Scientists decide Pluto’s no longer a planet
Historic new guidelines approved by astronomers in Prague
The IAU therefore resolves that planets and other bodies in our SolarSystem be defined into three distinct categories in the following way:
(1) A planet1is 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 orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
IAU Resolution: Pluto
The IAU further resolves:
Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.
The following sentence is added to Resolution 6A:
This category is to be called "plutonian objects."