The issue "what is a planet?" will have to be visited again as it becomes more clear just how inadequate the current nomenclature of orbiting objects is. And yes, in many ways it is a trivial concept. And judging how it has grasped the public's imagination, it is also emotional, not just clinical. But as we map more and more orbiting objects, it would be nice to have a system that isn't arbitrary and confusing, like the International Astronomical Union's current decision is.
Some cities, like New York, have an easy-to-grasp layout. Someone who has lived in New York for 10 years is as just as capable as someone who has spent fifteen minutes in Midtown Manhattan in navigating themselves from one address to the next: it's a simple grid, with each avenue and street simply numbered in one direction. Now compare that to Boston, where streets loop around and meet in triangles, and have different arbitrary names... madness. So nomenclature does matter, in the end, when you are dealing with hundreds or thousands of objects. Rather than the mere dozens we are dealing with when it comes to our current portfolio of planet-like objects that we know of. Politics and historical comfort may be a valid basis for the IAU's decisions today, but something more scientifically rigorous needs to come to the fore if we are to deal with our growing portfolio of known objects tomorrow.
The IAU not only allowed Pluto to remain a planet but they said that the moon of Pluto, Charon, should be considered a planet as well. Why? Because Charon and Pluto orbit a common center of gravity, rather than one orbiting the other. So while Earth's own Moon might be larger than both Pluto and Charon put together, and while some consider the Earth and its Moon just as much of a double planet system as Pluto and Charon, it still remains that the common center of gravity between the Earth and the Moon is well inside the Earth. Grumble... seems a rather puny technicality for elevating such a wimpy object as Charon to planethood.
Also, Ceres, the asteroid, got promoted to planethood as well. Why? Well, it's circular. And it's big for its environment, it contains a third of the mass of the entire asteroid belt. And I guess it just wouldn't look good to promote Pluto and Charon and not Ceres. OK, on that rationale: welcome Ceres to the pantheon of planethood. We've known you for awhile, but we just never gave you the recognition you deserved. Fine. But now elementary school students will need a new mnemonic for remembering your place I guess: "My Very Excellent Method: Just Say YoU KNow Planets" would become... "My Very Excellent Memory Concept: Just Say YoU KNow Planets Changing"?
Then there's the cornucopia of new planetoids Mike Brown and his colleagues have been discovering for the last few years: Xena, Sedna, Quaoar, 2002 AW197, etc., and probably a bunch more just lurking in deep exposure pictures on hard drives at some university but not yet properly sifted through computer algorithms and human eyes to be revealed yet. The Oort Cloud/ Kuiper Belt perhaps contains hundreds, maybe thousands of such bodies.
Now the IAU DID propose a new sub-classification for planets: Plutons. Well what is a Pluton? Any small icy object in the Oort Cloud/ Kuiper Belt would make sense to be called so. But no: Ceres is a Pluton says the IAU. Huh? So the IAU's move, in the end, seems to be very arbitrary and politically related rather than scientifically satisfying. Their Pluton designation will help with separating new, small discoveries from what we consider to be the classical eight planets, but it leaves one unsatisfied.
Consider the problems of classifying all orbiting objects we already know of according to their various qualities: Is it a gas giant (is it mostly gas)? Has it undergone nuclear fusion before? Could it? Is it currently undergoing nuclear fusion (like a binary/ tertiary star system)? Does it rotate clockwise or counter clockwise (consider Neptune's captured moon, Triton, and its retrograde orbit)? Does it orbit in the same plane of the other objects? Does it have a noncircular orbit or a complicated multifocal orbit? Is it a Lagrange point companion? Does it have an atmosphere? Is it spherical? Is it unstable? Is it transient? Is it it ice? Is it rock? Is it metal? Is it rock/ice/metal (like comets/ Oort Cloud/ Kuiper Belt objects)? Is it one of thousands/ millions like it (an asteroid belt, or a planetary ring)? What does it orbit: A star? A planet? A moon? A moon of a moon? Is it a binary object? Is it a comet/ does it shed a lot of material? Etc, etc, etc...
As you can see, things get rather mindboggling very quickly. All of these considerations mean that we need to harden our astronomical nomenclature into something less dependent on folk cultural artifacts of Pluto's planethood or not. I've been thinking about this for awhile, so how about this, my homebrew nomenclature, for what it's worth:
And finally... drumroll please... most importantly... all of these objects should be defined independently of what they orbit. And, also, importantly, all of these objects should be defined independently of their size (I suppose you could talk about moonlets, or planetesimals, or planetoids, or minor planets, etc., if something falls under a certain size threshold that renders them less interesting and more numerous than certain large moons/ planets of obvious greater interest).
- Planet = mostly rock/metal/ice sphere with a significant atmosphere. What "significant" means becomes a point of contention then of course. Pluto's atmosphere for example is existent but tenuous, so again Pluto screws with our attempts at classification.
- Moon = rock/metal/ice sphere without an atmosphere.
- Asteroid = solid rock/metal/ice object that is not spherical.
