What I don't understand is how Pluto is no longer a planet.

[Helm]

It's round and it orbits the Sun, size is a complete irrelevance beyond that. Look how much smaller the Earth is compared to Jupiter, but they're both still planets because they're round and they orbit the Sun, it doesn't need get anymore technical than that.

[John I Tzimisces]

This seriously could be answered by wiki-ing the issue.

Pluto is no longer considered a planet because it's been determined that beyond it's orbit there are larger bodies that are similar in composition and have equally eccentric orbits, so, it's not just a small rocky planet that would look like mars or earth or venus or mercury or whatever were it closer.

So, lets recap:

It is much smaller than any rocky planets.
It's not a gaseous planet.
It is not on the same orbital plane as the planets.
There are bodies which are closer to the size of the already pretty small rocky planets further out that also fit into the previous descriptions.

Thus, although it was logical to place Pluto as a planet initially, the relatively discovery of other bodies like it means it's probably better to create a new category rather than classify them all as planets.

Heck, Ceres (a planetoid in the asteroid belt) is a lot closer and isn't classified as a planet either due to its size and orbit eccentricity (I believe). Those seem to be key.

[Trax]

Ceres was considered a planet for decades after it's discovery, only after it became clear that there are countless number of asteroids did it change. Exactly the same thing happened to Pluto, it turned out to be the first representative of a new class of celestial bodies in the Solar System.

[B5C]

Seriously, do you really want to study over 20 planets? Also note back in the 1600-1800s, Astronomy used list the solar system too have 100 planets! That is until we found out about the Asteroid belt.

Watch this:

[Sphere]

Look how much smaller the Earth is compared to Jupiter, but they're both still planets because they're round and they orbit the Sun, it doesn't need get anymore technical than that.

It would be cruel to force school children to remember the names of several thousand planets. There are several objects orbiting the sun that are actually larger than Pluto and a whole mess of them that are similar in size. Like Pluto they are small icy bodies orbiting on the very edge of our solar system outside of the plane of the planets. It would be very odd to lump them in with the actual planets.

No it makes much more sense to talk about the inner terrestrial planets, the asteroid belt, the gas giant planets and then the distant icy bodies of which Pluto was the first to be discovered.

[Helm]

School kids are already very familar with Pluto so we may as well just keep it as a planet. You can call all the other round objects that orbit the Sun as planets as well because that's what they are but it's probably not necessary to make school kids remember them all, just stick the 9 planets including Pluto for historic reasons.

[Elfdude]

Science isn't guided by tradition. So no.

[Helm]

Sceince is guided by facts and the fact is Pluto is a planet, a really tiny little planet and there are other tiny planets much like it but we don't have to teach those in school class, they deserve a mention perhaps.

[John I Tzimisces]

No, Helm, because that's stupid.

[Elfdude]

Lets go through the definition slowly Helm. A planet by science is A.) A satellite which is large enough to be round by it's own gravity. Check. B.) A satellite which orbits a star. Check. C.) A planet that has cleared the orbit of planetesimals and debris. Fail. Pluto is not a planet. Learn english.

[Helm]

A planet that has cleared the orbit of planetesimals and debris. Fail. Pluto is not a planet.

They added this definition recently but I can't see why it should matter. A planet will clear a space of debris in proportion to it's own mass and gravitional pull surely? No doubt Jupiter has cleared a bigger area than Earth because it's a much larger planet. Pluto being a tiny planet won't have cleared much if anything at all. It also happens to be in a region of the solar system where there is a lot of debris if it had been in a region free from debris this would not even be an issue.

[Elfdude]

I know this has been mentioned before but since you ignore information so often here it is again: The problem is Pluto doesn't have a regular orbit so it can't clear the area of planetesimals. Earth can. Neptune can. Mars can. Mercury can. Pluto can't. You're wrong. You're ignorant. There's no problem with the definition, there's no problem with the idea, there's only a problem with you.

[Helm]

So it's a tiny planet with an irrelegular orbit, but so what it's still a planet of some description. It even has an atmosphere and a moon that orbits around it. I could settle for calling it a planetoid at a pinch.

[John I Tzimisces]

Pluto's gravity is so insignificant that Charon does not orbit around Pluto so much as Pluto and Charon orbit around each other.

[Col. Tartleton]

Helm by your logic everything that Orbits may as well be a planet because it just means something that wanders through the sky.

However they've got a more specific definition for what science has determined are true planets.

Pluto is a Planetoid; a Plutoid. It's like a planet but smaller. Just like Earth's moon is like a planet but smaller. Large moons and small planets. There really isn't a difference other than having its own clear orbit. Titan is comparable to Earth in size. But it's a satellite of Saturn.

A Planetoid is a non planet, not comet. That would include moons and asteroids.

It's a hunk of rock. They're all hunks of rock.

[Elfdude]

So it's a tiny planet with an irrelegular orbit, but so what it's still a planet of some description. It even has an atmosphere and a moon that orbits around it. I could settle for calling it a planetoid at a pinch.

Which is why pluto is a dwarf planet but not a planet

[Trax]

I could settle for calling it a planetoid at a pinch.

Planetoid is the same as asteroid, Pluto is not an asteroid.

Pluto is called dwarf planet now, so I really can't see what's your problem.

[Phier]

Calling it a dwarf planet is just to get people too fixated on the old way to quiet down.

The asteroid belt would have several dwarf planets, that would make sense as they formed out of the same disk that the main planets formed out of. Ceres should be a dwarf planet, putting Pluto in there was just to appease those who didn't like the change.

Pluto is still a Kuiper belt object.

[Elfdude]

Right, but in this case that was exactly the purpose of calling it that

[Trax]

Ceres should be a dwarf planet,

It is.

The IAU currently recognizes five dwarf planets—Ceres, Pluto, Haumea, Makemake, and Eris.