Pluto the Planet :D

[vger]

Even if there was an Earth size planet way out in the Kuiper Belt it still wouldn't be a planet IMO.

If there was an Earth size planet out in the Kuiper Belt, there probably wouldn't be a Kuiper Belt anymore. Though I guess there might be a Kuiper Ring.

[peadar1987]

Except the biggest thing is that 'it hasn't cleared its orbit.' According to certain definitions, EARTH hasn't cleared its orbit.

It all depends where you put the critical Stern-Levison parameter. It's currently set at 100. If you set it to 1000, Mars is no longer a planet. If you set it to 0.001, Pluto, Eris, Makemake, Ceres, Quaoar, Orcus, Haumea, Ixion and almost certainly many others become planets. At the moment, it is in a relatively sensible place, as you don't have to start including things like Pallas, Vesta, Chaos and Huya,

[kiwi1960]

I am old enough to have been taught that Pluto is a planet.

Lets look at the facts....

Any object orbiting a planet is a moon.

Any object that does not have a real orbit is not a planet.

Any object that has anything resembling a moon must be a planet if it has an orbit and is not a moon.

Any object that is small, like pluto, without the above, and only consisting of rock MIGHT be classed as a dwarf planet... (but Pluto, the Planet has ice!)

Pluto's orbit was stable enough for a probe to take years to fly to it and send greetings from all human kind.

As for those others... mentioned in the above post.... they do not have orbits, not like Pluto's... OK, so Pluto has a wonky orbit compared to the other planets, but its still an orbit.

The rocks.... they just float.... although they do move as all objects in space do, they have no momentum (which translates to an orbit) to have them classed as a planet.

I rest my case...

[Dkmdlb]

I object to your third fact.

[Brotoro]

So all asteroids that have moons would be planets? Welcome to the list of planets, Ida.

[kiwi1960]

So all asteroids that have moons would be planets? Welcome to the list of planets, Ida.

No, if they are rock, and do not have a moving orbit then its still a rock. Asteroids often have other asteroids orbiting them... re read my list....doesn't make the asteroid a planet OR a moon.

[peadar1987]

So all asteroids that have moons would be planets? Welcome to the list of planets, Ida.

Dactyl is one of my favourite names for a celestial body

- - - Updated - - -

Also, weirdly, this whole thing could have been killed at the start, if only the IAU had known the trouble it would cause. There were two motions voted on. The first was to classify Pluto, Ceres et al as dwarf planets. That was passed. The second was to clarify that dwarf planets were still planets, just a subset thereof. It was voted against.

[Albert VDS]

The rocks.... they just float.... although they do move as all objects in space do, they have no momentum (which translates to an orbit) to have them classed as a planet.

They move but have no momentum? Really?

[Camacha]

At the moment, it is in a relatively sensible place, as you don't have to start including things like Pallas, Vesta, Chaos and Huya,

Why is that sensible? We just discovered we have lots of planets. It seems rather odd to come up with an artificial boundary to cull it to a handy size that can be taught in school.

[peadar1987]

Why is that sensible? We just discovered we have lots of planets. It seems rather odd to come up with an artificial boundary to cull it to a handy size that can be taught in school.

Because it has to be set somewhere, and it makes sense to have it in the middle of a large gap. There are hundreds, possibly thousands, of dwarf planets out there filling every possible gap in the scale from Pluto down to the smallest Neptune Trojan. The gap between Pluto and Mars is the biggest gap in the Stern-Levison parameter, which is prmarily why the cut-off has been put there.

[Camacha]

If we just, today, discovered everything we know about in the solar system, how would we classify it? Pluto makes sense as a planet when you're shown a simple diagram in school, but not so much when you're looking at a more realistic representation of the solar system.

It does, but it really just turns out we have lots of planets. The fact that telescopes in the olden days had trouble picking smaller and more distant things up seems a rather shoddy basis for defining your system. While I do not have problems with demoting Pluto as such, I do have a problem with how the definition seems to be crafted to fit a view.

It seems the scientific community is not really in agreement either way, which means the subject is likely to change shape later on as opinions change or solidify, or until more data is collected to shift views.

Other systems won't change anything about Pluto, it will only enforce that it's a dwarf planet.

The simple reason is that dwarf planets will never have a significant gravitational interaction.

