I figure having been on a bit of an offensive over the definition of planet in a few places, I owe those I've been debating with a chance to shoot back at my position. So to the best of my ability, I'm going to spell it out here.

The foundation of my position on what makes a planet, and my opposition to the current trend in trying to define one, is that a body is a body, no matter where it is. A star is a star, whether it forms inside a galaxy, between a galaxy, orbits a galaxy, or is shot from a galaxy. No matter where it is, a star is a star is a star, the physical properties of the body define its label.

With brown dwarves we're pretty much in the same situation. Whether the brown dwarf is a body within a star system, or an independent body, if its a non-fusing deuterium burner, its a brown dwarf. Size doesn't matter, where and how it was born doesn't matter.

Defining what is a planet, for reasons I see poorly bound in centuries of tradition, drags in this belief that where a body is formed and currently located is as critical to the definition as what it actually is physically. It completely contradicts almost every other definition on the books.

From my perspective, the definition of a planet should be the result of non-arbitrary, defined attributes that are unique to planets. Where they are, how they formed, what caused their formation, and where they happen to be at the time of discovery is irrelevent. With rare exception, and object is what it is, or its something else. (I acknowledge the fact that comets are in fact KBOs/Oort objects that have changed status without substantial change in physical make up. Exceptions do exist, but they should be strenuously avoided where possible, and corrected when feasible).

Ok, enough rambling, here's my list.

1) Spheroid shape.
2) Differentiated internal structure. A defined and distinct core, and a crust.
3) Must either be an independent body or of at least equal status to a co-orbital body. Meaning, its either on its own, or its not the satellite of another major body. By establishing mass equilibrium as a clearly defined, non-arbitrary break point, you can look at a binary set of solid objects like Earth-Moon and Pluto-Charon and clearly say "Earth is the planet" and "Pluto is the planet". The actual sizes of the bodies isn't relevent, its how they relate to their companions in their particular circumstances.

Now #3 to me is actually negotiable. I don't see any particular crime in saying we have some satellite planets in the solar system. The Moon, Io, Europa, Ganymeade, Callisto, Titan, Triton and probably some of the larger moons of Uranus, are potentially deserving of planet status because of their very clearly planet like composition. Keep in mind, as I see it, location and orbit aren't relevent to the definition because it creates unnecessary overlap. A body is, or it is not.

Some time ago on Space.com, I read about some objects that were observed in some manner in the Orion nebula, that were pretty clearly NOT brown dwarves, but were independently formed bodies of a physically planet-like nature (the size of Saturn approximately, as I recall). The controversy at the time was, are they a new type of planet born free of a stellar disk, or are they brown dwarves by right of being formed independently. To me, this was a silly arguement. If they weren't burning deuterium, calling them brown dwarves muddies the waters that definition needlessly. If they are substellar bodies that aren't burning deuterium, they aren't brown dwarves, its that simple.

Recently, I was introduced to the arguement against planetary status for Pluto known as "Multiplicity". This, quite honestly, is a joke. That's like saying that the 40 plus billion objects in the Milky Way aren't stars because there's just too many of them in the same place so using the term "star" for all of them is too indistinct to be meaningful. I didn't know whether to laugh or cry. With this argument, some eight planet defenders are straying into the same realm of bogus logic as geocentrists, making up rules more in line with protecting the traditional status quo, than truly and meaningfully creating a definition that can be equally applied to all situations where objects of a given type are found.

As I see the situation, we're now face to face with one of the last spectres of primitive planetary astronomy. The fight to defeat these spectres of ignorance started centuries ago with the acceptance of Earth as one of nine originally known planets, not something unique and special in the universe.

Then with the discovery of pulsar planets in 1992, and the discovery of 51 Pegasus B in 1995, we've learned definitively that the nine planets in this system were no longer the only ones out there. The planets we've discovered elsewhere have also shaken the very foundation of what we believed a "normal" planetary system would be in form and structure (from post supernova pulsar planets to "normal" main sequence stars with planets with migrating gas giants practically hugging the central star to red giants with protoplanetary disks to multistar systems with stably orbitting planets).

