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I love that people are still upset about this! Since it's all just made up names and "rules" anyways, they should just set the exacting requirements for what makes a planet, and then just add "except for Pluto".

Sound like a good idea Mars Base?
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That's one of the best thought out your mom jokes ever.
The reason Pluto was ultimately dropped from planet status was not because of size. At least not in a direct sense.

See to qualify as a planet it has to orbit our sun. So by definition the IAU definition of a planet disqualifies 99.999999999999999999999999999999... ok for sake of arguement it disqualifies 100% of the universe from having planets. Ouch. But Pluto orbits our sun so it qualifies for this requirement.

Second qualification is that it has sufficient mass to assume hydrostatic equilibrium. This means it has to be round. Well Pluto makes this qualification too. So 2/3 so far for Pluto as a planet. Also this means it is BIG ENOUGH.

Third qualification though... has "cleared the neighbourhood" around its orbit. So nothing else can orbit in the same orbit of said planetoid. If anything else orbits the sun in the same orbit of the planetoid it is no longer considered a planet. See this is were Pluto falls apart. It is in the Kuiper Belt, so there are lots of other objects that also orbit the sun in the same orbit as Pluto. This is also the reason Ceres isn't a planet (which it should be considered a planet). But this is were the whole thing falls apart, see technically there are other objects orbiting the sun in Earth's orbit. So technically (which is what this definition is all about) Earth isn't a planet either. In fact we look at most planets there is debris in their orbits that orbit the sun. Including Mariner 10 around Mercury.

So to be honest, there is no such thing a planet. A planet is a myth, it cannot exist in science. Now I know this post is fairly long, but if you really want to debate look up the definition for a continent. You will find that the definition of a continent is much different than what a continent is actually considered by most people. Anyways, that is why Pluto isn't considered a planet.
I think the real reason that Pluto was dropped, and more directly, why the rule was created that caused Pluto to be dropped, is because without that one rule, we would have something like 30 planets in a solar system and counting. I think the board just felt that trying to explain that to the public and school children specifically would get a little confusing.
So we are trying to encourage our students to learn less? See I have heard that excuse before and that makes no sense. We are either saying you students should learn less, or we are saying that we are making learning more confusing by creating unnecessary categories for stuff to be classified in. Either way it's not a good message.
I said nothing about whether or not the decision was a good thing. But if you take a look at committees in most organizations, they assume everyone outside are idiots who need to be protected from themselves and complicated ideas.
Never has the International Astronomical Union (IAU) has so much publicity for a ruling. This is a touchy subject obviously. If they had instead not included the 'cleared it's own orbit' part and upped the count to 12 or more would people have been up in arms? If they had instead kicked Mercury out of the planet list, would people be as upset?

I've been asked about this many times over the years. Yes it hurt that the planet I grew up with is considered a 'dwarf-planet' now. In essence it never was much like the other planets though. All the other planets have a nice circular orbit, while Pluto's orbit actually means it is closer to the Sun than Neptune for approximately 20 years in each orbit of 248 years. (I enjoyed being snarky about this in school growing up) It's orbit is more than 10° off of any other planets rotation. It's moon is so large they sort of orbit each other.

Lets face it, science changes, and in the last few decades we have suddenly seen more and more planets around other stars our there in the universe ... this fact complicated the issue. Suddenly we realized that it couldn't just be the unspoken rule that a 'planet' was a big rock orbiting our sun.

Don't get me wrong I don't want the death threats some some astronomers have gotten over this issue. (for real) They now call Pluto a 'dwarf-planet' ... it's still a planet, just in a sub-category. I almost view it as thinking a Siamese cat isn't a cat just because it has a more specific definition.

A rose by any other name ... Pluto is the same rock orbiting our sun as it was even before it was discovered. I think it's kind of a good thing the fact that this ruling brought about this much publicity. Really it created a lot of talk in and out of the classroom about the solar system and what planets are.

So, dear people passionate about Pluto. I hope you can remain passionate about science, but maybe without the death threats. Keep talking about and analyzing things that the science community says and rules on with friends and family. I mean if neutrinos might be traveling faster than the speed of light you never know where science might take us.
It did create a lot of talk out of the classroom, but that's not always good. I am often exposed to anti-science people who use this decision as further evidence that scientists don't know what they are doing or that they lie. As someone who was working on a M.S. in physics, they've always regarded me as some sort of anti-Christ, especially since I am an atheist. Trying to explain to them that this was actually about language rather than science doesn't help. They won't accept the idea that science is a process, not some sort of global conspiracy perpetrated by Satan and "demoting" Pluto really fuels some of their arguments.

The fact that it is a controversial matter in the scientific community actually seems to help polarize anti-science Americans even more. They see it as some sort of "shady backroom deal" in the same way they characterize, not coincidentally it would seem, discussions had by Democrats that influence political decision. Science is a left-wing tool to these people. We have a Congress that felt it necessary to reaffirm a cold-war era motto for our country to protect it from a liberal threat that didn't exist.

We have a very large group of people in America who are so sure of their world view, and so scared of it being challenged, that they must play Zeno's Paradox with the missing link. make personal relationships about religious morality, and can't accept a solar system with only eight planets. Maybe Europe doesn't have these problems but in America, it's worth considering when creating an arbitrary definition. Of course, that can be problematic as well since we now have pizza, made of grain, dairy product, and fruit minimum, now defined as a vegetable so parents and schools don't have to feel guilty about being cheap and lazy.
I believe is not NASA but the IAU which makes pluto not a planet
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