La Unión Astronómica Internacional, en su asamblea plenaria delebrada en Praga, finalmente ha establecido una definición del
término planeta, al menos en lo referente al Sistema Solar. Según la misma, Plutón deja de ser un planeta, para pasar a ser el prototipo de un nuevo tipo de objetos, los "planetas enanos". Sustantivo y nombre van juntos. Tal vez un guión uniéndolos sería lo más apropiado, o el acuñar una palabra en castellano. La propuesta denominándolos "plutonianos" no ha sido aceptada.
El "nuevo Sistema Solar", donde claramente se distingue ente los ocho planetas y los "planetas enanos" (Ceres, Plutón, y 2003UB(313) -nombre provisional). Crédito IAU. Dentro de la categoría de "planeta enano" se encuentran Plutón, Ceres y 2003UB(313).
Por tanto, el Sistema Solar se queda con ocho planetas: Mercurio, Venus, la Tierra, Marte, Júpiter, Saturno, Urano y Neptuno.
The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defined as 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.
This means that the Solar System consists of eight "planets" Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called "dwarf planets" was also decided. It was agreed that "planets" and "dwarf planets" are two distinct classes of objects. The first members of the "dwarf planet" category are Ceres, Pluto and 2003 UB313 (temporary name). More "dwarf planets" are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate "dwarf planets" are listed on IAU's "dwarf planet" watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.
The "dwarf planet" Pluto is recognised as an important proto-type of a new class of trans-Neptunian objects. The IAU will set up a process to name these objects.
Below are the planet definition Resolutions that were passed.
RESOLUTION 5A 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 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) shape
2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects
3 except satellites 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.
RESOLUTION 6A 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.