There might be solar systems which are filled with dwarf planets and it wouldn't change that view.

Pluto has enough gravitational interaction to collect a whole system around it. Or are you referring to the orbit clearing demand, which seems rather arbitrary?

Thinking it will change is like thinking spiders will be categorized as insects in the future.

There are clear and simple characteristics why they are arachnids and that wont change if any other insect or spider species are found.

While that sounds like a great story, the dataset we have of planets is many times smaller than that of arthropods. We have a pretty good overview of what goes in making a bug, while we only grasp fairly basic things about solar system formation. Most of that information comes just from one solar system, which is a rather small subset in the greater scheme of things.

Comparing planet categorization to entomology does not fly. We know, in relative terms, next to nothing, and new discoveries will almost certainly mean overhauling our definitions yet again. Even without new discovery, definitions seem to be as much a cultural thing as a scientific thing. Certain schools of thought rise up, others die off, almost like political movements. Meanwhile, nothing actually changes about the body or even the knowledge of that body, just the opinions on how to structure the subsequent metadata.

Because it has to be set somewhere, and it makes sense to have it in the middle of a large gap. There are hundreds, possibly thousands, of dwarf planets out there filling every possible gap in the scale from Pluto down to the smallest Neptune Trojan. The gap between Pluto and Mars is the biggest gap in the Stern-Levison parameter, which is prmarily why the cut-off has been put there.

If that seems to hold true in our system and a fair amount of other systems, yes, then it makes sense. Using it as a definition for a single partially known system makes it a temporary or preliminary definition per... er... definition

[tater]

The very notion of ranking planets is by definition arbitrary. The IAU picked a new arbitrary standard trying to make it look less arbitrary, but it still is. They wanted a size limit, and chose hydrostatic equilibrium (rough spheroid), but obviously what the body is composed of will change the mass required for this. "Clearing the neighborhood?" Incredibly vague.

If they want a non-arbitrary standard, show us the math. I want to be able to put in the shape of the body, and data for the orbits, etc, and have it tell me if it is a planet. If a single new body enters the neighborhood (or is discovered)... it gets instantly demoted.

Better to have a vague standard, and just vote bodies in since the current standard is also arbitrary.

The whole point seems to be to limit the number of "planets" to something a 1st grader could memorize.

[Cydonian Monk]

Better to have a vague standard, and just vote bodies in since the current standard is also arbitrary.

So it's a... thing, orbiting this other..., bigger thing which has lots of other... things orbiting it. And there's these other... things that nobody can quite explain or agree upon, so we call them dwarf things. [Please insert hand waving and pauses for the ... elipses in the above thing. And a finger snap and a smile at the end.]

[LittleBlueGaming]

It does, but it really just turns out we have lots of planets. The fact that telescopes in the olden days had trouble picking smaller and more distant things up seems a rather shoddy basis for defining your system. While I do not have problems with demoting Pluto as such, I do have a problem with how the definition seems to be crafted to fit a view.

It seems the scientific community is not really in agreement either way, which means the subject is likely to change shape later on as opinions change or solidify, or until more data is collected to shift views.

Should we just call all planets, moons, asteroids, and comets 'planets' and get it over with?

It's part of our basic nature to seek to understand things, and to notice differences. When we notice differences, we start classifying. It's a useful tool. Pluto hasn't changed, so calling it a planet or dwarf planet or minor planet or trans-Neptunion object... what we decide to classify it as won't change the actual thing. What it will change is how we understand and teach things. It's no different than any other classification system.

I'd rather my children learn the differences between planets, moons, comets, asteroids, etc... they'll surely have a better understanding than if they were just taught that there are all these Orbital Objects revolving around the Big Bright Central Orbital Object, ranging from the size of a grain of dust to larger than Our Orbital Object.

[peadar1987]

The very notion of ranking planets is by definition arbitrary. The IAU picked a new arbitrary standard trying to make it look less arbitrary, but it still is. They wanted a size limit, and chose hydrostatic equilibrium (rough spheroid), but obviously what the body is composed of will change the mass required for this. "Clearing the neighborhood?" Incredibly vague.

If they want a non-arbitrary standard, show us the math. I want to be able to put in the shape of the body, and data for the orbits, etc, and have it tell me if it is a planet. If a single new body enters the neighborhood (or is discovered)... it gets instantly demoted.

