why do they call pluto a dwarf planet

Pluto's planet status remains a matter of debate among astronomers. Dr. Alan Stern, now Principal Investigator of NASA's New Horizons mission to Pluto, initially coined the term "dwarf planet" in 1991 to designate a third class of planets in addition to terrestrials and jovians--small planets large enough to be rounded by their own gravity but not large enough to gravitationally dominate their orbits. He never meant for dwarf planets to not be considered planets at all. The 2006 IAU decision represents only one viewpoint in an ongoing debate on what constitutes a planet. It reflects the position of one camp of astronomers, the dynamicists, who believe that an object has to gravitationally dominate its orbit to be a planet. Only four percent of the IAU even voted on this.


Their decision was immediately opposed in a petition of hundreds of professional astronomers. The latter group, the geophysicists concentrate on individual objects themselves rather than on how those objects affect other objects. To geophysicists, if an object is large enough to be rounded by its own gravity, a state known as hydrostatic equilibrium, and it orbits a star, it is a planet. Dwarf planets are simply small planets. Because Eris and Ceres are also in hydrostatic equilibrium, geophysicists consider them planets too. Pluto is estimated to be about 70% rock, and is geologically differentiated into core, mantle, and crust, just like Earth is. It has geology and weather, all characteristics of planets and not of shapeless asteroids.


Additionally, Pluto has four moons: its large moon Charon and three tiny moons, two discovered in 2005, named Nix and Hydra, and one discovered in 2011, known as P4. Pluto can be considered to be both a planet and a Kuiper Belt Object. The first tells us what it is, and the second tells us where it is.
Pluto was discovered by the astronomer Clyde Tombaugh in February 1930. It was given the status of the ninth planet of the solar system. As telescopes, particularly in on satellites, improved, more objects were discovered which caused a problem that they were quite small and some astronomers didn't think they qualified as being planets. The International Astronomical Union (IAU) had a vote which was very close. They defined three criteria which a planet must satisfy.


It must be large enough for gravity to overcome structures of materials and make it spherical. Most bodies are flattened spheroids due to rotation. It must orbit. It must have cleared its orbit of other bodies other than moons. The IAU created a new definition of an object called a dwarf planet which only satisfies the first two criteria. Pluto fails the third criterion, so it was demoted to a dwarf planet. Many people, including myself, still consider Pluto to be the ninth planet. To be pedantic, Jupiter has a lot of asteroids in its orbit at its two Lagrange points. They are called trojan asteroids. So, this means that Jupiter fails the IAU's third criterion and should be a dwarf planet, which it is certainly not!

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