Pluto the Planet :D

[rkman]

In other words, a planet wouldn't be in the Kuiper or Asteroid Belt because it would have cleared it's orbital zone.

The question remains: would a planet the size of Ceres that has cleared its orbit be called a dwarf planet or not?

Iow: does the designation "dwarf" pertain to size or does it pertain to it not being a planet, or perhaps either depending on the situation - in which case here would be dwarf planets that are planets and dwarf planets that are not planets.

And if "dwarf planet" means "not a planet", why does it have "planet" in the name anyway?

I don't care very much which category Pluto is in, my point is that the designation "dwarf planet" is poorly chosen - unless "dwarf planet" is a subset of "planet".

Science doesn't work by voting. Even so, the IAU cast the vote with a quorum of a tiny subset of members.

Deciding on the names of categories/designations, even if relating to science, is not science. It's not like they voted on what the composition or the mass of Pluto is.

[Albert VDS]

The question remains: would a planet the size of Ceres that has cleared its orbit be called a dwarf planet or not?

Something the size of Ceres can't be a planet because it's not big enough to clear it's neighbourhood in it's orbits/orbital zone or is gravitationally dominant.

Iow: does the designation "dwarf" pertain to size or does it pertain to it not being a planet, or perhaps either depending on the situation - in which case here would be dwarf planets that are planets and dwarf planets that are not planets.

And if "dwarf planet" means "not a planet", why does it have "planet" in the name anyway?

I don't care very much which category Pluto is in, my point is that the designation "dwarf planet" is poorly chosen - unless "dwarf planet" is a subset of "planet".

There are lot's of things which are "misnamed":

Red panda's aren't pandas.

A Guinea pig is not a pig.

A flying lemur is not a lemur.

An elephant shrew is not an elephant or a shrew.

Crayfish aren't fish.

Mountain chicken is not a chicken and doesn't live in the mountains.

An aardwolf is not a wolf.

Killer whales are dolphins.

A mountain goat is not a goat.

Dutch clocks are from Germany.

Rice paper isn't made from rice.

Sealing wax isn't wax.

Honeydew is neither honey nor dew.

Salt is not a salt but a chloride of sodium.

Cuttle-bone isn't bone.

Sun rise or set as the Sun doesn't actually rise or set

Big bang was not a bang.

[peadar1987]

The earth shares its orbit with thousands of asteroids. I read the definition, and I don;t see any mention of trojans, or in fact any co-orbital bodies, just "neighborhood."

Better to say µ must be >100 or something.

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They are in jupiter's neighborhood, are they not? They orbit the sun, right? What does the definition say? Nothing.

Like I said, imprecise.

Third time I've said this, but "clearing the neighbourhood" doesn't mean that nothing comes near to something's orbit, ever. It is a measure of whether something is likely to become gravitationally dominant over its orbit in less than a Hubble Time. The term "clearing" is a little misleading, but the problem is mainly people refusing to find out what it actually refers to.

Larger objects with smaller orbits and fewer things crossing them have a greater power to clear their orbits, and therefore have a greater Stern-Levison parameter, which is mathematically well-defined.

Earth has things that share its orbit. There's the moon, which orbits around us. There's 2010TK7, which is the only known Earth Trojan. There's Cruithne, which is a quasi-satellite of earth, locked in an orbital resonance. Earth is gravitationally dominant over all of these bodies. Any earth-crossing bodies, asteroids such as Apophis, have their orbits perturbed by Earth, until eventually they will be forced into a stable resonance, or ejected from earth's orbit.

The Stern-Levison parameter is, in part, a measure of how quickly this is likely to happen.

As Pluto is far out, and also shares its orbit with a huge body like Neptune, it is extremely unlikely to become gravitationally dominant over its orbit.

Although Ceres is very small, it will tend to have these effects on all of the asteroids that share its orbit. However, due to its small mass, the orbit-clearing effect is very, very slow, and will take over the lifetime of the universe to happen, therefore it is not gravitationally dominant.

Mercury, although small, has a small, fast orbit, so perturbs objects around it more quickly. It also doesn't have the supply of Mercury-crossing objects replenished as quickly as further-out planets, as Jupiter is less likely to throw stuff at it.

