Is Pluto a Planet?

The discovery of Eris forced the IAU to act on a definition. In October 2005, a group of 19 IAU members, which had already been working on a definition since the discovery of Sedna in 2003, narrowed their choices to a shortlist of three, using approval voting. The definitions were:Michael E Brown, discoverer of Eris

• A planet is any object in orbit around the Sun with a diameter greater than 2000 km. (eleven votes in favour)
• A planet is any object in orbit around the Sun whose shape is stable due to its own gravity. (eight votes in favour)
• A planet is any object in orbit around the Sun that is dominant in its immediate neighbourhood. (six votes in favour)^[46][47]

Since no consensus could be reached, the committee decided to put these three definitions to a wider vote at the IAU General Assembly meeting in Prague in August 2006,^[48] and on August 24, the IAU put a final draft to a vote, which combined elements from two of the three proposals. It essentially created a medial classification between planet and rock (or, in the new parlance, small Solar System body), called dwarf planet and placed Pluto in it, along with Ceres and Eris.^[49][50] The vote was passed, with 424 astronomers taking part in the ballot.^[51][52][53]

The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

(1) A “planet“¹ 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 into either “dwarf planet” and other categories.
³ These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.