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The Pluto Debate

Planet X?

In the year of 1930 the planet Pluto was originally discovered .Once it was found, its faintness and failure to show a visible disc cast doubt on the idea that it could be Lowell's Planet X. Lowell had made a forecast of Pluto's position in 1915 which had turned out to be fairly close to its actual position at that time; but Ernest W. Brown accomplished almost immediately that this was a coincidence, and this view is retained today. Lowell had also made earlier, different predictions of Planet X's position beginning in 1902.

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

In an endeavor to reconcile Pluto's small apparent size with its identification as Planet X, the theory of specular reflection was anticipated. 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 remarkably high density.

The ambiguity was decisively resolved by the discovery of Pluto's satellite Charon in 1978. This made it possible to establish the combined mass of the Pluto-Charon system which turned out to be lower even than that projected by skeptics of the specular reflection theory, which was then rendered completely untenable. The conventional 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.

In recent times, 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 in fact has no role in what were believed to be anomalies in Neptune and Uranus' motion. Pluto's discovery was mostly due to the meticulousness and diligence of Tombaugh's search, which he sustained for some time after the discovery and left him contented that no other planet of a comparable magnitude existed.

Although 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?

An image of Pluto and Charon taken with a 61" telescope; note the intricacy in resolution despite telescope size. Pluto's planetary status is debated for the reason of its small size.

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 sustained discovery of these objects, particularly 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 well thought-out 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. The view that Pluto might be "demoted" to non-planet status created an expressive 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. As a result "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 unintentionally 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. 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. Ultimately, the number 10,000 was assigned to an "ordinary" asteroid, 10000 Myriostos.

The debate focuses on how a "planet", from the Greek for "wanderer", is a designation 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 affinity to become round under its own gravity, should be known as a major planet; this would comprise 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.

See about:

vHistory of Pluto
vPluto Discovery And Naming
vPluto Physical Characteristics
vPluto's Moon
vExploration Of Pluto
vThe Pluto Debate
vPluto New Discoveries