What do you think Constitutes a Planet?

[Perfection]

A sphere formation, of rock or gas, that orbits around a star, that's at least 2000km in diamiter,

Why 2Mm? What makes that number so special. Seems like an arbitrary characteristic to me...

that's not a moon or part of an astoriod belt. Pluto is simply a big astoriod ya know, it's in an astoriod belt, it's not a planet. If it is then there's humdreds of planets in our solar system.

Pluto is not an asteroid, it's a KBO. KBOs are not considered asteroids. (There should be a better term for these icy bodies).

OK, fine Perfection, maybe your gravitational limit is better than mine. Happy?

No, because "maybe" still indicates that you have doubts about the superiority of my system. I'd like to adress them so that I can either convince you or rework my system.

 

[Bozo Erectus]

Bozo, Roundness in and of itself is not that good a metric, I say so for the following reasons:
1. Making roundess the sole measure would more than double the current amount of planets and reverse the ruling that Ceres is not a planet.
2. What makes a body round is not completly gravity centered. A larger much heavier rock can be irregular when a smaller chunck of ice exhibits very smooth gravitional rounding.

Ok I see your points. Seems to me that the debate has more to do with semantics than science. So heres my solution: for the sake of convenience, lets stay with the traditional 9 planets. We can call them Class 1 planets. When we say 'nine planets', we mean Class 1. Any additional bodies we find that approach any of them in size, we refer to as Class 2 planets.

 

[Perfection]

Ok I see your points. Seems to me that the debate has more to do with semantics than science.

Yeah, it is largely a semantics debate, however we need to discuss science because we want the word to have scientific meaning.

So heres my solution: for the sake of convenience, lets stay with the traditional 9 planets. We can call them Class 1 planets. When we say 'nine planets', we mean Class 1. Any additional bodies we find that approach any of them in size, we refer to as Class 2 planets.

Then what do you do with the extrasolars?
Or what about a hypothetical large mass solar system object?
I don't think order of discovery makes a good model of
"planetness".

I think my definition produces a more physically-based system that fits better with our intuitive ideas of what a planet should be.

 

[Cheezy the Wiz]

Ok I see your points. Seems to me that the debate has more to do with semantics than science. So heres my solution: for the sake of convenience, lets stay with the traditional 9 planets. We can call them Class 1 planets. When we say 'nine planets', we mean Class 1. Any additional bodies we find that approach any of them in size, we refer to as Class 2 planets.

This was the kind of thing I was looking for, a system of greater and lesser planets, so we aren't stuck with that confusing "planets and funny rocks" lack of a system we have now.

Now we need to come up with where to draw the line at: should we take the nine we have now, and call them the Class 1s, and then just everything else in the Solar System we find call it a Class 2? (The idea you proposed), or do we draw the line somewhere else? Also, where will we draw the line between Class 2 planets and asteroids/KBOs, or do you think a Class 3 will be required?
We've still got work to do, but this progress is good.

EDIT: I also think (for semantics' sake) we ought to come up with some more romantic names for our new classes of planets, you know, something better than just numbers.

EDIT II: Crosspost with Perfection..again

 

[Kan' Sharuminar]

Have no scientists just sat down and officially discussed what makes a planet? Seems like a pretty big deal to me.

Maybe we should invite them to CFC and join in the debate

 

[Perfection]

EDIT: I also think (for semantics' sake) we ought to come up with some more romantic names for our new classes of planets, you know, something better than just numbers.

Well, I dislike the whole notion of different classes of planets, that are traditionally bodies lacking in planetary qualities. Calling stuff planetoids and planetesimals would be preferable to words like "Class 2 planets" and "Minor Planets" IMO.

Have no scientists just sat down and officially discussed what makes a planet? Seems like a pretty big deal to me.

Maybe we should invite them to CFC and join in the debate

Currently the IAU is debating this, they're the official authority on the matter, they claim that they'll have thier act together come September.

 

[Kan' Sharuminar]

Currently the IAU is debating this, they're the official authority on the matter, they claim that they'll have thier act together come September.

