Before the discoveries of the early 21st century, astronomers had no strong need for a formal definition of a planet. With the discovery of Pluto in 1930, astronomers considered the Solar System to have nine planets, along with thousands of significantly smaller bodies such asasteroids and comets. For almost 50 years Pluto was thought to be larger than Mercury,[12][13]but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure Pluto's mass accurately and determine that it is much smaller than the initial estimates.[14] It was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was still more than ten times as massive as the largest object in the asteroid belt, Ceres, it was one-fifth that of Earth's Moon.[15] Furthermore, having some unusual characteristics such as large orbital eccentricity and a high orbital inclination, it became evident it was a completely different kind of body from any of the other planets.[16]
In the 1990s, astronomers began to find objects in the same region of space as Pluto (now known as the Kuiper belt), and some even farther away.[17] Many of these shared some of the key orbital characteristics of Pluto, and Pluto started being seen as the largest member of a new class of objects, plutinos. This led some astronomers to stop referring to Pluto as a planet. Several terms including minor planet, subplanet, and planetoid started to be used for the bodies now known as dwarf planets.[18][19] By 2005, three other bodies comparable to Pluto in terms of size and orbit (Quaoar, Sedna, and Eris) had been reported in the scientific literature.[20] It became clear that either they would also have to be classified as planets, or Pluto would have to be reclassified.[21] Astronomers were also confident that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly if Pluto were to remain a planet.[22]
In 2006, Eris (then known as 2003 UB313) was believed to be slightly larger than Pluto, and some reports unofficially referred to it as the tenth planet.[23] As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006.[24] The IAU's initial draft proposal included Charon, Eris, and Ceres in the list of planets. After many astronomers objected to this proposal, an alternative was drawn up by Uruguayan astronomer Julio Ángel Fernández, in which he created a median classification for objects large enough to be round but that had not cleared their orbits of planetesimals. Dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, since they have not cleared their orbits.[25]
The IAU's final resolution preserved this three-category system for the celestial bodies orbiting the Sun. Fernández suggested calling these median objects planetoids,[26][27] but the IAU's division III plenary session voted unanimously to call them dwarf planets.[1] The resolution read, in full:
The IAU ... resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:
(1) A planet1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects3, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies.”
- Footnotes:
- 1 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
- 2 An IAU process will be established to assign borderline objects either dwarf planet or other status.
- 3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
Although there were concerns about the classification of planets in other solar systems,[10] this issue was not resolved; it was proposed instead to decide this only when such objects start being observed.[25]
The 2006 IAU's Resolution 6a[28] recognizes Pluto as "the prototype of a new category of trans-Neptunian objects". The name and precise nature of this category were not specified but left for the IAU to establish at a later date; in the debate leading up to the resolution, the members of the category were variously referred to as plutons and plutonian objects but neither name was carried forward.[1] On June 11, 2008, the IAU Executive Committee announced a name, plutoid, and a definition: all trans-Neptunian dwarf planets are plutoids.[29] On July 18, 2008, the Working Group for Planetary System Nomenclature reclassified the object then known as (136472) 2005 FY9 as a dwarf planet, and renamed it Makemake.[30
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