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Pluto's official status as a planet has been a constant subject of controversy since its discovery since the 1990s, when hundreds of smaller objects were found the area of Pluto.

Omission from museum models

Museum and planetarium directors occasionally would create 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 8 planets without Pluto. The controversy made headlines in the media at the time.

New discoveries ignite debate

Continuing advances in telescope technology allowed for further discoveries of Trans-Neptunian objects in the 21st century, some of comparable size to that of Pluto. In 2002, 50000 Quaoar was discovered, with a 1,280 kilometers diameter, making it a bit more than half the size of Pluto. In 2004, the discoverers of 90377 Sedna placed an upper limit of 1,800 kilometers on its diameter, near Pluto's diameter of 2,320 kilometers.

On July 29, 2005, a Trans-Neptunian object called 2003 UB313 (nicknamed "Xena") was announced, which on the basis of its magnitude and simple albedo considerations is assumed to be slightly larger than Pluto. This was the largest object discovered in the solar system since Neptune in 1846. Discoverers and media initially called it the "tenth planet", although there was no official consensus at the time on whether to call it a planet. Others in the astronomy community considered the discovery to be the strongest argument for reclassifying Pluto as a minor planet.

The last remaining distinguishing feature of Pluto was now its large moon, Charon, and its atmosphere; these characteristics are probably not unique to Pluto: several other Trans-Neptunian objects have satellites; and 2003 UB₃₁₃'s spectrum suggests that it has a similar surface composition to Pluto, as well as a moon (nicknamed "Gabrielle"), discovered in September 2005. Trans-Neptunian object 2003 EL61 (nicknamed "Santa") has two moons (one of which is nicknamed "Rudolph") and is the fourth largest TNO behind 2003 UB313, Pluto, and 2005 FY9 (nicknamed "Easterbunny").

IAU 2006 decision

There are three main conditions for an object to be called a 'planet', according to the IAU resolution in 2006.

1. The object must be in orbit around a star, but not be a star itself.
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 neighborhood around its orbit.

Pluto fails to meet the third condition, but will act as a prototype for a yet to be named category of Trans-Neptunian objects.

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". 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. 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." Stern's current 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. 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. 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. 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."

Among the general public, reception is mixed. Some have 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. The U.S. state of New Mexico's House of Representatives passed a resolution declaring that, in honour of Tombaugh, a longtime resident of that state, Pluto will always be considered a planet while in New Mexican skies, with March 13 being known as "Pluto Planet Day". Others reject the change for sentimental reasons, citing that they have always known Pluto as a planet and will continue to do so regardless of the IAU decision. Some observers view this rejection as an attempt to bend the rules in order to keep the only planet discovered by an American classified as such.

Impact of the IAU decision

The news of Pluto's reclassification by the IAU was met with a mixture of bemusement in the press alongside stories of an astronomer backlash against the decision. Multiple petitions exist online asking the IAU for reinstatement.

Alan Stern (leader of the NASA "New Horizons" mission to Pluto) has derided the IAU decision, stating "the definition stinks, for technical reasons." Stern's contention is that the new definition should also exclude Earth, Mars, Jupiter and Neptune, all of which share their orbits with asteroids.

Commemoration as a planet

Pluto is shown as a major planet on the Pioneer plaque, an inscription on the space probes Pioneer 10 and Pioneer 11, launched in the early 1970s. The plaque, intended to give information about the origin of the probes to any alien civilization that might in the future encounter the vehicles, includes a diagram of our solar system, showing nine planets. Similarly, an analog image contained within the Voyager Golden Record, included on the probes Voyager 1 and Voyager 2, also launched in the 1970s includes data regarding Pluto and again shows it as the ninth planet.

Elements 92, 93, and 94 are named uranium, neptunium, and plutonium respectively after the planets Uranus, Neptune, and Pluto.

1. http://www.space.com/scienceastronomy/solarsystem/tyson_responds_010202.html
2. "IAU 2006 General Assembly: Result of the IAU Resolution votes". IAU. 24 August 2006.
3. http://www.nytimes.com/2006/08/24/science/space/25pluto.html?ei=5087&en=cfe4d03207c823f2&ex=1172030400&adxnnl=1&excamp=GGGNpluto&adxnnlx=1156820936-x7vi0zUxIJHoKC1TQ0qrMA
4. http://space.com/scienceastronomy/060824_planet_definition.html
5. http://msnbc.msn.com/id/14489259/
6. http://www.newscientistspace.com/article/dn9846-new-planet-definition-sparks-furore.html

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