Pluto was discovered in 1930 and was considered the ninth planet of the Solar System for more than 70 years.
However, in 2006, the International Astronomical Union (IAU) decided to change the definition of a planet and demoted Pluto to a dwarf planet. What was the reason behind this controversial decision?
The IAU is the organization that is responsible for naming and classifying celestial objects, such as stars, planets, asteroids and comets. In 2006, the IAU held a general assembly in Prague, where they voted on a resolution that defined a planet as a celestial body that:
– is in orbit around the Sun,
– has sufficient mass to assume a nearly round shape,
– and has cleared the neighbourhood around its orbit.
According to this definition, Pluto did not qualify as a planet because it shares its orbital neighbourhood with other icy objects in the Kuiper Belt, a region beyond Neptune that contains thousands of small, frozen bodies.
Pluto is also smaller than some of these objects, such as Eris, which was discovered in 2005 and sparked the debate about Pluto’s status.
The IAU’s decision was not universally accepted by astronomers and the public, who felt that Pluto deserved to keep its planetary status for historical and sentimental reasons.
Some argued that the IAU’s definition was too narrow and arbitrary, and that there could be other criteria to classify planets, such as their geology, atmosphere or potential for life.
However, the IAU’s decision also reflected the advances in technology and knowledge that have revealed the diversity and complexity of the Solar System.
By recognizing Pluto as a dwarf planet, along with four other similar objects (Ceres, Eris, Haumea and Makemake), the IAU acknowledged that there are different types of worlds that deserve to be studied and appreciated for their own characteristics.
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| A closer look on the majestic mountains and frozen plains of Pluto as captured by NASA’s New Horizons spacecraft on July 14, 2015. NASA/JHUAPL/SwRI |
In 2015, NASA’s New Horizons mission flew by Pluto and captured stunning images of its surface features, such as mountains, glaciers and a giant heart-shaped plain. The mission also revealed that Pluto has five moons, including Charon, which is almost half its size.
Pluto may have stopped being a planet 17 years ago, but it has not stopped being an important and beloved member of the Solar System family.