I dont want to ride on the coattails of Macro Mouses excellent topic, but I was literally woken up from sleep just now with an idea for a planet definition that could perhaps take into account potential future discoveries as well as the ambiguities within our solar system.

Here it is:

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A *planetoid* is an object not of stellar metallicity that displays neither ongoing geological activity nor a permanent self-generated atmosphere.

A *planet* is an object not of stellar metallicity that displays ongoing geological activity, a permanent self-generated atmosphere, or both.

A *brown dwarf* is an object of stellar metallicity that does not produce energy through the nuclear fusion of hydrogen.
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*of stellar metallicity* implies a make-up of mostly hydrogen and helium, and, depending on the level of rock at their cores, any or all of the gas giants could be promoted to *brown dwarf* category under this definition.

the necessity for ongoing geological activity or a permanent self-generated atmosphere would exclude all asteroids and KBOs from the list, and also Pluto, whose atmosphere is not permanent, most satellites, including our Moon, and perhaps Mercury, depending on the results of the MESSANGER probe.

The removal of the necessity that a planet must orbit a star opens the way for large satellites of gas giants (or brown dwarfs) to be included as planets, allows for any *interstellar planets* that may be discovered, and also removes any potential ambiguity should we ever discover a terrestrial planet-sized object in orbit round a large gas giant planet.

In our own solar system, this definition would mean that as few as three of our current planets (Venus, Earth and Mars) would retain their planethood, but a large number of other objects, such as the Gallilean satellites, Titan, Triton and possibly a number of Saturns smaller, geologically active moons, would be admitted into the category. In other solar systems, this definition would include the *roasters* observed around other stars, since they are composed mainly of rock and metal.

What do you think?

Bit too complicated

How about:
"A planet is a object in orbit around a star that is, or would be, visible by human naked eye from any other planet around that star."


That excludes Pluto etc., includes all the 'classic' Sun planets and all Earth or other planet satellites.

Job Done.

John

But Neptune isnt visible with the naked eye from Earth, and none of the terrestrial planets would be visible with the naked eye from Neptune.

I wonder if regarding the gas giants as brown dwarfs is sensible. There must be good reasons why they were given a special name in the first place...

You need to define what is 'geological activity' and...

Since Pluto's atmosphere evaporates and solidifies periodically, shouldn't that process generate some geological transformations?

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In my definition, *gas giant* could still apply to any of the gas giant planets if the current theory that Jupiter and Saturn have rocky cores is correct. If they dont, then there is little to separate them from brown dwarfs and for all intents and purposes, that is what they would be. Uranus and Neptune could still be considered *gas giants*, since they are not entirely of stellar metallicity, though they do approach it. I suppose it depends how loosely one defines *of stellar metallicity*

Geological activity, in my definition, would be activity beneath the rocky surface of a planet- ie, eruptions, plate techtonics, and internal heating. Mars, though it no longer displays geological activity, would still qualify as a planet since it has a permanent self-generated atmosphere, and a number of geologically active moons without permanent atmospheres, such as Ganymede, would also qualify.

Pluto could qualify if, like Triton, it is shown to desplay geological activity under my definition. Geological transformations via the atmosphere, like those created by impacts, are not generated from the interior of the planet and therefore wouldnt qualify.

But count the words in mine, and in yours.

A planet is a planet is a........... That's an infinite definition.
John

Sorry, but where do I say that? I have three definitions up there, one for planet, one for planetoid, and one for brown dwarf. Since any definition of planet demands that it be made distinct from planetoids and brown dwarfs, it was impossible to define planet without placing it into that context.

Any definition of planet that even has the possibility of excluding Mercury is probably doomed.
He said, visible "from any other planet"

But then I'm surprised that he hasn't noticed that recursiveness of his own definition. Or, was he talking about his own--was that changed in the OP?

Bigger than Pluto--its a planet--smaller--its a planetoid.

No escape from some arbitary system. They will continue to orbit no matter what we call them.

From what I've read in wikipedia, planetoid is a synonym of minor planet, and Pluto is a minor planet, thus a planetoid

It will likely be grandfathered in--and its combined mass with Charon is still greater than that of this body (perhaps) so it is a good cut off point. It has an atmosphere, a moon, etc. So it deserves to be called a planet.

Otherwise it's a slap in the face of Tombaugh IMHO.

Pluto has a resonance of 3:2 with Neptune. This characteristic is shared by a legion of bodies called Plutinos. No other 2 planets of the solar system have resonance between them, so that is a indication that Pluto is a Plutino, not a planet

I think it might be a good reward to largest of these objects with the name "planet" as long as it is as large as pluto or larger.

The 8 planets just doesn't have that ring to it.

It really gets me that people is reluctant to deny the title of planet to Pluto only because historical reasons. History is there to be modelled and changed, if we always follow traditions then there will not be advance...
We would still living in caverns...

"Caverns"? Now that we've discovered Hole-In-Mountain Object 24B6, I say they should be downgraded and reclassified as "caves".

<ducking and running>

I just don't see a reason not to have dozens of planets. Since there's no official formal definition, I personally have adopted the "round" definition. We have no problem calling the behemoths in other solar systems planets when they're in highly elliptical orbits, or in orbital resonances, so I don't havea issues doing it here, either.

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You can't take the sky from me.

What about Earth and Venus? I think they have that next two fibonacci numbers thing going.

Not in our Solar system...

But several of the newly found planetary systems have resonances of some sort. And some of the planets in our Solar system have probably been in resonances (at least if their orbital distances have changed since their formation). So resonances are bad criterion for planethood.

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Science is a way of trying not to fool yourself. The first principle is that you must not fool yourself, and you are the easiest person to fool.
-- Richard Feynman

If you're talking about Bode's law, that is not an indicative of resonance, only a serindipitous coincidence

Why not in our Solar sytem?. I read from this page...
http://www.answers.com/topic/plutino

plutino
In astronomy, a plutino is a Pluto-like object, insofar as it has the same relative orbit as Pluto. These orbits are stabilized by an orbital resonance with Neptune, similar to Pluto's 3:2 orbital resonance. This means that plutinos complete 2 orbits around the sun in the time it takes Neptune to complete 3 orbits. Plutinos form the inner part of the Kuiper belt. About 1/4 of known Kuiper Belt are Plutinos