In 2006, the IAU (International Astronomical Union) re-classified Pluto as a dwarf planet. This caused a lot of confusion at the time and is still confusing a lot of people. The IAU came up with three rules that an object in our solar system must follow for it to be considered a planet:
The object in question must orbit the Sun.
This is one of the most you don’t say statements I have ever heard. For an object to be classified as a planet, it MUST directly orbit the sun. Pluto does, and therefore this criterion is satisfied for the tiny world.
The object must be massive enough to be nearly spherical in shape by virtue of its gravity.
An object with sufficient enough gravity will be able to maintain a nearly spherical shape. Many objects smaller than Pluto are able to this, as is evident in some moons of Saturn, Jupiter, Uranus and Neptune.
The object in question must have cleared its neighborhood.
Now this is where it all falls apart for Pluto as a planet. Pluto is surrounded by a huge collection of debris originating from the birth of the Solar System called the Oort Cloud. It passes through the orbit of another planet , i.e. Neptune, and there is evidence that Pluto is NOT the biggest of the bunch (though data from the New Horizons says otherwise). In fact, many other objects between 900km – 2300 km were discovered, leading to a question- If Pluto is a planet, so are these objects. If Pluto is not a planet, than these objects are not either. The latter was implemented, and thus, we have eight planets. Pluto was unceremoniously kicked out from the set of Planets due to the discovery of other tiny worlds, namely- Eris, Sedna, Makemake, Haumea and much more.
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