Now back here at home, we're seeing the existance of candidate planets with orbits that are not the comfortably near circular, ecliptically aligned type we've known since antiquity. Now that new discoveries are forcing us to put some meat on the skeletal definition that for so long was "understood" faces us with the possibility that our handhold on tradition may have been a mistake. Its interesting to me that some astronomers seem to be so fascinated with the wildly variable nature of other star systems, yet to challenge the regularity of this system is somehow offensive. Where is it written that the star system orbitting Sol MUST be this rigidly defined, clockwork mechanism into which the elevation of bodies to planetary status that defy that mechanistic tranquilty are forbidden? Why must we write special rules for defining the term planet that absolutley respect the "traditional" structure of this star system as a baseline? What's the crime in shaking up the family a little? Because its definitely a crime NOT to shake it up through the use of artificial rules a geocentrist could take a shine to. Its the same mindset, the difference being that instead of just the Earth being special, its the Earth and these seven other massive bodies that exist here in the Solar system with it that are special, and its just as wrong.


There she is, folks, the official standing of Doodler on what makes a planet. The bull's eye's drawn for you, have at it.

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I'm not completely heartless, the doctor who removed it told me he'd never be able to get it all.

While listening to the Stardust return press conference, they mentioned that Pluto exhibits planetary processes within itself (not just cold on the outside). It might go along with your #2.
What processes?

(I don't want to start a debate, I'm just wondering what they are talking about).

The transfer of heat through the atmosphere in the reverse greenhouse effect that made it colder than it should be, given its distance from the Sun was the only new one I've heard lately.

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In my opinion #3 is all that you need to define a planet. I´d prefer establishing a mass threshold (the only arbitrary parameter) for defining a double planet or a planet-satellite system

If #2 was a requirement, the classification of any new object would be pending until direct assesment of its internal structure was carried out. We couldn´t tell a differentiated internal structure through a telecope or remote sensing.

And if #3 is adopted as the only rule, it follows that an spheroid shape is not necessary.

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I don't see how #3 even came up. It flat out contradicts your position (as you stated it). Must be Tradition's corrupting influence ;-)

Good post. I've been thinking about this as well, and my general idea is similar to yours.

However, I'd scatch #2 for the reason Argos mentioned. Also, you can't discount the possibility of a large but undifferentiated object: Callisto, for instance, is just barely.

I'd definately set the lower limit of a planet to where it's large enough to become spheroid. Now, there are a few overlaps here: Vesta, Pallas, and Proteus are all larger than Mimas, yet they are irregularly-shaped, whereas Mimas is spheroid. Thus, Mimas would count as a planet, but the other larger ones wouldn't. But even much larger objects such as Iapetus and 2003 EL61 aren't quite spheroids either. The latter could fit as a highly oblate spheroid, but I have to admit I'm at lost about Iapetus. For now, let's discard that one and worry later.

So, if it's a spheroid, it's a planet. If it's not, it's an asteroid, comet, KBO, debris, etc. The upper boundary is whether it burns deuterium or not. I don't care about orbit, composition, mass, any of that.

What IS an important question, IMO, is whether to allow moons of planets as planets themselves. Are we going to call Ceres a planet but not Ganymede just because of what kind of body they orbit? This is a tough question for me. It might be confusing to hear in the news that "Neptune has another planet." Or would it? I think calling spheroid moons as planets would help seperate them from the other heaps of rubble also called moons (which I think should be called moonlets or moonoids instead). Perhaps it could go like this:

All spherical moons are planets, but not all planets are spherical moons.

Just like how all squares are rectanges, but not all rectangles are squares. Might sound confusing at first glance, but in a brief summary of, say, Titan, it would be introduced: "Titan, a satellite of Saturn, is a very interesting planet to say the least." I don't see why this couldn't work. Afterall, we already refer to the moon as one of the terrestrial planets. So, I accept spheroids orbiting planets as planets too...moon or satellite would just describe an important orbital aspect of the planet.

