Pluto

In the following decades estimates of the Plutonian mass and diameter were the subject of debate as telescopes and imaging systems improved. The consensus steadily favored smaller masses and diameters as time passed. Indeed, one observer waggishly pointed out that if the trend were extrapolated, the planet seemed to be in danger of vanishing altogether, a remark which proved possibly prophetic in light of later debates over Pluto's status as a "planet".

In an attempt to reconcile Pluto's small apparent size with its identification as Planet X, the theory of specular reflection was proposed. This held that observers were measuring only the diameter of a bright spot on the highly reflective surface of a much larger planet which could thereby be massive without having an exceptionally high density.

The uncertainty was conclusively resolved by the discovery of Pluto's satellite Charon in 1978. This made it possible to determine the combined mass of the Pluto-Charon system which turned out to be lower even than that anticipated by skeptics of the specular reflection theory, which was then rendered completely untenable. The accepted figure for Pluto's diameter today makes it considerably smaller than the Moon, with only a fraction of the Moon's mass on account of its being largely composed of ice. More recently, measurements of the path of Voyager 2 have shown that Neptune has a lower mass than previously believed and that when this lower mass is taken into account there is no anomalous movement of Uranus or Neptune.

Thus Pluto's discovery and Lowell's 1915 prediction were largely coincidental as Pluto actually has no role in what were believed to be anomalies in Neptune and Uranus' motion. Pluto's discovery was mostly due to the thoroughness and diligence of Tombaugh's search, which he continued for some time after the discovery and left him satisfied that no other planet of a comparable magnitude existed.

While Pluto's identification as Planet X began to be doubted soon after its discovery, and for some decades afterwards some considered that a hypothetical tenth planet might be the true Planet X which supposedly caused anomalies in Uranus and Neptune's position, Pluto's identity as the solar system's ninth planet was unquestioned until the 1990s.

Minor planet?

Starting in September of 1992 scientists began discovering hundreds of other bodies in the area of the solar system beyond the orbit of Neptune. The second of these, after Pluto and Pluto's moon Charon, was (15760) 1992 QB1. The continued discovery of these objects, especially of plutinos, rekindled a debate that goes on to this day: is Pluto a major planet or simply one of the largest trans-Neptunian objects?

Trans-Neptunian objects are considered to be minor planets, so the question arose whether to consider Pluto to be one too. This planetary sciences debate landed in newspaper headlines, editorials, and on the Internet in the mid- to late-1990s. Thoughts that Pluto might be "demoted" to non-planet status created an emotional response in certain sectors of the public. Such news outlets as the BBC News Online, the Boston Globe, and USA Today all printed stories noting that the International Astronomical Union was considering dropping Pluto's planetary status. "Save Pluto" websites sprang up, and school children sent letters to astronomers and the IAU.

On February 3, 1999, Brian Marsden of the Minor Planet Center inadvertently fueled the debate when he issued an editorial in the Minor Planet Electronic Circular 1999-C03 noting that the 10,000th minor planet was about to be numbered and this called for a large celebration (the IAU celebrates every thousandth numbered minor planet in some way). He suggested that Pluto be honored with the number 10,000, giving it "dual citizenship" of sorts as both a major and a minor planet.

Between the media reports and the Minor Planet Electronic Circulars, IAU General Secretary Joannes Anderson issued a press release that same day, stating there were no plans to change Pluto's planetary status. Eventually, the number 10,000 was assigned to an "ordinary" asteroid, 10000 Myriostos.

The debate centers on how a "planet", from the Greek for "wanderer", is an appellation that depends upon an object's particular size, formation, or orbit. Some argue that not only is Pluto a major planet but also some moons like Titan, Europa or Triton, or even the larger asteroids. Some argue that an astronomical object more than about 360 km in diameter, at which point the object has a tendency to become round under its own gravity, should be known as a major planet; this would include several moons and a handful of asteroids. Isaac Asimov suggested the term mesoplanet be used for planetary objects intermediate in size between Mercury, the smallest terrestrial planet with a diameter of 4879.4 km and Ceres, the largest known asteroid with a mean diameter of 950 km, which would include Pluto but not most moons.

New discoveries

Continuing discoveries in the transneptunian region keep rekindling the debate. In 2002, 50000 Quaoar was discovered, with a 1280 km diameter, making it a bit more than half the size of Pluto. Another recent discovery, 90482 Orcus, is probably even larger. In 2004 the discoverers of 90377 Sedna, an extremely distant object well beyond the other known transneptunian objects, placed an upper limit of 1800 km on its diameter, close to Pluto's 2320 km.

