The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:
(1) A planet^1 is a celestrial body that (a) is in orbit around the Sun,(b) has sufficient mass fo 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) shape^2 and (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects^3 orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
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.
RESOLUTION 5B Insert the word "classical" before the word "planet" in Resolution 5A, Section (1), and footnote 1. Thus reading: (1) A classical planet^1 is a celestrial body... 1 The eight clsssical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
>>35の前に来る IAU Resolution: Definition of a Planet in the Solar System
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particlar, to the designation 'planets'. The word 'planet' originally described 'wonderers' that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently avaiable scientific information.