In the solar system we know many planets, either the named rocky planets, as the Earth, the gaseous planets, as Jupiter, or the dwarf planets, as for example Pluto. The gaseous planets have very large sizes, and the rocky planets contain many minerals, but a celestial body has to fulfil certain requirements to be considered a planet.
Since the initial proposal of this question, in 1999, Pluto has been demoted to be considered as a dwarf planet, instead to be considered as a planet.
Before the XXIth century, even at the beginnings of the year 2000, it still didn’t exist an appropriate definition to the concept of planet. The term “planet” was only defined as a body that orbited around a star.
To try to solve this problem, in 2006, the International Astronomical Union (IAU), which has the last word within the questions of astronomical nomenclature, voted to a formal definition of what constitutes a planet. According to their decision, a celestial body has to fulfil 3 different requirements to be considered a planet:
- It has to be an object that orbits around a star in an independent form. This means that the moons can’t be considered as planets, as they orbit around other celestial bodies such as planets.
- It has to have the sufficient mass for its own gravity to cause it to have a spherical shape.
- It has to be big enough to be able to “dominate” its orbit, that to say, its mass has to be greater than any other body that went across its orbit.
Because Pluto is not big enough to be able to “dominate” its orbit, it isn’t considered as a planet. For this reason, Pluto is catalogued as a dwarf planet. However, Neptune is approximately 8000 times more massive than Pluto, reason for which it accomplishes this requirement to be considered as a planet.
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