Mars is going to have a killer apparition later this summer-magnitude -2.7.
I was just repeating Chronos.
And Ganymede can just be an ice/rock ball.
Some people are trying to argue that Neptune and Uranus are enough different that Jupiter and Saturn that they should have a different name. I disagree. They’re all gas giants, if you like you could just say that Neptune and Uranus are cold gas giants. Slap a modifier on there.
The real answer of course is that every planet and star and ball of stuff is unique, and all classification systems are approximations. Earth isn’t like Venus, Mars isn’t like Mercury, Ceres isn’t like Europa, Neptune isn’t like Luna.
Jovian gas giants like Jupiter and Saturn are predominately hydrogen, with an outer core of metallic hydrogen. Neptunian ‘icy giants’, on the other hand, are composed predominately of heavier elements and little which should give much more complex (or at least different) atmospheric and chemical dynamics. There is good reason to differentiate between hydrogen gas giants and more complex icy giants just on the basis of how these planets are formed and their planetary evolution as well as the kind of environment they provide and phenomena they exhibit.
Stranger
I’d hate for Earth to declared a “non-planet,” because it would kill one of my favorite “Dad jokes.” Sometimes my kids would look at a starry sky and ask me if any planets are visible. I’ll say “yes”…and point down at the ground.
It must be a much bigger type than a 747… Plus refuelling in space is tricky… 
That sounds like a great reason to make it a non-planet.
Spaceball would be a reasonable inclusive term for rockball, gasball, and iceball, if it wasn’t an old Mel Brooks comedy.
I assume Ganymede is a rockball, if iceball refers to ice giants. Ices are 46-50% of total mass on Ganymede. I see it has a subsurface ocean, possibly the largest ocean in the solar system. Huh: I thought subsurface oceans were unique to Europa.
Nothing to see here, move on. 
The distinctions ‘rock’, ‘ice’ (and to a certain extent, ‘gas’) are a bit Earth-centric. Rock and ice are both just solid minerals. What’s the meaningful difference between silica pebbles on a beach at the edge of a water ocean, and water-ice pebbles on the beach of an ocean of liquid methane?
See post 23.
Planet originally just meant some celestial body that primarily orbits the sun. Science is science though, and things get a little more confusing when people start talking about planets in other solar systems. So a formal definition was needed. The appropriate groups got together and settled on a naming scheme based on an solar orbiting objects size (and I believe size alone). Several moons of Jupiter are actually referred to as planets in astronomy literature. Pluto doesn’t make the cut. This has been true for a long time. Neil de grasse Tyson just took his unique opportunity to “correct” the public.
Edit, outdated: looks like the iau changed the definition to include something about “clearing its sky”. What’s the definition of clearing?
The difference is the processes by which they are formed, the envrionment they exist in, and how they interact with other conponents of the planetary environment to influence the climate and behavior of the world.
The notion that planets and other planet-like bodies can simply be discreted into any small number of arbitrary clades is just not correct or very useful. Even within our solar system each of the planets has distinct characteristics that make it unique not only in relative composition but in fundamental behavior. Lumping tiny Mercury in with the other “rocky” worlds ignores that it is actually smaller than Titan and Ganymede and barely larger than Callisto. Mars is only barely more than a largish moon, but is one of four silicate objects to have enough of an atmospere to bother about. Earth and Venus are often described as “twin planets” because of the relative closeness of mass, but practically nothing else about them is similar, from atmospheric composition and rotation rate to a magnetiosphere (Venus has none) indicating that under the crust Venus may have very different mechanisms driving its geology for reasons we can only guess at. Even Uranus and Neptune, which at first glance appear to be quite similar in size, appearance, and general location within the solar system have some fundamental differences in composition and character which indicates that they may have been formed in different ways and in very different locations so even the similarity of these planets is superficial.
