The definition of a dwarf planet
Astronomy, like all scientific disciplines, is continuously evolving. Along with it, even its classification structure is subjected to periodic up-dates. The latest has raised a great stir because it has closely affected the image that the general public has of our Solar System, consisting of one star, nine planets and different kinds of minor planets.
In fact, from 24 August 2006, Pluto has been “demoted” from planet to dwarf planet, a new category created specifically by astronomers to take into account what the systematic and continuous exploration of the Solar System seems to highlight: some of Pluto’s characteristics are not unique but also belong to other bodies that orbit around the Sun. Actually, it is just a question of nomenclature, more than a conceptual revolution. Pluto becomes the progenitor of a new class of objects, rather than a particular case of an old class of bodies.
What is the difference between a planet and a dwarf planet? The fact that gravity, being a central force, tends to originate spherical shaped bodies is used to define a planet. Astronomers have therefore decided that for a ‘planet’ to be recognised as such it must be an object that orbits around the Sun and that has sufficient mass (and therefore sufficient gravity) to have a spherical shape and to have “cleared out” all the small bodies from the neighbourhood around its orbit. Pluto had been placed in this category but it did not satisfy the last requisite, being situated within the Kuiper Belt. Hence the new category of dwarf planets was defined, spherical-shaped bodies that orbit around the Sun in the company of neighbours.