IAU Definition of Planet - Alternative Proposal

Alternative Proposal

According to Alan Boss of the Carnegie Institution of Washington, a subgroup of the IAU met on August 18, 2006 and held a straw poll on the draft proposal: only 18 were in favour of it, with over 50 against. The 50 in opposition preferred an alternative proposal drawn up by Uruguayan astronomer Julio Ángel Fernández.

“ (1) A planet is a celestial body that (a) is by far the largest object in its local population, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) does not produce energy by any nuclear fusion mechanism .

(2) According to point (1) the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane are the only planets of the Solar System. All the other objects in orbit around the Sun are smaller than Mercury. We recognize that there are objects that fulfill the criteria (b) and (c) but not criterion (a). Those objects are defined as "dwarf" planets. Ceres as well as Pluto and several other large Trans-Neptunian objects belong to this category. In contrast to the planets, these objects typically have highly inclined orbits and/or large eccentricities.

(3) All the other natural objects orbiting the Sun that do not fulfill any of the previous criteria shall be referred to collectively as "Small Solar System Bodies".

Definitions and clarifications
  1. The local population is the collection of objects that cross or closely approach the orbit of the body in consideration.
  2. This generally applies to objects with sizes above several hundred kilometers, depending on the material strength.
  3. This criterion allows the distinction between gas giant planets and brown dwarfs or stars.
  4. This class currently includes most of the Solar System asteroids, Near-Earth objects (NEOs), Mars-, Jupiter- and Neptune-Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs), and comets.
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Under the alternative proposal the current Solar System would have remained unchanged, but Pluto would have been demoted to a dwarf planet.

Read more about this topic:  IAU Definition Of Planet

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