Quizá no haya 12 planetas después de todo

Se ha hablado mucho de la propuesta de la Unión Astronómica Internacional que pretende terminar con la polémica que rodea al estatus planetario de Plutón. El texto recoge una nueva definición de planeta que abre la puerta a la inclusión de una miríada de cuerpos pequeños que cumplan las condiciones impuestas, que pueden ocasionar que dentro de mucho tiempo (hablamos de millones de años) la Luna alcance por méritos propios el título de planeta. En todo este cacao montado para que Plutón no “se salga de la foto” puede haber motivos políticos, al haber sido este planeta el único del Sistema Solar descubierto por un estadounidense (Clyde Tombaugh, en 1930).

Pero se ha presentado otra propuesta que elimina a Plutón de la lista de los 9 planetas que todos nos aprendimos de pequeños y que resulta, a mi entender, más racional. El texto de esta nueva propuesta especifica que:

(1) A planet is a celestial body that (a) is by far the largest object in its local population[1], (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) does not produce energy by any nuclear fusion mechanism [3].

(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 our 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’.[4]

[1] The local population is the collection of objects that cross or close approach the orbit of the body in consideration.

[2] This generally applies to objects with sizes above several hundreds km, 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.

[tags]unión, astronómica, internacional, plutón, planeta, definición, espacio[/tags]