Saturday, August 29, 2009

Classification


The Earth Dysnomia Eris Charon Pluto Makemake Haumea Sedna Orcus Quaoar Varuna File:EightTNOs.png
Pluto compared to Eris, Makemake, Haumea, Sedna, Orcus, Quaoar, Varuna, and Earth (all to scale).

Since Pluto's place within the Kuiper belt was determined, its official status as a planet has been controversial. Many have since questioned whether Pluto should be considered together with or separately from its surrounding population.

Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the solar system. Some omissions were intentional; the Hayden Planetarium reopened after renovation in 2000 with a model of only eight planets. The controversy made headlines at the time.[106]

In 2002, the KBO 50000 Quaoar was discovered, with a diameter then thought to be roughly 1280 kilometres, about half that of Pluto.[107] In 2004, the discoverers of 90377 Sedna placed an upper limit of 1800 km on its diameter, nearer to Pluto's diameter of 2320 km,[108], although Sedna's diameter was revised downward to less than 1600 km by 2007. [109] Just as Ceres eventually lost its planet status after the discovery of the other asteroids, so, it was argued, Pluto should be reclassified as one of the Kuiper belt objects.

On July 29, 2005, the discovery of a new Trans-Neptunian object was announced. Named Eris, it is now known to be slightly larger than Pluto.[110] This was the largest object discovered in the solar system since Triton in 1846. Its discoverers and the press initially called it the "tenth planet", although there was no official consensus at the time on whether to call it a planet.[111] Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet.[112]

Remaining distinguishing features of Pluto were its large moon, Charon, and its atmosphere. These characteristics are probably not unique to Pluto: several other Trans-Neptunian objects have satellites, and Eris's spectrum suggests that its surface has a composition similar to Pluto's.[113] It also possesses a moon, Dysnomia, discovered in September 2005.

2006: IAU classification

Main article: 2006 definition of planet

The debate came to a head in 2006 with an IAU resolution that created an official definition for the term "planet". According to this resolution, there are three main conditions for an object to be considered a 'planet':

  1. The object must be in orbit around the Sun.
  2. The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.
  3. It must have cleared the neighbourhood around its orbit.[114][115]

Pluto fails to meet the third condition, since its mass was only 0.07 times that of the mass of the other objects in its orbit (Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit).[116][117] The IAU further resolved that Pluto be classified in the simultaneously created dwarf planet category, and that it act as the prototype for the plutoid category of trans-Neptunian objects, in which it would be separately, but concurrently, classified.[118]

On September 13, 2006, the IAU included Pluto, Eris, and the Eridian moon Dysnomia in their Minor Planet Catalogue, giving them the official minor planet designations "(134340) Pluto", "(136199) Eris", and "(136199) Eris I Dysnomia".[119] If Pluto had been given a minor planet name upon its discovery, the number would have been a little over a thousand rather than over 100,000. The first minor planet to be found after Pluto was 1164 Kobolda, a month later.

There has been some resistance within the astronomical community toward the reclassification.[120][121][122] Alan Stern, principal investigator with NASA's New Horizons mission to Pluto, has publicly derided the IAU resolution, stating that "the definition stinks, for technical reasons."[123] Stern's contention is that by the terms of the new definition Earth, Mars, Jupiter, and Neptune, all of which share their orbits with asteroids, would be excluded.[124] His other claim is that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community.[124] Marc W. Buie of the Lowell observatory has voiced his opinion on the new definition on his website and is one of the petitioners against the definition.[125] Others have supported the IAU. Mike Brown, the astronomer who discovered Eris, said "through this whole crazy circus-like procedure, somehow the right answer was stumbled on. It’s been a long time coming. Science is self-correcting eventually, even when strong emotions are involved."[126]

The ongoing debate over the status of Pluto continues to be acknowledged by the Jet Propulsion Laboratory which, as recently as January 2008, continued to reference it on JPL Photojournal webpages dedicated to Pluto.[127] Researchers on both sides of the debate gathered on August 14-16, 2008 at The Johns Hopkins University Applied Physics Laboratory for a conference that included back-to-back talks on the current IAU definition of a planet.[128] Entitled "The Great Planet Debate"[129], the conference published a post-conference press release indicating that scientists could not come to a consensus about the definition of a planet.[130] Just before the conference, on June 11, 2008, the IAU announced in a press release that the term "plutoid" would henceforth be used to describe Pluto and other objects similar to Pluto which have an orbital semimajor axis greater than that of Neptune and enough mass to be of near-spherical shape.[131][132][133]

Public reaction to the change

Protesters of the reclassification of Pluto, with counter-protesters on a different corner.

Reception to the IAU decision was mixed. While some accepted the reclassification, others seek to overturn the decision with online petitions urging the IAU to consider reinstatement. A resolution introduced by some members of the California state assembly light-heartedly denounces the IAU for "scientific heresy," among other crimes.[134] The U.S. state of New Mexico's House of Representatives passed a resolution in honor of Tombaugh, a longtime resident of that state, which declared that Pluto will always be considered a planet while in New Mexican skies and that March 13 2007 will be "Pluto Planet Day".[135][136] The Illinois State Senate passed a similar resolution in 2009, on the basis that Clyde Tombaugh, the discoverer of Pluto, was born in Illinois. The resolution asserted that Pluto was "unfairly downgraded to a 'dwarf' planet" by the IAU.[137]

Some members of the public have also rejected the change, citing the disagreement within the scientific community on the issue, or for sentimental reasons, maintaining that they have always known Pluto as a planet and will continue to do so regardless of the IAU decision.[138] Others view this rejection as an attempt to bend the rules in order to keep the only planet discovered by an American classified as such.[139]

"Plutoed"

CRIRES model-based computer-generated impression of the Plutonian surface by ESO — L. Calçada, with atmospheric haze, and Charon and the Sun in the sky
Creative Commons Attribution 3.0 Unported

The verb "to pluto" (preterite and past participle: "plutoed") was a neologism coined in the aftermath of its transition from planet to dwarf planet in the aftermath of the 2006 IAU decision. In January 2007, the American Dialect Society chose "plutoed" as its 2006 Word of the Year, defining "to pluto" as "to demote or devalue someone or something", "as happened to the former planet Pluto when the General Assembly of the International Astronomical Union decided Pluto no longer met its definition of a planet."[140][141]

Society president Cleveland Evans stated the reason for the organization's selection of "plutoed": "Our members believe the great emotional reaction of the public to the demotion of Pluto shows the importance of Pluto as a name. We may no longer believe in the Roman god Pluto, but we still have a sense of connection with the former planet."[142]

See also

Solar System portal

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