*rushes off to do a quick research of what IAU is*

Ah...good to see. Does that mean we could be seeing a downgrading of Pluto this year then? I remember reading a while back that some group had debated what it should be classed as recently, and decided it was a planet.

 

[Perfection]

*rushes off to do a quick research of what IAU is*

Ah...good to see. Does that mean we could be seeing a downgrading of Pluto this year then?

We very well could

I remember reading a while back that some group had debated what it should be classed as recently, and decided it was a planet.

Well back in 1999 the IAU ruled that Pluto was indeed a planet. However, with the discovery of several large KBOs, including one larger than Pluto there's a reasonable chance that it will reverse its previous decision.

 

[Bozo Erectus]

Perf and Cheezy, I think 'Class 2' could be reserved for KBOs beyond the orbits of Neptune and Pluto, that would otherwise have been classed planets if theyd been closer in. So you could draw a boundary around the solar system at the outermost orbits of Neptune/Pluto and say that all planet like bodies outside of it are Class 2's

Extra Solar planets: planets around other stars could be categorized in the same way. Bodies that are closer in to their stars are Class 1's, and those that are farther out are Class 2s.

Oort Cloud objects: Class 3's (Im sure they arent all comets)

So distance from the star should be the determining factor.

 

[warpus]

So distance from the star should be the determining factor.

But how would you determine where the line between class 2 and class 3 would be? Or class 1 and class 2?

Seems pretty arbitrary to me.

 

[Bozo Erectus]

But how would you determine where the line between class 2 and class 3 would be? Or class 1 and class 2?

Seems pretty arbitrary to me.

To me its obvious. We can tell the difference between Kuiper Belt objects and Oort cloud objects, right? Simple: KBOs = Class 2, Oort objects = Class 3.

edit: as I said, we draw the Class 1 boundary at the outermost orbit of the nine traditional planets.

 

[Cheezy the Wiz]

But how would you determine where the line between class 2 and class 3 would be? Or class 1 and class 2?

Seems pretty arbitrary to me.

Not as random as you think: Neptune is without question the largest thing that far out in the Solar System. The Ort Cloud is a defined field, and is completely separate from the Kuyper belt. I think these could be good definers as Class 2 and 3, as objects in the Ord Cloud are going to by nature be smaller, as they are farther out than a Class 1 or 2, and something larger would probably have been lost by the Sun's gravity out there.
However, as I understand it, most objects in the Ort Cloud don't follow the regular orbits like closer objects do, they tend to be much longer ellipses, ducking in much closer to the Sun, and then slingshotting out to the Ort Cloud again.

Also, isn't one of the definitions of a planet than it crosses no other planet's orbit? If it is, we might have to discontinue Pluto as a planet. I don't know where I'm getting this from, I just recalled it for some reason.

 

[EdwardTking]

We very well could Well back in 1999 the IAU ruled that Pluto was indeed a planet..

Quite.

However, with the discovery of several large KBOs, including one larger than Pluto there's a reasonable chance that it will reverse its previous decision.

I can not see why the subsequent discovery of anything else should
(i) change Pluto or (ii) the definition of what is a planet and therefore
reopen the judgement as to Pluto being a planet.

There may be an issue as to what is the distinction between a large comet
and a small planet, but Pluto is clearly nearer a planet than a comet.

The definition of a planet to me would be a:

(a) an object that is and has been primarily illuminated by external visible spectral light rather than its own incandescence

(b) having a spheroidal shape due to its own gravity


Criteria (a) excludes most stars, black holes, galaxies, gas clouds

Criteria (b) eliminates small comets, small asteroids

This definition would include many moons (orbiting other planets)
and also any free roaming interstellar planets. Now the mathematicians
could propose a qualifying test for spheroidity, the alternative being an arbitrary limit on mass. The biggest problem in the definition is at the small brown dwarf/giant planet boundary where it might be unclear whether the majority of its light was emitted by is own heating up or by reflection, for which an arbitrary upper mass limit might be a more useful distinction.

