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Why on earth is Pluto no longer considered a planet?

Large map of Pluto generated from several images captured by New Horizons at 1800 km altitude. Credits: NASA/JOHNS HOPKINS UNIVERSITY APPLIED PHYSICS LABORATORY/SOUTHWEST RESEARCH INSTITUTE

A 6 year old little girl who lives in Ireland wrote to NASA with an unusual request: to make Pluto a planet in our solar system once again. This reminds us of the reason for why Pluto ceased to be considered a “classic” planet over a decade ago. 

The New Horizons probe, which aimed to study Pluto and its satellites, took off from Earth in 2006. At this time Pluto, which was then the ninth planet in our solar system, was “declassified” and re-classified as a dwarf planet a few months later. However, the probe’s observations of Pluto only started in 2015!

On the 17th February 2018, the Washington Post published an endearing story. Cara Lucy O’Connor, an Irish little girl of just 6 years old, was helped by her teacher to write to NASA, to ask for Pluto to be given back its status as one of the planets in our solar system. In her letter, the little girl provided precise scientific data, relating in particular to the Kuiper belt (a ring situated beyond Neptune), an area in which several dwarf planets are found.

Credits: NASA/JHUAPL/SwRI

But why on earth is Pluto no longer considered a planet in the same way as Earth or Mars? In reality, although it orbits around the sun and has a relatively round shape, an argument put forward by researchers puts paid to any hope. In fact, its gravitational force is not great enough to either attract and bring together bodies around it, or to repel them far away.

Therefore, Pluto was declassified, which gave birth to a new class of dwarf planet: the plutoids, a category incorporating Eris, the largest known dwarf planet, situated beyond the Kuiper belt.

The declassification of Pluto was also a great blow to the United States, as it had been the only planet in the solar system to be discovered by an American!

Sources: Washington PostSciences et Avenir