Better to have a vague standard, and just vote bodies in since the current standard is also arbitrary.

The whole point seems to be to limit the number of "planets" to something a 1st grader could memorize.

So it's a... thing, orbiting this other..., bigger thing which has lots of other... things orbiting it. And there's these other... things that nobody can quite explain or agree upon, so we call them dwarf things. [Please insert hand waving and pauses for the ... elipses in the above thing. And a finger snap and a smile at the end.]

I literally just posted about this. It's not vague or hand-wavey, it's very well-defined, and doesn't mean that nothing is allowed to cross your orbit:

"Cleared the neighbourhood" doesn't means "Nothing sharing the orbit", it means that everything else in the orbit is dominated by gravitational interactions with that body. So Neptune has cleared its orbit, because the only things sharing it are Trojan asteroids and Plutinoids locked into a 3:2 orbital resonance (and minor Neptune-crossers which haven't been fully cleared). The same with Earth, everything earth-crossing is either locked in orbital resonance with earth (like Cruithne), or it is slowly being perturbed either towards an orbital resonance or ejection from earth's orbit.

There is a mathematical parameter used to determine how dominant a planetary body is over its orbit, and how likely it is to clear its neighbourhood over astronomical timescales. It is called the Stern-Levison Parameter, and is anything but arbitrary. The major planets have Stern-Levison parameters from 942 (Mercury) to 1.3E9 (Jupiter). The most gravitationally dominant dwarf planet over its orbit is Pluto, with a Stern-Levison parameter of 2.95E-3. So Jupiter is 7 orders of magnitude more dominant than Mercury, and Mercury is 5 orders of magnitude more dominant than Pluto. Pluto, Ceres, Eris, Haumea and Makemake all have S-L parameters of roughly the same order of magnitude, so if you let one in you have to let them all in. Not that I'm against that, but the IAU defined a very clear cut-off point of a Stern-Levison parameter of 100 for full planetary status. It's not a subjective "yeah that orbit is probably clear enough" sort of a thing.

[Camacha]

So it's a... thing, orbiting this other..., bigger thing which has lots of other... things orbiting it. And there's these other... things that nobody can quite explain or agree upon, so we call them dwarf things. [Please insert hand waving and pauses for the ... elipses in the above thing. And a finger snap and a smile at the end.]

As I have said many times before, using system hierarchy as a naming convention would probably be a good option. You would need to take a long hard look at complicated nested systems (binary stars that circle a barycentre together with a third star, each of which has several layers of binary and tertiary planets and moons, for instance). This is already partly true, and eliminates issues with objects that do not quite fall into this category or that. I can imagine that things still need hydrostatic equilibrium to have a baseline, so that all the loose floaty rocks and dust will be cut out. Another baseline cap like certain processes, planetary differentiation perhaps, or something else, might be suitable too, though you need a great deal more information to establish that.

[LittleBlueGaming]

This might be helpful to show just how big the gap is between the 8 current planets and Pluto/others. Yes, 100 is technically an arbitrary line, but it's an arbitrary line that makes a lot of sense.

[Camacha]

This might be helpful to show just how big the gap is between the 8 current planets and Pluto/others. Yes, 100 is technically an arbitrary line, but it's an arbitrary line that makes a lot of sense.

*For our system, which means the part of it we know, excluding the parts we do not know and all other systems out there.

It only seems to make sense to use something like that as a defining boundary when you actually can substantiate whether than boundary has any actual significance for planetary systems.

I literally just posted about this. It's not vague or hand-wavey, it's very well-defined, and doesn't mean that nothing is allowed to cross your orbit:

The definition itself might be clear, the chosen parameter or cut-off just it not, which means the whole thing still is hand-wavey.

Should we just call all planets, moons, asteroids, and comets 'planets' and get it over with?

It's part of our basic nature to seek to understand things, and to notice differences. When we notice differences, we start classifying. It's a useful tool. Pluto hasn't changed, so calling it a planet or dwarf planet or minor planet or trans-Neptunion object... what we decide to classify it as won't change the actual thing. What it will change is how we understand and teach things. It's no different than any other classification system.

I'd rather my children learn the differences between planets, moons, comets, asteroids, etc... they'll surely have a better understanding than if they were just taught that there are all these Orbital Objects revolving around the Big Bright Central Orbital Object, ranging from the size of a grain of dust to larger than Our Orbital Object.