As for hydrostatic equilibrium, it is not based on pure "roundness", as any object can be coincidentally quite round, it is based on the gravity and mechanical strength of the body. If the body is round because of coincidence or design, it is not in hydrostatic equilibrium, it is just round. Hydrostatic equilibrium occurs when the material of which the body is made is not strong enough to stop its gravity collapsing it into a sphere with less than a certain amount of aberration. For rock, ice, and gas, this value is measured in hundreds of kilometres. Under certain conditions, you could probably get a sphere of liquid mercury with a diameter of a few hundred to a few thousand metres, but that is extremely unlikely, and probably not stable over planetary timescales. Unlikely enough that we absolutely don't need to tear up the rulebook in order to account for it.

[rtxoff]

I really don't understand why this is still a subject, after discussing it already in a few threads one would think that this topic is over. There's a good reason why there is such an organization like the IAU and if they decide that Pluto isn't a planet anymore they had good reasons for it and everybody not being a member of IAU will have to accept that or become a member and try to change things. Chewing this over and over in this Forum won't change a anything about it. Regardless of my private opinion about Pluto i am accepting their judgement and decision about it because if we would start to object every such resolution things would rather fast develop into chaos. Things evolve and change over time, you can't expect things to stay the same for all of your life guys. So Pluto is not a planet anymore, i couldn't care less because it's still up there and it didn't change it's physical properties. I just have to learn not to call it a planet anymore but this is obviously a very big pain for some of you and it's IMO very hypocritical given the fact that humanity has to worry about much greater problems.

[Superfluous J]

Red panda's aren't pandas.

A Guinea pig is not a pig.

A flying lemur is not a lemur.

An elephant shrew is not an elephant or a shrew.

Crayfish aren't fish.

Mountain chicken is not a chicken and doesn't live in the mountains.

An aardwolf is not a wolf.

Killer whales are dolphins.

A mountain goat is not a goat.

Dutch clocks are from Germany.

Rice paper isn't made from rice.

Sealing wax isn't wax.

Honeydew is neither honey nor dew.

Salt is not a salt but a chloride of sodium.

Cuttle-bone isn't bone.

Sun rise or set as the Sun doesn't actually rise or set

Big bang was not a bang.

I read each of these in my head in Linda Richmond's voice, and added "Discuss" after it. And added my favorite, "A cubic octagonal strut is not a cube, an octagon, nor a strut. Discuss."

I really don't understand why this is still a subject, after discussing it already in a few threads one would think that this topic is over.

Like all topics of no import and 0 chance that they'll change, we get quite up in arms about our opinions.

[rkman]

There are lot's of things which are "misnamed":

Right. But since in the case of Pluto a bunch of planetary scientists sat down to discuss at length what Pluto is and what it is not, could they not do better than come up with a misnomer?

[Darnok]

So science is about bargain about what number means what? Or what planet parameter is more important... is it radius, orbit, mass, shape etc?

How about create classification that is based on universal mathematical values or patterns?

[lajoswinkler]

This isn't a case of "growing up" or being immature.

I think it's exactly that. Pluto was always weird and astronomers and astrophysicists always suspected there's more to it. Kuiper belt was proven in early nineties so after like a quarter of century it's time to acknowledge it. Pluto is a remnant, along with an ever growing number of similar bodies. There are probably thousands or more such bodies. Is it useful to have a system with thousands of planets? Of course not. The name "planet" would lose its meaning and function. Therefore a new class of bodies has been proposed and accepted. Idea is very old but is now official.

No room for romanticism and nationalism in science. Pluto is fascinating no matter what its official designation is.

[rkman]

So science is about bargain about what number means what? Or what planet parameter is more important... is it radius, orbit, mass, shape etc?

No, there is no scientific meaning in the specific name of a classification.

Or what planet parameter is more important... is it radius, orbit, mass, shape etc?

No, designations for (non) planets are not based on only one of those parameters. Pluto is not a planet (in spite of being a dwarf planet).

How about create classification that is based on universal mathematical values or patterns?

That is where the actual science is, but it is only loosely related to the designation.

[tater]

There are ways to calculate the degree of which a body has cleared its neighborhood, such as the Stern-Levison parameter. It would be trivial for the IAU to choose a value to determine the cutoff point for being a planet or dwarf planet, or a range in which individual cases would be decided. In any case, the value between the most messy planet and cleanest dwarf planet differs by many orders of magnitude, regardless of what specific formula is used.

https://en.m.wikipedia.org/wiki/Clearing_the_neighbourhood

Yes, I've read that, which is why I mentioned setting a value for µ.

The definition does not set a value for µ, however, so it's not on the table. Its a bad definition, period. Sloppy.

