynamics surrounding celestial objects as opposed to the composition of the objects themselves, Consolmagno said. According to dynamicists, planets are objects that control what happens outside their regions, such as perturbing the orbits of other planets, something Pluto does not do. Political considerations entered the equation during the debate because most planetary scientists are American while most dynamicists are European, Consolmagno explained. “The US is not popular in most parts of the world.” The compromise created during the General Assembly established three classes of planets: classical planets, dwarf planets, and small solar system objects. However, a resolution affirming that both classical planets and dwarf planets are subcategories of the larger classification “planet” was voted down. As a result, “Pluto is a dwarf planet but not a planet,” Consolmagno said.
He admitted that the planet definition adopted by the IAU, which requires a planet to clear its orbit of all other objects, is “sloppy language” that needs to be refined. Consolmagno currently serves on a committee whose goal is accomplishing this. Consolmagno also acknowledged that the demotion of Pluto to dwarf planet led to outrage and horror as well as questions over how to explain the situation to children. Ultimately, it will be the public who decide whether the new definition sticks, he said. “Whether this will fly in popular speech is up to the public.” The change in Pluto’s classification also sparked a backlash among planetary scientists, many of whom were not present at the General Assembly and therefore could not vote. Dr. Ken Kremer, a NASA JPL Solar System Ambassador, said he believes the decision was not representative because fewer than five percent of IAU members took part in the voting. “It was basically a rush to judgment.” Many astronomers maintain that Pluto is a planet, the first representative of a new class of planets known as Ice Dwarfs, Kremer said. He also agreed that the new planet definition is sloppy, noting it technically excludes four of the classical planets because they too “do not clear their orbital paths." The tools for studying dwarf planets are those used to study standard planets as opposed to those used for asteroids, which have far less density, Consolmagno said. Studying planets requires the tools of geology because planets have activity on their surfaces. “The compositional difference between dwarf planets and classical planets is not that much,” Consolmagno said. “Dwarf planets have a lot of ice, but so do Uranus and Neptune. The majority of their mass and size is ice.”
As for what to tell children, Consolmagno evoked the story, “The Ugly Duckling.” “The way of presenting it to kids is that as a planet, Pluto is an ugly duckling, but it is its own thing—a perfect example of a beautiful dwarf planet.”
Last update: 12-01-2008 13:37
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