 

[Cheezy the Wiz]

There may be an issue as to what is the distinction between a large comet
and a small planet, but Pluto is clearly nearer a planet than a comet.

The definition of a planet to me would be a:

(a) an object that is and has been primarily illuminated by external visible spectral light rather than its own incandescence

(b) having a spheroidal shape due to its own gravity


Criteria (a) excludes most stars, black holes, galaxies, gas clouds

Criteria (b) eliminates small comets, small asteroids
.

Well, Comets (see my earlier post) follow regular, but very odd orbits. They come in very close to the Sun, and then slingshot out all the way to the Ort Clould, before coming back in for another pass. The distance from the Sun determines the length of the Comet's tail (heat and Solar wind), that's why they are plenty pretty when they get to where we can eyeball them. So comets exclude themselves as planets.

 

[Perfection]

You really need a better system cutoff then approacing Pluto's size because there's a continium of masses from pebble to Pluto size. Since there is no sharp natural deliniation you'd have to come up with some arbitrary one, and in my view arbitrary is best avoided. That's why I make planethood based on how it effects bodies in its orbital locus. It ensures that we don't have to make such deliniations in belt disk and cloud situations.

Perf and Cheezy, I think 'Class 2' could be reserved for KBOs beyond the orbits of Neptune and Pluto, that would otherwise have been classed planets if theyd been closer in. So you could draw a boundary around the solar system at the outermost orbits of Neptune/Pluto and say that all planet like bodies outside of it are Class 2's

What about bodies of around Pluto mass in about the same orbit as Pluto such as Orcus? Are they considered Class 1s?

I really can't see why you make this the differentiation between "class 1" and "class 2". Pluto and Orcus share a wole lot more of similarities between what you've described as "class 2" then any of the 8 innermost planets.

Extra Solar planets: planets around other stars could be categorized in the same way. Bodies that are closer in to their stars are Class 1's, and those that are farther out are Class 2s.

Well, where do you make the cutoff? The clarification you add in post 71 can't apply to extrasolars because they may not have the same configureation as our solar system.

So distance from the star should be the determining factor.

No, it shouldn't. You end up with a more complex mess then you started with.

Really, I find that my definition provides all the simplicity needed for a planetary definition without all sorts of messiness.

 

[Cheezy the Wiz]

Well, where do you make the cutoff? The clarification you add in post 71 can't apply to extrasolars because they may not have the same configureation as our solar system.

I hadn't thought at all of that. I wonder, can you see on Earth-sized planet in another system? How about an asteroid? If you cannot see an asteroid-sized object, or even one the size of Pluto or the Moon, then one definition for extrasolars could be that "if you can't identify it from another system, then is isn't a planet." That is more of an excpulsionary rule than a definitive one, though.

 

[Bozo Erectus]

What about bodies of around Pluto mass in about the same orbit as Pluto such as Orcus? Are they considered Class 1s?

If they fall within the Class 1 boundary, then theyre Class 1. No thinking necessary, either they do, or they dont.

I really can't see why you make this the differentiation between "class 1" and "class 2". Pluto and Orcus share a wole lot more of similarities between what you've described as "class 2" then any of the 8 innermost planets.

I dont think thats important. What similarities are there between Jupiter, and Mars? We call them both planets because they are close in to their star (where we are).

Well, where do you make the cutoff? The clarification you add in post 71 can't apply to extrasolars because they may not have the same configureation as our solar system.

No they wont have the same configuration, but using our own as a guide, we can order other solar systems along the same lines. Planets close in to their stars are Class 1, those we determine to be farther out from their star are Class 2.

No, it shouldn't. You end up with a more complex mess then you started with.

I dont think so. My solution may be messy for scientists, but using distance from the primary as a yardstick is easily understood by non scientists, I think.

Really, I find that my definition provides all the simplicity needed for a planetary definition without all sorts of messiness.

I think your system makes sense, but might not be as easily understood and assimilated by the average person.