- Comet = loose agglomeration of ice and rock and dust such that it can shed when on a violent orbit... but called a comet regardless of its current violent/ peaceful trajectory. So I am including not just those we see streaming towards the sun on a regular basis, for as we explore the Oort Cloud/ Kuiper Belt, we'll find plenty of these "dormant" comets.
- Gas Giant = considered as something different than a planet (mostly gas, obviously spherical). A star is simply a gas giant that has achieved thermonuclear fusion, and in fact, you have binary, tertiary, quaternary, etc. star systems where that is exactly what has happened. And in between you have your brown dwarfs, white dwarfs, and other objects occuring at the end of a star's life time/ before it's lifetime/ malformed and never quite fully fusing, etc. So what I am saying most importantly is that the whole notion of "gas giant" should not only be divorced from being a type of planet, but that it should also be combined with the notion of being a type of star. A gravitationally unignited/ dormant star. Arthur C. Clarke knows what I am talking about.
- Star = the root gravitational focal point for a family of rotating objects (besides the galactic center of course, which everything loosely rotates). For now, a star is a large bright fusing object, until we develop the technology in some far off future where we can discover and classify dark, unignited gas giants out there floating by themselves in the galaxy, and their menagerie of what would be very, very cold satellites.
So Mercury isn't a planet, it's a moon of the Sun. Likewise, Pluto is a moon of the Sun. And Ceres and Vesta are moons of the Sun (small perfectly spherical "asteroids"). Titan isn't a moon, it's a planet of Saturn (it has a significant atmosphere). Deimos and Phobos are called moons of Mars, but they are clearly NOT moons, but captured asteroids of the Sun. So, represent that: call them asteroids of Mars. Why call them moons? They are not on the same order of objects we can rightfully consider to be moons, ie, spherical but devoid of atmosphere.
It all boils down to this: no matter what nomenclature is agreed upon, as we discover weirder extrasolar objects and orbital arrangements out there, the "what it's made of" part of an object's identity should come to mean something more important than "what it orbits". And for the same reason (the many different exotic configurations), size should no meaning, or much less important tertiary meaning. Then we have to come to grips with direction of orbit, orbits outside the orbital plane, orbits with bizarre shapes, binary/ tertiary objects, binary/ tertiary/ quaternary star systems, ring systems, etc.
How about a tertiary star system orbiting a common gravitational barycenter, each star with it's own planetary system... and one planet that, via natural harmonics between the three stars, switches orbital allegiance every now and then? Unlikely but possible. Well, what do we call such an object then with a nomenclature dependent first and foremost on what something orbits, rather than what it is made of?
How about a trojan planet? Usually objects that are trojans are tiny, a requirement of objects existing at Lagrange points for two much larger objects. But what if those two objects were so massive that they allowed for the existence of a mass large enough to gravitationally become a sphere and retain an atmosphere at the LaGrange point? Yes, a trojan planet. Again, unlikely, but plausible.
You can probably think up some of your own exotic scenarios. So anything with an appreciable atmosphere that is NOT a gas giant should be called a planet ...REGARDLESS of what it orbits. Some other nomenclature can address what it orbits ("a moon of the Sun" or "a planet of Saturn"). And the nomenclature doesn't have to be some German compound word ("thisisamoonnotaplanetbecauseitistooclosetothesuntohaveanatmosphere"). What I mean is that it's just a matter of accepting that you can not capture the object's complete characteristics in one single word, like we can do now with so few objects in our portfolio.
The whole issue is finding and agreeing upon a convenient yardstick for calling a planet a planet. We should detach the pretext for calling something a planet from the classical understanding of things, and we should get a greater pretext of taxonomical rigor on the question. Of course my conception is radical... but only from the classical point of view. But think about it: look at Saturn's or Jupiter's menagerie of objects. In complexity and quantity and variety they rival the Solar System itself. Those ecosystems are quite big, and should get that recognition they deserve. So let's give these large moons with atmospheres there the status of planet. They are moons just because of what they orbit? Nah. I say promote Titan to planet, and demote Mercury to moon.
As we find weirder systems out there: moons orbiting planets orbiting moons orbiting gas giants orbiting gas giants orbiting stars, stars with ring systems, true binary planets of equal size with a shared atmosphere, etc., we are going to need a nomenclature system that is more scientifically grounded, and certainly grounded more than the IAU's recent foray into political appeasement over scientific rigidity.
You can have asteroids of asteroids (Ida and Dactyl) and moons of moons (Pluto and Charon's Nix and Hydra). We can have a binary moon of the sun (Antiope the "asteroid", Pluto and Charon). I firmly believe this nomenclature would be more useful as we will come to see much more planetary systems out there, some quite exotic, with secondary planetary systems orbitting gas giants (which you can say Saturn and Jupiter already have) and maybe even TERTIARY systems (moon of a moon of a moon?). I expect to find planets orbiting nonspherical large asteroids. All sorts of lunacy (no pun intended).
But I am sorry: Pluto is just NOT a planet like the Earth. And Titan is just NOT a moon like some forlorn rock. So we need a new nomenclature for classifying planets, and mine simply stated is:
- composition first (spherical, solid, atmosphere: planet),
- orbital focus second (planet of Saturn),
- size third, or not at all (major planet of Saturn... Titan).