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answers:
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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Dump the liberals into Jupiter say: Pluto IS what it has always been. The map is not the territory. The description is not the thing. The label is not the contents.
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say: You are 11 years late
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Poseidon say: Hi Speed,
Hi Speed,
According to the new definition, a full-fledged planet is an object that orbits the sun and is large enough to have become round due to the force of its own gravity. In addition, a planet has to dominate the neighbourhood According to the new definition, a full-fledged planet is an object that orbits the sun and is large enough to have become round due to the force of its own gravity. In addition, a planet has to dominate the neighbourhood around its orbit.