 

[Perfection]

Not as random as you think: Neptune is without question the largest thing that far out in the Solar System. The Ort Cloud is a defined field, and is completely separate from the Kuyper belt.

Don't be so sure about that. Sedna certainly messes with the boundries.

I think these could be good definers as Class 2 and 3, as objects in the Ord Cloud are going to by nature be smaller, as they are farther out than a Class 1 or 2, and something larger would probably have been lost by the Sun's gravity out there.

Not really, a large object could be very easily retained by the sun. In fact it's easier for a larger object because small objects can get ejected when they come near large ones.

Also, isn't one of the definitions of a planet than it crosses no other planet's orbit? If it is, we might have to discontinue Pluto as a planet. I don't know where I'm getting this from, I just recalled it for some reason.

Orbital uniqueness is definitely an important theme in this definition debate, and a good definition probobly should address it. Mine does indirectly by making all Masses of size large enough to have great effects on thier orbital locuses. That prevents long-term coorbital planets under most conditions, and certainly prevents field objects from being planets.

I can not see why the subsequent discovery of anything else should
(i) change Pluto or (ii) the definition of what is a planet and therefore
reopen the judgement as to Pluto being a planet.

Well the reason is that it shows the existance of a new field of objects that need to be dealt with.

There may be an issue as to what is the distinction between a large comet
and a small planet, but Pluto is clearly nearer a planet than a comet.

Well it's not an either or thing, I'd call Pluto a planetoid or an icetroid or something else besides a comet or planet

The definition of a planet to me would be a:

(a) an object that is and has been primarily illuminated by external visible spectral light rather than its own incandescence

(b) having a spheroidal shape due to its own gravity


Criteria (a) excludes most stars, black holes, galaxies, gas clouds

Criteria (b) eliminates small comets, small asteroids

This definition would include many moons (orbiting other planets)
and also any free roaming interstellar planets. Now the mathematicians
could propose a qualifying test for spheroidity, the alternative being an arbitrary limit on mass. The biggest problem in the definition is at the small brown dwarf/giant planet boundary where it might be unclear whether the majority of its light was emitted by is own heating up or by reflection, for which an arbitrary upper mass limit might be a more useful distinction.

That definition clearly violates common notions of what a planet is.

I don't think the solar system should have 50-odd planets. That's the biggest problem, it shatters old notions of what a planet should be.

 

[EdwardTking]

Well common notions vary.

As Pluto was called a planet for 60 years, I find it strange that
it should suddenly be redefined as not being a planet.

And I regard our moon as a planet, of the solid, moon and airless type.

As orbits and source of luminosity can change, and size and shape
are difficult to determine, I am coming to the conclusion that the best way forward would be for a planet to be defined as a gravitationally bound astonomical object within a particular mass range to be defined by the IAU.

 

[Perfection]

If they fall within the Class 1 boundary, then theyre Class 1. No thinking necessary, either they do, or they dont.

Orcus is on the boundry...

I dont think thats important. What similarities are there between Jupiter, and Mars?

They both are large mass bodies in unique (non-field) orbits that clear away debris in thier orbital path and that directly orbit the sun.

We call them both planets because they are close in to their star (where we are).

Then why don't we call Ceres a planet? (Hint: it's becase distance isn't the determining factor)

No they wont have the same configuration, but using our own as a guide, we can order other solar systems along the same lines. Planets close in to their stars are Class 1, those we determine to be farther out from their star are Class 2.

But how much closer in and how much farther out?

I dont think so. My solution may be messy for scientists, but using distance from the primary as a yardstick is easily understood by non scientists, I think.

Well, I've had no formal astronomy training besides a basic unit in 8th grade and I still understand it. Hell, I even proposed it! You understood it just fine and IIRC you're not an astronomer either. So, I don't think ease of understanding is that big an issue.

I think your system makes sense, but might not be as easily understood and assimilated by the average person.

Teach them a few new words and they'd understand. And if they can't be bothered to learn those then there's no point in trying to tell them anyways, they simply aren't interested.