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Nyx say: This is why. And its nothing to worry about.
https://www.iau.org/public/themes/pluto/
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R say: Pluto is a dwarf planet. It is part of the Kuiper belt, which is beyond the orbit of Neptune.
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Clive say: Because it doesn't fit the criteria.
The thing is that with the discovery of so many Pluto-like objects further out, either there are only eight planets, or there are dozens and dozens of them. Pluto's orbit was always known to be different from the rest (it comes closer to the Sun than Neptune sometimes) and all in all, it looks more like one of those Kuiper Belt objects further out than a "proper" planet.
So the International Astronomical Union decided it was time to fix on a proper definition of a planet, and the definition is this:
1. It orbits the Sun.
2. It is in hydrostatic equilibrium (its own gravity has squashed it into a round shape).
3. It has cleared its neighbourhood (its gravity dominates over every other object in its orbit).
Pluto fails on point 3. Neptune dominates over it and in particular, the two are in 3:2 orbital resonance - Neptune orbits the Sun 3 times in exactly the same time Pluto goes round twice. They're affecting each other and Neptune is clearly boss.
This puts Pluto in its proper perspective - the closest of the Kuiper Belt objects rather than a proper planet. It was called a planet before because for over half a century, it was the only one we knew, but even so it looked out of place.
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Leicester City say: Pluto left planet like Brexit, because i am the king of there
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say: its too small to be classified as a planet
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