When Did Pluto Stop Being a Planet? Why Was It Reclassified and Demoted?

Pluto used to be considered the solar system's ninth planet but ended up getting reclassified and demoted in the year 2006.

Pluto Was Once the Solar System's Ninth Planet

Aside from being previously considered the solar system's ninth planet, Pluto is also the biggest known dwarf planet in the Earth's solar system. It is situated in the Kuiper Belt, an area beyond Neptune's orbit that contains hundreds of thousands of icy and rocky objects that span over 100 kilometers across. The Kuiper belt is also rich in comets, with its comet count totalling to roughly 1 trillion or more.

The existence of Pluto was first suggested in 1905 by American astronomer Percival Lowell. The astronomer noticed some unusual deviations in Uranus and Neptune's orbits and thought that there may have been another object whose gravity was affecting the orbits of these ice giants. He was able to predict Pluto's location in 1915, but ended up dying before the actual discovery of Pluto.

In 1930, Clyde Tombaugh discovered Pluto in the Lowell Observatory. The discovery was grounded on the predictions made by Lowell and other astronomers.

The cosmic world was demoted in 2006 when it was reclassified into a dwarf planet. Its demotion became a subject of controversy and debate in the scientific community and beyond.

Why Was Pluto Reclassified?

It has been a long-standing belief that Pluto was unique and distinct in the Kuiper Belt. However, as specialists delved deeper into the Zone and the asteroid belt situated between Jupiter and Mars, they discovered that there were several other Pluto-like objects. These objects were more similar to Pluto rather than Pluto being like other planets.

The discovery of the new planets prompted astronomers to have a clearer criteria for classifying planets and for deciding Pluto's category.

The International Astronomical Union then came up with three rules for standardizing the definition of a planet. For one, the object should be orbiting the sun. Secondly, the object should be huge enough to have a roughly spherical shape. Lastly, the object should have an orbit that is clear of objects that have the similar mass as its own. The third point means that the object should be gravitationally dominant within its orbit.

While Pluto meets the first two criteria, it falls short when it comes to the third. One of Pluto's Moons, known as Charon, is roughly half of its size.

Hence, Pluto has thus been demoted to a dwarf planet. Nevertheless, it is still considered the largest known dwarf planet in our solar system.

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