How Many Planets, Exactly?

I’m dismayed by today’s vote by the IAU (International Astronomical Union) on their definition of a planet. Apparently, I’m not the only one… the 9,500 or so of the 10,000-ish total of astronomers who didn’t get to vote on this are a tiny bit upset as well. What’s upsetting me this time? Is it that Pluto got demoted? Honestly, maybe a little (I’m more disappointed that “Xena” never got a chance and that Ceres got it hopes raised and dashed yet again). What I’m more upset about is the definition itself and the politics that went into it (but stick around…I’ll give you my definition in a bit).

Here is the new definition of what a planet is:

(1) A classical planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit. [The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.]

(2) A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite. [An IAU process will be established to assign borderline objects into either dwarf planet and other categories.]

(3) All other objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”. [These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.]

I basically read this as a classical human attempt to elevate us. My team is better than your team. My country is better than your country. My God is better than your god. My planet is better than your planet. So much of the argument has revolved around whether or not Pluto “should be” called a planet that I think some basic scientific objectivity was lost. Mike Brown, discoverer of “Xena” (officially 2003 UB313 [officially now called Eris]) commented how an explosion of planets would diminish their romance. Are you kidding me? Does having hundreds of moons and satellites in our own solar system diminish their romance?

Focusing on the resolution, I think the most troubling bit concerns the orbit-clearing requirement. Uh…vague enough? We’ve gone from “Is it big enough to be a planet?” to “Is the orbit clear enough for it to be a planet?” Not only has nothing been cleared up, but the water itself has been muddied. How much of the orbital neighborhood must be cleared, and of what must it be cleared of? Lots of rocks, big and small, still populate Earth’s orbit. Jupiter has countless rocks in its orbit. And who’s to say which of Neptune or Pluto is the intruder in the other’s orbit? Seriously, by the definition given it’s not outside of the realm of possibility for Earth, Mars, Jupiter, Neptune (and maybe the rest) to lose their claim for the title of planet. Move the Earth out into Pluto’s orbit and it suddenly becomes a “dwarf planet”? If you move a lone bird on a wire into a flock, does it suddenly become a “dwarf bird” just because there are other birds around it?

Then there’s the “dwarf planet” thing. In our “politically-correct” culture, shouldn’t the phrase be “little-planet”? Or “clear-neighborhood-challenged planet”? Although I prefer to simply just have definitions for planets and SSSBs, if we do classify these almost-planets, shouldn’t we be a little creative? Call them Lilliputs.

Then we also have the problem with this vote that it was so focused on our small little speck of the universe that there didn’t seem to be much talk about all of the myriad possibilities that likely exist that we don’t think about. What about two or three Earths sharing exactly the same orbit? What about systems strewn with asteroids, but with enough large planet-like bodies to shepherd them like a system-large equivalent of Saturn’s rings? What about systems whose “planets” orbit off the ecliptic, and who often cross orbits in a non-collision sort of way? There are so many possibilities that simply haven’t been considered in the rush by some to make our large wayward ice-ball into something less than a planet.

As I’m always big on the proposing a solution instead of only complaining about it, here’s may proposal for a definition of a planet:

(0) For the purpose of clarity, these definitions refer only to bodies in an established stellar system which has sufficient structure and temperature to be stable. This is not an unambiguous definition, so the example of the collision of a Mars-sized object with Earth would be an instance of a transition gray-area from the cataclysmic stage of formation to the stable stage of formation.

(1) A traditional planet is a celestial body that (a) is in orbit around its star or stars, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) is not so large that it can naturally initiate or sustain nuclear fusion, and (d) has cleared the neighborhood around its regular and/or local orbit of any non-stellar bodies capable of destroying it through impact or gravitational forces.

(2) A satellite planet is a celestial body that (a) orbits around another traditional planet where its barycenter rests within the physical mass of the traditional planet, or shares an orbit with another body that could be called a traditional planet except that they orbit around a barycenter outside of the physical mass of either body; (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; (c) is not so large that it can naturally initiate or sustain nuclear fusion, and (d) has cleared the neighborhood around its regular and/or local orbit of any non-stellar bodies capable of destroying it through impact or gravitational forces excepting its parent traditional planet or of its satellite planet sibling(s).

(3) A satellite (or moon) is a celestial body which does not meet the requirements of a satellite planet and which (a) orbits around a larger object with a barycenter located within the larger object, or (b) is orbitally bound to the barycenter of a local multi-body system of a traditional planet, with or without one or more satellite planets, or of a satellite planet system that lacks a traditional planet.

(4) A lilliput is a celestial body which does not achieve the requirements for a traditional planet or a satellite planet but which (a) is in orbit around its star or stars; (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) is not so large that it can naturally initiate or sustain nuclear fusion.

(5) All other objects orbiting within the star or stellar system shall be referred to collectively as “Small System Bodies”.

OK. It’s a little wordy, but I think it’s clears up a lot of potential exceptions. There’s no reason why it can’t be simplified for the public, but I think the astronomical community needs to have a definition that covers reasonable possibilities.

So, what would my definition result in? Well, I don’t have tons of data around me, but this is my guess (TP=traditional planet; SP=satellite planet; SM=stellite/moon; L=lilliput; SSB=small system body):

Mercury (TP)
Venus (TP)
Earth (TP), Moon/Luna (SP)
Mars (TP), Deimos/Phobos (SM)
Ceres (TP)
Vesta (L/SSB)
Pallas(L/SSB)
Jupiter (TP), Io/Europa/Ganymede/Callisto/etc (SP) other non-round Jovian (SM)
Saturn (TP), Titan/Rhea/Iapetus/Dione/etc (SP) other non-round Saturnian (SM)
Uranus (TP), Oberon/Titania/Umbriel/Ariel/etc (SP) other non-round Uranian (SM)
Neptune (TP), Triton (SP) other Neptunian (SM)
Pluto (TP/SP), Charon (SP), Nix/Hydra/P4 (SM?)
~~2003 EL61~~ Haumea (TP), Hi’iaka/Namaka (SM)
Makemake (TP)
~~“Xena”~~ Eris (TP/L), Dysnomia (SM)
Sedna (TP/L)
Quaor (TP/L)

With this, Pluto (maybe…due to the barycenter issue with Charon) and Ceres make planet status. The large round moons get promoted to satellite planets. The definition of ~~“Xena”~~ Eris, Sedna, and the other outer bodies are dependent on how much in danger they are from other bodies in their locale (doesn’t have to be clear, exactly, but definitely safe for them to be a planet…which, given the age of the solar system, can be inferred).