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Debate: Is Pluto a planet?

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*[ "Pluto is a planet"] *[ "Pluto is a planet"]
- +*[ "The Case For Pluto". Sky And Telescope.]
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Revision as of 22:01, 30 July 2009

Is Pluto a Planet or was it properly classified as a dwarf-planet in 2006?

Background and context

Characteristics: Does Pluto fit the characteristics of a planet?


  • Pluto's planetary characteristics diverge in just the same way as other planets. Robert Naeye, Senior Editor for "The Case for Pluto" 9/6/06 - "Clearly, Pluto is a different type of object than the 8 solar system planets in the official IAU definition. But if we look at those 8, we see an extreme range of diversity as well. Mercury and Jupiter differ in mass by a factor of 5,750, and in volume by 25,000. Their compositions could hardly be more different. Jupiter's composition is more like that of a star; it's a giant ball of mostly hydrogen and helium. It also has a family of at least 63 moons, and several tenuous rings. In contrast, Mercury is a ball made of heavy elements, with no appreciable atmosphere and zero moons. Mercury is more than 13 times closer to the Sun. About all that Jupiter and Mercury have in common is that they are spherical objects orbiting the Sun. So if astronomers are comfortable lumping Jupiter and Mercury into the same category, it's not at all obvious that Pluto should be excluded from this club...If Mercury and Jupiter are considered similar enough to fall under the same category, it's not crazy or unscientific to think that Pluto should also be included, especially since Pluto shares the same properties with Jupiter and Mercury that give these two objects their commonality."
  • A "planet" should not have to "dominate" its locality. Robert Naeye, Senior Editor for "The Case for Pluto" 9/6/06 - "If astronomers don't make a distinction based on location, mass, and size for galaxies, stars, and black holes, why should planets be different? Why should it matter whether a spherical object orbiting a star can clear out its zone of space? A galaxy is a galaxy whether it dominates its cluster or is being devoured by a bigger galaxy. A star is a star whether it's alone, inside a cluster, or part of a binary. Just because a 50-solar-mass star can't eject a red-dwarf binary companion doesn't mean that we stop calling it a star! As described above, astronomers have historically categorized almost all classes of objects by their properties, not their locations.
  • "Planets" should not have to "clear" their area. David Tholen, Astronomer at the University of Hawaii's Institute for Astronomy 11/06 - "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."


The arguments in this section need to be converted to look like the arguments on the pro side of thsi debate article.

Argument that Pluto fits the description of a Kuiper belt comet more than of a planet: Neil deGrasse Tyson, a famous American Astrophysicist, "Pluto is Not a Planet" Natural History Magazine 2/99 - "The more we learned about Pluto, the more it did not fit any reasonable classification scheme that applied to the other planets. It was in a class by itself. But can you have a class of one? Should you have a class of one? In 1992, David Jewitt of the University of Hawaii and Jane Luu of Harvard began to discover icy bodies just beyond the orbit of Neptune. Since then, nearly a thousand such objects have been discovered with similar properties: They are small, they are icy, they all orbit just beyond Neptune, they have somewhat eccentric paths, and their orbits are tipped out of the plane of the solar system. This new class of objects was duly named the Kuiper belt, in honor of the Dutch-born American astronomer Gerard Kuiper, who in the 1950s advanced the idea that such a belt of comets might exist. Alas, Pluto, which is small and icy and orbits just beyond Neptune and has an eccentric orbit that is tipped out of the plane of the solar system, is none other than a Kuiper belt object—a leftover comet from the solar system’s formation. If Pluto’s orbit were ever altered so that it journeyed as close to the Sun as Earth, Pluto would grow a tail and look like a jumbo comet. No other planet can make this (possibly embarrassing) claim. I must vote—with a heavy heart—for demotion."

Argument that a "planet" needs to dominate its locality, and that this was the primary justification for the IAU's demotion of Pluto: National Geographic News, "Pluto not a planet, astronomers rule" 8/24/06 - "Pluto has been demoted because it does not dominate its neighborhood. Charon, its large 'moon,' is only about half the size of Pluto, while all the true planets are far larger than their moons. In addition, bodies that dominate their neighborhoods, 'sweep up' asteroids, comets, and other debris, clearing a path along their orbits. By contrast, Pluto's orbit is somewhat untidy."

Argument that the new definition of a planet adopted by the IAU was needed and that it is much more scientific than anything before it: Astrophysicist Alan Boss of the Carnegie Institution - "reversed course completely, and offered up a definition that's much more scientifically palatable. They reworked it and it has become a much superior definition. I think this will stand the test of time."

Argument that the new definition under the IAU will eliminate the possibility that dozens of new "planets", under the old definition, be found and that an unmanageable and less meaningful list of "planets" emerges: "The Case for Demoting Pluto" - "The Pluto-Charon system is too small and common in the outer solar system to be rightly categorized as a planet, and should be removed from the repertoire. Otherwise the panoply of planets in our solar system -- and in other planetary systems -- would grow at an unmanageably rapid pace as we discover more and more 'Plutons,' and our astronomy textbooks would become hopelessly out of date with every new discovery."

Social effects: Will the downgrading of Pluto's status cause negative social effects?


  • Pluto's new status will diminish attention paid to dwarf planets. A likely educational effect of the IAU definition stating that dwarf planets are not planets at all is the limitation of what is taught to elementary school children to only the eight "major" planets. How many of us learned about Ceres in school? Had it been classified, as we now know it ought to be due to its roundness, as a planet, all of us would likely have learned about it in lessons on the solar system. Teachers are constrained by having to meet state and federal mandates and have children achieve good scores on required standardized tests. This means their time is seriously limited, and anything not on these tests is likely to be given short shrift. The result, in a practical sense, is that the amount of information children learn about the solar system will be diminished, and awareness of Pluto will likely fall into obscurity. Now if dwarf planets are considered planets, as they should be, the solar system curriculum in elementary school would likely be expanded to include Ceres, Eris, and Makmake as well as Pluto, meaning children would learn more instead of less.

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Committee of the Division for Planetary Sciences of the American Astronomical Society said in 2006 - "This action would undoubtedly be viewed by the broader scientific community and the general public as a reclassification of Pluto from a major planet to a minor planet. We feel that there is little scientific or historical justification for such an action."


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Pro/con resources


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