There never was a formal definition of Word "Planet" in the astorology books so IAU decided to fill the void and came up with the proposal that increases the tally of planets in our Solar System to a dozen..
This is what International Astronomical Union defined what is a plant :
(1) A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape1, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.2
(2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a "dwarf planet."3
(3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call "plutons".
(4) All non-planet objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".4
1 This generally applies to objects with mass above 5 x 1020 kg and diameter greater than 800 km. An IAU process will be established to evaluate planet candidates near this boundary.
2 For two or more objects comprising a multiple object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto-Charon a double planet.
3 If Pallas, Vesta, and/or Hygeia are found to be in hydrostatic equilibrium, they are also planets, and may be referred to as "dwarf planets".
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. In the new nomenclature the concept "minor planet" is not used.
As a result of this definition we have 12 planets now which includes :
- The asteroid Ceres, which is round, would be recast as a dwarf planet in the new scheme.
- Pluto would remain a planet, and its moon Charon would be reclassified as a planet. Both would be called "plutons," however, to distinguish them from the eight "classical" planets.
- A far-out Pluto-sized object known as 2003 UB313, currently nicknamed Xena, would also be called a pluton.
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