A non-fusing spheroid not orbiting a star can be called a rogue planet. I don't see what the debate is about that type of object not being a planet; this is going back to the arguement that where the object is or what it orbits should not matter.

If we call moons planets, this obviously makes the debate of whether a pair is a double planet or a planet-moon system a mooter point. Since my definitions allow the term moon to still be used to specify where the planet orbits, it still needs to be defined. Maybe depending on where the central point of gravity is (if it's on the surface of a body, that body is the parent one...if it's in between both objects, they're double planets). I haven't read much about this area so this may be stupid. But I'm sure something can be reached.

Finally, adjectives to group planets based on location and structure is allowed, but not a necessity. Mercury, Venus, Earth, Luna, Mars are terrestrial planets...Jupiter, Saturn, Uranus, Neptune are gas planets...Triton, Pluto, Quaoar, Orcus, etc are Kuiper Belt planets. Sedna could fit either there or as an Oort cloud planet. As for the planets orbiting the gas planets, most of them seem intermediate between terrestrial and Kuiper, so how about Intermediate planets? Ceres, a spheroid having an ice mantle like the Galileans, would also be an Intermediate planet.

Let me repeat that tagging adjectives is NOT a necessity under my classification. Sort of like how bears can be grouped into black bears, grizzly bears, polar bears...but you don't HAVE to specify what kind of bear when you scream there's one in your garage. It's more common to just say BEAR rather than what type. Some planets may be debateable or hard to put in a group...in this case, debate is more acceptable since it won't make the difference between planet or no planet.

To sum up:

-A planet is simply a gravationally relaxed spheroid and a non-fusor.
-A planet in orbit around another planet may also be called a moon or satellite.
-Planets can be subdivided by location and/or structure (terrestrial, intermediate, kuiper belt, gas).

-A non-spheroid object is called a planetoid.
-A non-spheroid object in orbit around another planet may also be called a moonlet or moonoid.
-Planetoids can be subdivided by location and/or structure (asteroid, centaurs, kuiperoids, comets).

So the planets in our solar system would include the 9 traditional ones, plus all the spheroid moons, the spheroid KBOs, and Ceres. *Edit*Since I'm trying to stay technical with no ifs/ands/buts...Iapetus and maybe EL61 are planetoids under this scheme because they're irregular. I'm not going to try to make up an excuse as to why they should be exceptions just because they're too big. But they may end up being the tomatoes of the planetary business and be regarded as planets anyways.

For those who are going, holy crap no way...I'll close my post with something that an older (but still smart) person told me. She said she remembers when there were two dozen or so elements, and how they memorized them in school. I guess if something similar to this or what the topic poster suggested takes hold, there won't be any fun memorizing anymore, but that's just like what happened with the elements. This happens in science all the time, and it's time to expand planet classification to account for our new understanding. There's no way I can look at our solar system and say there's 9 planets anymore...the line has to be drawn somewhere less arbitrary. Spheroid is the least arbitrary so it's the best IMO. And it's probably time we start accepting moons as planets too. I never learned about Europa, Io, Titan as a kid (I was a kid in the 90s, so yes they were important), just because they happened to orbit a planet and not the sun. While my suggestion results in a ton of planets...the definition itself is extremely simple. It'd also be easy to maintain as we discover new objects around the fringes of our solar system, and allow the news to say "New planet" rather than the stupid "New planet-like object".

But hey, it's certainly open for debate.

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I think the physics-minded people pretty much agree, planets should be defined by their internal physics. I and others have argued that on other threads, too. So that means #1 and #2, but drop #3 (who said two planets can't orbit each other, two stars can). That gives us a lot of planets, so what (we can always reserve the term Major Planets for our faves). Note that just because we don't always know the internal physics is not a reason to avoid that approach, as this is the central issue in all of astronomy and never stops us in other situations. Also note that the internal physics is by far dominated by mass and composition, just like stars, so in effect this approach is a lot like a mass limit but is not arbitrarily chosen, like the absurd "greater than Pluto mass" approach.