For casual reference, referring to anything spherical, in a regular and cleared orbit, and above a certain mass threshold as a “planet” is adequate but from the standpoint of planetary scientists there a wealth of distictions that go beyond superficial appearance and basic composition, hence the need to refine and sometimes change how planets are defined. And as we learn more about extrasolar planets that are unlike any planets in our solar system, we’ll probably have to do that again.
Stranger
Categorizations exist because they are useful for some purpose. A definition of ‘planet’ which includes Mercury, a hubcap, and the Red Hot Chili Peppers is not a useful categorization.
When we first discovered Pluto, it sure looked like a planet like the others. So we developed a categorization for planets that included the common features of the things we considered planets -they had to be in orbit around the sun, and not another body, which got rid of the pesky large moons which clearly weren’t planets. Then we decided that to be a planet the object had to be big enough for its gravity to cause it to become roughly spherical. That took care of most of the asteroids and comets. Finally, after discovering Ceres which was large, spherical, and in orbit around the sun, we decided that a planet had to have ‘cleared its orbit’. That seemed to cover all the bases.
But over the decades, we found other objects that were like Pluto, but not like other planets. They were in highly eccentric orbits, and seemed to be part of the Kuiper Belt. Suddenly Pluto looked less like Mercury or Mars, and more like other Kuiper belt objects, some of which were roughly the same size as Pluto. So either we had to start calling lots of other things ‘planets’ even though they seemed substantially different from the 8 other major planets, or we had to create a new category and put Pluto in that one instead.
All of these distinctions are arbitrary and only useful in that they help us think about and study the solar system. As we continue to find more exoplanets, we are discovering that there are huge variations in solar systems and planets can take of many different forms and sizes.
Point taken, but bold mine, the inner ‘planets’ seem substantially different then the big 4. The arbitrary lines seemed to be drawn to keep earth in the center of the universe so to speak, even though it is not there. In other words the inner planets plus Pluto seem more identical to each other then the ‘real’ planets of J,S,U and N which dominate the solar system (under the Sun).
Pluto is not materially like any of the inner planets, and in fact the other celestial body it most represents is Triton, which is very likely a captured Kuiper Belt Object.
Stranger
Again, it depends on the purpose of the classification. Every planet differs from other planets in substantial ways. But they are all big round things fairly close to their star (conpared to Kuiper Belt and Oort Cloud objects) and in roughly circular, clean orbits.
Why isn’t Ceres a planet? The simple answer is because Ceres is just the largest of a whole bunch of bodies in the Asteroid belt, so it doesn’t make sense to allow it into the planet club while excluding other similar objects. Likewise, Pluto is just one of many thousands of Kuiper Belt objects. They can’t all be ‘planets’. Other Kuiper Belt objects lke Sedna, Makemake and Eris are also large enough for gravity to pull them into a sphere, and have eccentric orbits. Should they also be planets?
For all those bodies, we have a new classification; “Dwarf Planet”. Works for me.
If people are obsessed with finding some “less arbitrary” definition, I recommend the Sotor definition.
A planet is any orbiting object whose mass is greater than the sum of all other objects sharing its orbital path.
Nope. There are only 9 slots, and all of them are already filled.
Now it could be, that if we knew 100 years ago what we know now, that some of those slots might have been filled differently; but what’s done is done, and that’s all there is to it.
There have always been 9 planets; we just didn’t realize it until 1930.
Thankfully, science doesn’t work this way.
One thing (of many) that isn’t clear to me is what characteristics are important when studying the matter in orbit around stars.
Intuitively, I’d say “Big and round”. “Big” among other things implies, “So big it must be round, or must be with 99+% confidence”. To me Haumea isn’t a planet: it’s an overgrown potato. Pluto has 3 times its mass.
But I’m getting a sense that spherical shape isn’t especially important to astronomers. (It also isn’t easy to measure at distance). Planetary differentiation might be one criteria, if you think that complicated geology separates planets from comets or asteroids. Or being big enough so that x% of bodies have planetary differentiation, x>>>50%.
So… what planetary characteristics are important to scientists? In rough order please.