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Few years back, our solar system had nine planets-Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Pluto. But today it consists of only eight planets. These may be classified into two groups- the first four rocky inner planets and the next four gas giants, the two groups being separated by the Asteroid belt. Pluto, the former ninth planet of the solar system was excluded from the list of planets in 2006 by the International Astronomical Union(IAU). What forced the IAU to take this step.
Pluto was discovered in 1930 by an US astronomer Clyde Tombaugh. It was known as the smallest planet in the solar system. Among the known nine planets at that time Pluto’s orbit was the most elliptical one while Mercury stands second. On an average, it is 39 AU far from the Sun. Its orbital period is 248 years. In 1978, Pluto’s first satellite “Charon” was discovered and it was found that Pluto and Charon are revolving around their common center of mass and thus can be treated as a binary planet system.
The story of nine planets remained as such for more than 70 years. In 2005, with the discovery of Eris( to be the tenth planet), the Pluto’s status as planet came in danger. As it was predicted that there exist a large number of such objects which orbits the Sun in the outer region of the solar system (now known as Kupier belt). Kupier belt(KB) is a disc beyond Neptune and upto 50 AU from the Sun just like the Asteroid belt between the Mars and Jupiter but is much larger and massive than it. Trillions of things are present in this KB. Nearly all of them composed of “ices” (frozen methane, water, ammonia etc) and rocks.
So discovery of Eris and prediction of many such objects led the IAU to redefine the term “PLANET”. This step was taken in fear of having 40-50 planets in our solar system in the coming years. In the 26th general assembly of IAU which was held in 2006, a planet is redefine as- A planet is a celestial body that (i) is in orbit around the Sun, (ii) has sufficient mass for its own-gravity to acquire a spherical shape, and (iii) has cleared the neighborhood around its orbit.
Pluto follows the first two criteria as it is round and also orbits the Sun. However, it does not follow the third criterian. This becomes clear from the fact that- Pluto is only 0.07 times the mass of everything else in its orbit while the Earth is 1.7 million times the mass of everything else in its orbit.
So, after redefining a “Planet”, Pluto and Pluto-like objects can’t be in the list of planets. At the same time when Pluto was degraded from being a planet, the IAU officially announced a third category of planets know as “Dwarf planets”.
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