Internal physics is a good idea; however, I still feel our knowledge of object interiors is too feeble to base classification upon it. We don't really know what's inside the known big KBOs, anything said is pure speculation really. Also, what do you mean by internal physics: grouping objects into different categories of planets, or just calling all objects with some kind of internal differentiation planets? If you do the latter, what would you call a large object like Callisto?

Another problem is that internal physics are extremely hard to analyze or even speculate for extrasolar planets. I really think any definition that is developed should be universal and able to be used for other stellar systems.

This is why I prefer size; it's much easier to gauge for bodies both in our solar system and outside. Neither the lower or higher limits are arbitrary. It's easy, and can be applied everywhere.

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Let me give an example. A star is defined by its ability to undergo nuclear fusion, not its mass. However, in practice, the distinction is not crucial, because we know what is happening in there well enough to have a good idea of what mass is needed to get fusion. We might be off a little, so we live with it. But ultimately, our effort is not to define stars by their minimum mass, but rather by their internal processes. It should be no different with planets, or planetary nebulae, or supernova remnants, plerions, pulsars, black holes.. need I continue?

I do see your points. But how would you seperate planets from non-planets by internal structure -- I assume differentiated vs homogeneous? If that's what you mean, that may not be too far off from the size scheme, since most spheroids do end up differentiating in one way or another (yes, I know there are some exceptions). Alternatively, you could group them into different classes based on interior...but again, this wouldn't be far from groups based on location.

When it comes down to the wire, it's primarily opinion. Your idea works well, though I still think size is better simply because it's easier to gauge and allows more flexibility with less confusion.

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I'm all for using internal processes. I'm particularly fond of the process of gravity caused by the internal mass.

edit:
Maybe we should have a sticky debate thread. The OP could be continuously re-edited to keep current with all the specific paradigms posted in the thread. Maybe it could have a poll and/or a consensus post that is kept current. This way all the ideas are kept in one place at the forefront, instead of having dozens of separate threads.

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I don't see why there needs to be a distinction. A man is a man, but from the point of view of his children, he is a father. A star is a star, but from the point of view of its planets, it is a sun. A planet is a planet, but from the point of view of its orbital planet, it is a moon. In short, a moon is a planet that orbits another planet.

Internal physics is a good idea; but it focuses mainly on the terrestrial worlds (and I think icy and rocky planets are both broadly terrestrial; Titan is more Earth-like than Venus is, but it is made of ice) rather than the gas giants. How it works in the substellar level is still unclear to me, as is the dividing line between star and planet.

I think the term "brown dwarf" has done a lot of damage to stellar classification. Brown dwarfs are not stars too small to fuse hydrogen; stars are brown dwarfs large enough to fuse hydrogen. The vast majority of "stellar objects" in the galaxy are brown dwarfs; they are therefore the standard, not the exception. Given this, it seems silly to make a distinction between stars and brown dwarfs. The one is simply a larger example of the other. Since we are almost certain within a few years to find a brown dwarf approaching jovian mass far away from any star, the term "jovian planet" may become obsolete, replaced by "orbital low-mass star".

That still leaves Uranus and Neptune, of course, but as we learn more about extrasolar planet physics, I think we may come to see them as super-terrestrial planets, rather than brown dwarfs.

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Yup, which is why I left #3 as negotiable. The break point between a planet and asteroid is pretty clear in my mind. The break point between planet and satellite is clearly relational.

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From a purely astronomical point of view, only the gravitational signature would be enough for defining a planet, imo.

Every body that is not a star would be a planet or a satellite (case the body orbits another body that orbits a star). That would include comets (active planetoids, in my definition) and asteroids (planetoids). There would be millions of planets, but so what? Astronomy is about big numbers.

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Which is kinda why I think the multiplicity arguement is funny. How can there be too many of something?

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I'm not completely heartless, the doctor who removed it told me he'd never be able to get it all.