Solar System
From the school level we are reading that we have nine planets in our Solar System. All the nine planets are moving around the Sun and they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto respectively according to their distance from Sun.

But now a day’s our largest distant planet Pluto is not considered as a planet in our Solar system. Now we have only eight planets in our solar system. According to the International Astronomical Union (IAU), Pluto is not a planet rather it is a Dwarf Planet as it don’t follow the three criteria to be a full-sized planet.

Solar System

History of Pluto:

Pluto was discovered as a planet on 18 February, 1930 by the astronomer Clyde W. Tombaugh with contributions from William H. Pickering. They discovered the planet at the Lowell Observatory in Flagstaff, Arizona. After discovery of Pluto it was considered that we have nine planets in our Solar system. Pluto was the longest distant planet from Sun.

Clyde W. Tombaugh
In 1894, Percival Lowell had founded the Lowell Observatory in Flagstaff, Arizona. In 1906 he started an extensive project in search of a possible ninth planet, which he named “Planet X”. By 1909, Lowell and Pickering had suggested a lot of possible celestial coordinates for the existence of such a planet. Lowell passed away in 1916 without knowing that his observatory had captured two fain images of the Planet X on 19 March, 1915. But Lowell was not the first to unknowingly photograph Pluto.

Percival Lowell
The job of the discovery of Planet X was handed to Clyde Tombaugh in 1929. Tombaugh’s task was to systematically image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any object s had shifted position. Using a ‘blink comparator’ machine, he rapidly shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. After one year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and 29 of that year.
The discovery of planet X made history. The Lowell Observatory received over 1,000 suggestions about the naming of the planet from all over the world. The name Pluto was proposed by an eleven year old schoolgirl Venetia Burney in Oxford, England. The Planet X became Pluto officially on 24 March, 1930. The name was announced on 1 May, 1930 and Venetia received five pounds (5) as a reward.
Orbit of Pluto

Criteria to be a planet:

According to the International Astronomical Union (IAU), to be a planet, anything needs to follow three criteria. Those are as follows,

1.       It needs to be in orbit around the Sun.

2.       It needs to be massive enough for its own gravity to make it almost round.

3.       It needs to have “cleared the neighborhood” of its orbit.

If all three criteria are followed by a body or an object, that will be called a planet. But if a body doesn’t follow the 3rd criteria then that is considered a Dwarf Planet.

Why Pluto is A Dwarf Planet ?

Kuiper Belt

Pluto follows only two of these criteria. When Pluto was discovered, astronomers thought that its mass is huge. They considered Pluto as a huge planet. But after years it was proved that Pluto is a very tiny planet and it doesn’t follow the third criteria of IAU to be a planet. It means the neighboring region of Pluto is not cleared. That means, “not clearing its neighboring region of other objects?”…..here the answer is Yes. Pluto shares its orbital neighborhood with Kuiper belt objects such as the plutinos. Pluto is only 0.07 times the mass of the other objects in its orbit. The Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit.

NASA had sent their New Horizons spacecraft off to visit Pluto from very nearly. After 9 years 5 months, on 15 January, 2015, it could able to see Pluto nearly. After researching briefly Pluto was considered a dwarf Planet by General Assembly.

New Horizons 


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