What about exoplanets? A robust definition needs to work for all bodies in all solar systems. I can certainly come up with endlees gray cases where we'd all be starting our heads (pretty easy when by the current definition it seems like Earth, Mars, Jupiter, and Neptune at the very least have problems with their planet status dues to their neighborhoods being somewhat less than empty (clear usually refers to a lack of contaminants in plain english, and it has no specific meaning in jargon for planetary science, just a few papers).

To be clear:

I'm fine with making an objective definition.

I'm fine with Pluto being bumped from planet status by an objective definition of planet vs dwarf planet vs small body, etc.

I just want a clear, unambiguous definition that includes everything needed to make a determination within the definition.

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I think it's exactly that. Pluto was always weird and astronomers and astrophysicists always suspected there's more to it. Kuiper belt was proven in early nineties so after like a quarter of century it's time to acknowledge it. Pluto is a remnant, along with an ever growing number of similar bodies. There are probably thousands or more such bodies. Is it useful to have a system with thousands of planets? Of course not. The name "planet" would lose its meaning and function. Therefore a new class of bodies has been proposed and accepted. Idea is very old but is now official.

No room for romanticism and nationalism in science. Pluto is fascinating no matter what its official designation is.

I agree completely, and the number of bodies in the solar system (any solar system, really) is going to be quite large. My real issue is how sloppy the definition is. You cannot argue that the current definition is unambiguous.

[Bill Phil]

I think it's exactly that. Pluto was always weird and astronomers and astrophysicists always suspected there's more to it. Kuiper belt was proven in early nineties so after like a quarter of century it's time to acknowledge it. Pluto is a remnant, along with an ever growing number of similar bodies. There are probably thousands or more such bodies. Is it useful to have a system with thousands of planets? Of course not. The name "planet" would lose its meaning and function. Therefore a new class of bodies has been proposed and accepted. Idea is very old but is now official.

No room for romanticism and nationalism in science. Pluto is fascinating no matter what its official designation is.

You think this is romanticism and nationalism? I don't care if an American discovered Pluto.

A planet is something that isn't easy to define. The original word meant "wandering star" which means just about everything in the solar system.

A system with thousands of planets is perfectly fine, because ours has only eight major planets, with thousands of minor planets.

According to the IAU, a dwarf planet is not a planet. Even though it should be a sub class of planets.

Not to mention that many people across the world still refer to Pluto as a planet, and many other bodies. People within NASA still call it a planet.

I'm fine if Pluto isn't a planet, but you wouldn't classify a human based on the environment it's in, right? So why classify a planet based on it's environment.

Edited July 17, 2015 by Bill Phil

[tater]

Something the size of Ceres can't be a planet because it's not big enough to clear it's neighbourhood in it's orbits/orbital zone or is gravitationally dominant..

The definition doesn't say this.

I'm not arguing that the result of the definition is wrong, I'm arguing the definition is bad.

[Bill Phil]

Right. But since in the case of Pluto a bunch of planetary scientists sat down to discuss at length what Pluto is and what it is not, could they not do better than come up with a misnomer?

No, a bunch of astronomers. People who named things, looked at them through a telescope, etc.

[tater]

Third time I've said this, but "clearing the neighbourhood" doesn't mean that nothing comes near to something's orbit, ever. It is a measure of whether something is likely to become gravitationally dominant over its orbit in less than a Hubble Time. The term "clearing" is a little misleading, but the problem is mainly people refusing to find out what it actually refers to.

Larger objects with smaller orbits and fewer things crossing them have a greater power to clear their orbits, and therefore have a greater Stern-Levison parameter, which is mathematically well-defined.

Earth has things that share its orbit. There's the moon, which orbits around us. There's 2010TK7, which is the only known Earth Trojan. There's Cruithne, which is a quasi-satellite of earth, locked in an orbital resonance. Earth is gravitationally dominant over all of these bodies. Any earth-crossing bodies, asteroids such as Apophis, have their orbits perturbed by Earth, until eventually they will be forced into a stable resonance, or ejected from earth's orbit.

The Stern-Levison parameter is, in part, a measure of how quickly this is likely to happen.

As Pluto is far out, and also shares its orbit with a huge body like Neptune, it is extremely unlikely to become gravitationally dominant over its orbit.

Although Ceres is very small, it will tend to have these effects on all of the asteroids that share its orbit. However, due to its small mass, the orbit-clearing effect is very, very slow, and will take over the lifetime of the universe to happen, therefore it is not gravitationally dominant.