On July 29, 2005, a Trans-Neptunian object called 2003 UB313 was announced, which on the basis of its magnitude and simple albedo considerations is assumed to be larger than Pluto. This caused its discoverers to call it as the "10th planet" of the solar system, although there is no consensus yet on whether to call it a planet, and others consider the new discovery to be the strongest argument yet for demoting Pluto to the status of a minor planet. 2003 UB313 could be the largest object yet discovered in the solar system since Neptune in 1846. The last remaining distinguishing feature of Pluto is now its large moon, Charon, and its atmosphere; these characteristics may not, however, be unique to Pluto: several other transneptunian objects (not including Sedna) are known to have satellites; and 2003 UB313's spectrum suggests that it has a similar surface composition to Pluto, as well as a moon discovered in September of 2005.

There is some historical precedent for "demoting" a "planet" in the light of subsequent discoveries. The first four asteroids (1 Ceres, 2 Pallas, 3 Juno and 4 Vesta) were considered to be planets for several decades (in part because their sizes were not accurately known at the time). However, in 1845, the first new asteroid in 38 years was discovered (5 Astraea), just one year before Neptune, and soon every year brought more asteroid discoveries. It was soon recognized that Ceres and the others were just the most prominent members of a populous asteroid belt, and although asteroids are also known as "minor planets", they are no longer considered "planets". Some see in this a precedent for noting that Pluto is just the most prominent member of the Kuiper belt.

On the other hand, it may very well be that regardless of future astronomical discoveries, Pluto will remain grandfathered as a planet in much the same way that Europe is considered a separate continent for historical reasons although geographically it makes more sense, from first principles, to consider both Europe and Asia to comprise the single continent of Eurasia.

See also: Definition of Planet.

Pluto in Fiction and Film

• In H. P. Lovecraft's Cthulhu mythos stories (1928–), the Mi-go are a fictional alien race with a base on Pluto, called Yuggoth and Iukkoth in the stories. There are some stories, though, that identify Yuggoth with a huge world situated beyond Pluto on an orbit perpendicular to the ecliptic.
  
• In Robert Heinlein's novel Have Space Suit-Will Travel (1958), Pluto is used by aliens as a remote base for Earth exploration.
  
• In Larry Niven's novel World of Ptavvs (1966), Pluto was theorized to have been a moon of Neptune until it was knocked out of orbit by an interstellar craft.
  
• Martinex of the Guardians of the Galaxy is the last survivor of a human colony in Pluto. His ancestors were African. The character first appeared in January, 1969 and operates in the 31st century.
  
• In the 1974 Japanese anime series Space Battleship Yamato, also known as Star Blazers, the eponymous starship destroys an alien base on Pluto and fights a subsequent battle in an asteroid belt beyond Pluto. Eighteen years later astronomers confirmed the existence of the real-life Kuiper belt.
  
• In the Doctor Who (1963–) serial The Sun Makers (1977), set far in the future, Pluto is covered with vast cities that are warmed by artificial suns, but access to sunlight is controlled by a sinister ruling elite.
  
• In John DeChancie's Starrigger series (1983), Pluto is the location of our solar system's dimensional gate to the interstellar Skyway.
  
• Pluto is featured in Kim Stanley Robinson's novel Icehenge (1985), in which a mysterious artificial structure is found on the planet's north pole.
  
• In the computer game Star Control II (1990), and consequently in The Ur-Quan Masters, the Spathi Captain Fwiffo can be found on Pluto.
  
• The final section of Dave Sim's graphic novel Minds (Cerebus the Aardvark, Volume 10, 1996) takes place on Pluto.
  
• Christine Lavin's song Planet X (1996) is a good-natured protest against suggestions that Pluto is not a planet.
  
• In the Japanese anime series Cowboy Bebop (1998), it is mentioned that a "supermax" maximum security penitentiary is located on Pluto.
  
• In the game Starsiege (1999), Pluto is destroyed at the end of the game.
  
• The television show Futurama (1999-2003) has featured Pluto on occasion, mainly as a habitat for penguins.
  
• In the television show Aqua Teen Hunger Force (2000), two of the show's "Villains", Oglethorpe and Emory (The Plutonians), are from Pluto.
  
• In the cartoon Fairly Oddparents (2001–) Cosmo destroys Pluto with a button the president lost.
  
• In the game Epoch Star (2004), Pluto is the home planet of the Anthropite civilization.
  

Links

Large trans-Neptunian objects