IAU Definition of Planet - Draft Proposal

Draft Proposal

The IAU published the original definition proposal on August 16, 2006. Its form followed loosely the second of three options proposed by the original committee. It stated that:

A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

This definition would have led to three celestial bodies being recognized as planets:

• Ceres, which had been considered a planet at the time of its discovery, but was subsequently treated as an asteroid
• Charon, a moon of Pluto; the Pluto-Charon system would have been considered a double planet
• Eris, a body in the scattered disk of the outer Solar System

A further twelve bodies, pending refinements of knowledge regarding their physical properties, were possible candidates to join the list under this definition. Some objects in this second list were more likely eventually to be adopted as 'planets' than others. Despite what had been claimed in the media, the proposal did not necessarily leave the Solar System with only twelve planets. Mike Brown, the discoverer of Sedna and Eris, has said that at least 53 known bodies in the Solar System probably fit the definition, and that a complete survey would probably reveal more than 200.

The definition would have considered a pair of objects to be a double planet system if each component independently satisfied the planetary criteria and the common center of gravity of the system (known as the barycenter) was located outside of both bodies. Pluto and Charon would have been the only known double planet in the Solar System. Other planetary satellites (like Earth and its moon) might be in hydrostatic equilibrium, but would still not have been defined as a component of a double planet, since the barycenter of the system lies within the more massive celestial body (the Earth).

The term "minor planet" would have been abandoned, replaced by the categories "small Solar System body" (SSSB) and a new classification of "pluton". The former would have described those objects underneath the "spherical" threshold. The latter would have been applied to those planets with highly inclined orbits, large eccentricities and an orbital period of more than 200 earth years (that is, those orbiting beyond Neptune). Pluto would have been the prototype for this class. The term "dwarf planet" would have been available to describe all planets smaller than the eight "classical planets" in orbit around the Sun, though would not have been an official IAU classification. The IAU did not make recommendations in the draft resolution on what separated a planet from a brown dwarf. A vote on the proposal was scheduled for August 24, 2006.

Such a redefinition of the term "planet" could also have led to changes in classification for the trans-Neptunian objects Haumea, Makemake, Sedna, Orcus, Quaoar, Varuna, 2002 TX₃₀₀, Ixion, 2002 AW₁₉₇, and the asteroids Vesta, Pallas, and Hygiea.

On 18 August the Division of Planetary Sciences of the American Astronomical Society, the world's largest international professional society of planetary scientists, endorsed the draft proposal.

According to the IAU, the roundness condition generally results in the need for a mass of at least 5×1020 kg, or diameter of at least 800 km. However, Mike Brown claims that these numbers are only right for rocky bodies like asteroids, and that icy bodies like Kuiper Belt objects reach hydrostatic equilibrium at much smaller sizes, probably somewhere between 200 and 400 km in diameter. It all depends on the rigidity of the material that makes up the body, which is in turn strongly influenced by its internal temperature.