May 25, 2022

A planet is a celestial body that orbits a star and which, unlike this, does not produce energy through nuclear fusion, whose mass is sufficient to give it a spheroidal shape, where its gravitational dominance allows it to keep its free. orbital belt from other bodies of comparable or larger size. This definition officially entered the astronomical nomenclature on August 24, 2006, with its official promulgation by the International Astronomical Union. Previously there was no precise definition, but an ancient indication deriving from ancient Greek astronomy, for which any celestial body with significant mass that moved on fixed orbits was considered a planet.

Origin and evolution of the term

In ancient times, as the etymology of the term planet reveals (in ancient Greek πλάνητες ἀστέρες plànētes astéres, wandering stars), all the stars that moved in the night sky with respect to the background of the fixed stars, i.e. the Moon, the Sun, were considered such , Mercury, Venus, Mars, Jupiter and Saturn, excluding comets, which were considered atmospheric phenomena.In the 16th century, with the emergence of the heliocentric system, it became clear that the Moon and the Sun did not actually share the physical nature and orbital characteristics of the other planets and that the Earth was also to be included in the list of planets.In 1781 Uranus was discovered, the first planet that was not known to Greek astronomers. Over the next 150 years, two other planets, Neptune and Pluto, would be identified in succession; the latter was counted among the planets from the discovery in 1930 until 2006, the year in which the new definition of planet was decided.Moreover, starting from 1801 more than one hundred thousand subplanetary-sized bodies were progressively discovered, orbiting the Sun mainly in the region of space between the Martian and the Jovian orbit, the so-called main belt. Although at first these bodies were designated as planets, by virtue of their ever-increasing number they were soon defined as a class of objects in their own right: asteroids. Among them, only a few dozen are characterized by an approximately spherical shape.

The promulgation of the new definition

The scheme of the nine classical planets remained unchanged until the nineties of the twentieth century; however, at the end of 2002, modern observational techniques had already allowed the identification of over one hundred bodies of this type, including exoplanets and icy planetoids orbiting in the peripheral regions of the outer solar system. In particular in the case of the latter, the discovery of bodies of comparable size or even greater than those of Pluto, the smallest of the nine planets, rekindled a strong debate on the need to provide a precise definition of a planet. The problem arose from the fact that the classification of celestial bodies derived in part from ancient Greek astronomy, which limited itself to clarifying that a planet was any celestial body moving along fixed orbits (or "patterns"). This description had been filed over time up to the current one, which however lacked vagueness and generality. In 2005, the International Astronomical Union (UAI) established the Committee for the Definition of the Planet (PDC), made up of seven globally recognized experts, to whom it assigned the task of providing a precise definition of the term. During the 26th General Assembly of the UAI, which took place from 14 to 25 August 2006, the resolution proposed by the committee was discussed and modified and on 24 August 2006 it was made official. Previously considered a planet, from this date Pluto was redefined, along with other recently discovered bodies, as a dwarf planet.


The names of the planets in