Read my post about hierarchy Though convenience of education is no argument for classification, as long as you can separate relevantly differing bodies out.

[peadar1987]

The definition itself might be clear, the chosen parameter or cut-off just it not, which means the whole thing still is hand-wavey.

Which is a problem you're going to have with any system of classification. Mass, radius, roundness, orbital characteristics, all of these are sliding scales, and you have to cut them off at some point if you're not going to include every grain of dust from Mercury to the Oort Cloud.

[LittleBlueGaming]

*For our system, which means the part of it we know, excluding the parts we do not know and all other systems out there.

It only seems to make sense to use something like that as a defining boundary when you actually can substantiate whether than boundary has any actual significance for planetary systems.

The definition itself might be clear, the chosen parameter or cut-off just it not, which means the whole thing still is hand-wavey.

Read my post about hierarchy Though convenience of education is no argument for classification, as long as you can separate relevantly differing bodies out.

Yeah, for our system. The IAU definitions are for our system only. And if we find out something new that makes this classification scheme no longer clear or points out a better one, it can change again.

I would say, as long as you get the same utility of out a classification system, then the simpler the better. It is important to make science education accessible and exciting, unless you want scientists to die out.

[Candlelight]

Look, Kaboom!

I have two simple solutions:

a) We make exceptions for Pluto.

We simplify our definition of a planet.

I rest my case.

[SAI Peregrinus]

Personally I feel that any body in hydrostatic equilibrium not orbiting another body that isn't the Sun should be considered a planet. Pairs of bodies orbiting a common barycenter outside both bodies should be multiple-planets. Bodies orbiting other stars should be exoplanets or multiple-exoplanets.

So Ceres should be a planet, as should Haumea, Makemake, and Eris. Others should probably be included (Orcus, Ixion, Quaoar, etc) but are not definitively known to be in hydrostatic equilibrium yet. Pluto-Charon should be a double-planet. This definition is simpler, but does lead to having lots of planets.

Also, the Stern-Levison parameter is nice, but it's not part of the official definition of having "cleared the neighborhood".

[kiwi1960]

They move but have no momentum? Really?

They move yes.... as all objects in space do, but seen from earth, if it doesn't move, then it doesn't have an orbit.

After all, its how Pluto was found by comparing it to other non moving objects using time lapse.

[Camacha]

It is important to make science education accessible and exciting, unless you want scientists to die out.

That is a very well rounded no A scientific system needs to provide a good and relevant framework for science. As soon as you start cutting or summarizing at the expensive of accuracy or for the sake of simplification, you are on a very slippery slope. A simpler system is a better system, but only if it does not sacrifice accuracy and relevance in the process

They move yes.... as all objects in space do, but seen from earth, if it doesn't move, then it doesn't have an orbit.

After all, its how Pluto was found by comparing it to other non moving objects using time lapse.

http://astro.hopkinsschools.org/course_documents/solar_system/icyworlds/pluto_discovery.jpg

So certain artificial satellites do not have an orbit? Also, given enough time (500 to 2000 years), even stars move relative to us and each other. Does that make them planets or comets?

It seems you need to check your definitions, as they seem a bit shaky

So Ceres should be a planet, as should Haumea, Makemake, and Eris. Others should probably be included (Orcus, Ixion, Quaoar, etc) but are not definitively known to be in hydrostatic equilibrium yet. Pluto-Charon should be a double-planet. This definition is simpler, but does lead to having lots of planets.

I can agree with that. There is nothing wrong with having lots of planets, you could always subdivide if you wish. They should have just done that in the first place. Call them dwarf planets, which are a subcategory of planets. Everyone is happy, no discussion, except for the obligatory sourpuss that does not agree

Edited July 14, 2015 by Camacha

[kiwi1960]

Seeing as we have a probe there now... wouldn't it be good manners to ask Pluto for his opinion?

"Hey you.... The big ball called Pluto.... are you a dwarf planet or a planet"

it would probably answer...

"As any woman will tell you, size isn't everything... they found me once, and called me a planet, it's their mistake, not mine, so why insult me by calling me a dwarf. Keep calling me a Planet because that's was how it was for around 80 of your years... argue anything new that comes along, but for the love of Mars, leave me alone....."