Most are hybrids anyway, right? So, what’s the criteria? I guess 50% volume or more of gas would be a gasball. And stars are just gasballs on fire. And black holes are noballs. 
Do we know for sure that Venus is a rockball? I thought it was more like a hot gasball than a rockball.
We’ve landed probes on the rocky part.
* pushes glasses up nose* Stars are not on fire.
It’s just a rockball with what I like to call a hotmosphere.
If Pluto is a planet, and that is fine with me, then so is Eris, then there’d be ten planets. There is either 8 or 10, not 9, unless this new one is found.
Makemake is just somewhat smaller than Pluto, so maybe 11. Then there is Gonggong. And Haumea.
Also seven moons larger than Pluto.
See, Pluto is a LOT smaller than they thought.
Size isn’t the reason.
Why is Pluto no longer a planet? | Library of Congress(IAU,neighboring%20region%20of%20other%20objects.%E2%80%9D
The orbits of objects in that part of the solar system are evidence that it has. All the objects that far out that we know of have orbits that keep them out of the way of this hypothetical planet. Either those objects started with such orbits or Planet 9 pushed them into one.
The orbits are not on the same plane. Their perihelion-aphelion lines all go in the same general direction. As far as I know, all known objects in that part of the solar system have such orbits, not just a selected number. It’s not a huge number of objects, originally 6, but there’s been at least one (and I think more than one) addition.
It’s Eris; Eros is a near-Earth asteroid. It turns out that Eris is slightly smaller than Pluto. By about 50 km. However, Eris is more massive (about 25% more), which means it’s composed of much more rock and less ice than Pluto.
Yes, actually it is. They were presented with quite a few trans Plutonian planets, most smaller than pluto. We also found out- Pluto was much smaller than thought.
So, rather than have two dozen planets, they had to come up with a reasonable scientific definition that would include most of the known planets and exclude the new, tiny ones.
- It is in orbit around the Sun.
- It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
- It has “cleared the neighborhood” around its orbit.
So they came up with those three rules, and with “cleared the orbit” Pluto failed. (And it is quite possible some of the Kuiper belt planetoids may clear their orbit, we dont know, they are very far).
They had to define planet, and yes, size was why they had to.
Size was one of (but far from the only) reasons for the debate, but not the reason for the reclassification.
Which one do you have in mind?
And as far as I know, Pluto is still officially a planet.
That works for our own solar system, where there are three quite distinct categories of objects. We don’t know if that would work as a general classification for extrasolar planets, where there could be more of a continuum, or even additional categories.
And then we have moons, whose only criterion is that they orbit a larger body. They vary a lot in composition so a simple classification would be difficult.
Since 2006, the International Astronomical Union has classified Pluto not as a planet, but as a dwarf planet.
This designation has always seemed to me to be absurd. Clearly Pluto is a different class of object from the inner planets, but calling it a dwarf planet was a kind of limbo. I think it was a sop to people who wanted to still call it a planet. If it’s different, call it something different. The definition of “cleared its orbit” for a planet seems to me contrived. Pluto has no more in common with Ceres, another dwarf planet, than it does with any of the rocky planets.
Planet is one of those classical terms like continent that has come to designate objects quite different in concept from the original definition. Originally the Sun and Moon were also planets, and the Earth wasn’t. As we have learned more about the solar system, the definition of planet has been tweaked to keep the their number between 5 and 9. Once we find more than 9 objects (like asteroids or Kuiper Belt objects) that fit the previous definition of planet the definition gets changed to keep the number small.
Goebbels doesn’t orbit the Sun.
I’m sorry, I didn’t quite hear you. A dwarf what?
So dwarf planets are planets, prairie dogs are dogs and sea lions are lions…
And your point? That dwarf rabbits (and perhaps even giant rabbits) are not rabbits?
That the inclusion of the term “planet” in the name “dwarf planet” does not mean that Pluto is a planet. It’s not.
I hate the term “dwarf planet”; it’s a sop and confusing. We should’ve kept the term “planetoid”.