page_title: IAU definition of planet
section_title: Background
hierarchical_section_title: IAU definition of planet / Background
context_page_description: The International Astronomical Union defined in August 2006 that, in the Solar System, a planet is a celestial body which:
  is in orbit around the Sun,
  has sufficient mass to assume hydrostatic equilibrium, and
  has "cleared the neighborhood" around its orbit.
  Among other things, this definition caused Pluto to no longer be classified as a planet, a change from how it had been widely considered until that point.
  A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet". According to the IAU, "planets and dwarf planets are two distinct classes of objects". In other words, "dwarf planets" are not planets. A non-satellite body fulfilling only the first criterion is termed a "small Solar System body". An alternate proposal included dwarf planets as a subcategory of planets, but IAU members voted against this proposal. The definition was a controversial one, and has drawn both support and criticism from different astronomers, but has remained in use.
  According to this definition, there are eight known planets in the Solar System. The definition distinguishes planets from smaller bodies and is not applicable outside the Solar System.
context_section_description: The process of new discoveries spurring a contentious refinement of Pluto's categorization echoed a debate in the 19th century that began with the discovery of Ceres on January 1, 1801. Astronomers immediately declared the tiny object to be the "missing planet" between Mars and Jupiter. Within four years, however, the discovery of two more objects with comparable sizes and orbits had cast doubt on this new thinking. By 1851, the number of "planets" had grown to 23 (the eight recognised today, plus fifteen between Mars and Jupiter), and it was clear that hundreds more would eventually be discovered. Astronomers began cataloguing them separately and began calling them "asteroids" instead of "planets". With the discovery of Pluto by Clyde Tombaugh in 1930, astronomers considered the Solar System to have nine planets, along with thousands of smaller bodies such as asteroids and comets. Pluto was thought to be larger than Mercury.
  Tombaugh discovered Pluto while working at the Lowell Observatory founded by Percival Lowell, one of many astronomers who had theorized on the existence of the large trans-Neptunian object Planet X, and Tombaugh been searching for Planet X when he found Pluto. Almost immediately after its discovery, however, astronomers questioned whether Pluto could be Planet X. Willy Ley wrote a column in 1956 titled "The Demotion of Pluto", stating that it "simply failed to live up to the advance publicity it received as 'Planet X' before its discovery. It has been a disappointment all along, for it did not turn out to be what one could reasonably have expected".
  In 1978, Pluto's moon Charon was discovered. By measuring Charon's orbital period, astronomers could accurately calculate Pluto's mass for the first time, which they found to be much smaller than expected. Pluto's mass was roughly one twenty-fifth of Mercury's, making it by far the smallest planet, smaller even than the Earth's Moon, although it was still over ten times as massive as the largest asteroid, Ceres.
  In the 1990s, astronomers began finding other objects at least as far away as Pluto, now known as Kuiper Belt objects, or KBOs. Many of these shared some of Pluto's key orbital characteristics and are now called plutinos. Pluto came to be seen as the largest member of a new class of objects, and some astronomers stopped referring to Pluto as a planet. Pluto's eccentric and inclined orbit, while very unusual for a planet in the Solar System, fits in well with the other KBOs. New York City's newly renovated Hayden Planetarium did not include Pluto in its exhibit of the planets when it reopened as the Rose Center for Earth and Space in 2000.
  Starting in 2000, with the discovery of at least three bodies (Quaoar, Sedna, and Eris) all comparable to Pluto in terms of size and orbit, it became clear that either they all had to be called planets or Pluto would have to be reclassified. Astronomers also knew that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly. They were also concerned about the classification of planets in other planetary systems. In 2006, the first measurement of the volume of Eris erroneously (until the New Horizons mission to Pluto) showed it to be slightly larger than Pluto, and so was thought to be equally deserving of the status of 'planet'.
  Because new planets are discovered infrequently, the IAU did not have any machinery for their definition and naming.  After the discovery of Sedna, it set up a 19-member committee in 2005, with the British astronomer Iwan Williams in the chair, to consider the definition of a planet.  It proposed three definitions that could be adopted:
  Cultural
  a planet is a planet if enough people say it is;
  Structural
  a planet is large enough to form a sphere;
  Dynamical
  the object is large enough to cause all other objects to eventually leave its orbit.
  Another committee, chaired by a historian of astronomy, Owen Gingerich, a historian and astronomer emeritus at Harvard University who led the committee which generated the original definition, and consis
caption: Plot of the positions of all known Kuiper belt objects (green), set against the outer planets (blue)
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