Mercury, although small, has a small, fast orbit, so perturbs objects around it more quickly. It also doesn't have the supply of Mercury-crossing objects replenished as quickly as further-out planets, as Jupiter is less likely to throw stuff at it.

As for hydrostatic equilibrium, it is not based on pure "roundness", as any object can be coincidentally quite round, it is based on the gravity and mechanical strength of the body. If the body is round because of coincidence or design, it is not in hydrostatic equilibrium, it is just round. Hydrostatic equilibrium occurs when the material of which the body is made is not strong enough to stop its gravity collapsing it into a sphere with less than a certain amount of aberration. For rock, ice, and gas, this value is measured in hundreds of kilometres. Under certain conditions, you could probably get a sphere of liquid mercury with a diameter of a few hundred to a few thousand metres, but that is extremely unlikely, and probably not stable over planetary timescales. Unlikely enough that we absolutely don't need to tear up the rulebook in order to account for it.

Then why not dump "clearing the neighborhood" and set a value of µ and be done with it?

µ > 1, 100, 100, 137.5, whatever is considered "clearing the neighborhood."

Had they done this, I'd not be arguing. Still arbitrary, but at least it's consistent, and everyone could do the math and say, "yep, not a planet."

You cannot argue about the specifics of clearing the neighborhood when it's not in the definition. It's sloppy.

I'd say the same about the hydrostatic equilibrium. I know what they mean, but the definition could include, you know, math, and there is no argument.

There is no requirement that the definition must fit within a twitter post.

Edited July 17, 2015 by tater

[peadar1987]

Then why not dump "clearing the neighborhood" and set a value of µ and be done with it?

µ > 1, 100, 100, 137.5, whatever is considered "clearing the neighborhood."

Had they done this, I'd not be arguing. Still arbitrary, but at least it's consistent, and everyone could do the math and say, "yep, not a planet."

You cannot argue about the specifics of clearing the neighborhood when it's not in the definition. It's sloppy.

I'd say the same about the hydrostatic equilibrium. I know what they mean, but the definition could include, you know, math, and there is no argument.

There is no requirement that the definition must fit within a twitter post.

I really don't know what your problem is with this. Stern-Levison parameters of greater than 1 count as planets. It is a hard cutoff. A S-L parameter of greater than 1 means it's a planet. Everyone can do the maths and work it out. There is no subjectivity at all. Argue with the cutoff, argue with the criterion they have decided upon, but you can't argue that it is subjective or open to interpretation now the rules have been set.

[tater]

I really don't know what your problem is with this. Stern-Levison parameters of greater than 1 count as planets. It is a hard cutoff. A S-L parameter of greater than 1 means it's a planet. Everyone can do the maths and work it out. There is no subjectivity at all. Argue with the cutoff, argue with the criterion they have decided upon, but you can't argue that it is subjective or open to interpretation now the rules have been set.

Where is this in the IAU definition of planet, exactly?

The IAU...resolves that planets and other bodies, except satellites, in the Solar System be defined into three distinct categories in the following way:

(1) A planet [1] is a celestial body that (a) is in orbit around the Sun, ( has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and © has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, ( has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], © has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects [3], except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

I see no mention whatsoever of the S-L parameter. If that is what they mean, then use it in the definition. That's the point of a definition.

I am arguing the definition is crap, and it unambiguously is. If you need to bring in any outside ideas to make sense of the definition, it's a bad definition.

Change (1) to (my changes bold):

1) A planet [1] is a celestial body that (a) is in orbit around a star, ( has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape (add in the required maximum eccentricity to count, because math), and © has an S-L parameter >1.

There, was that hard?

(note that you'd still get people arguing that >1 part, what about >0.99, or another, arbitrary value. Such definitions are always arbitrary, it's for "bookkeeping," not because there is necessarily physicality involved, and that is fine, we what a manageable number of major bodies to concern ourselves with.

Edited July 17, 2015 by tater

[pincushionman]

Tater isn't arguing that an S-L parameter of 1 isn't a good cutoff (well…at least not in that post). He's saying that the IAU's resolution doesn't anywhere state that the S-L parameter is the preferred measure of "clearing", let alone that 1 is an appropriate cutoff.

It also doesn't do a good job of specifying what the "neighborhood" is.

EDIT: ninja'd

Edited July 17, 2015 by pincushionman