Comments on Mike Brown's Planets: Don't try to blame it on Rio

Any chance for the term "Dwarf Planets" be changed...

2009-08-11T05:40:25.923-07:00

Any chance for the term "Dwarf Planets" be changed to "Planetoids"?

This seems like an unforunate side effect of the p...

2009-08-10T21:35:47.405-07:00

This seems like an unforunate side effect of the planet spat:

"Alan Stern, a planetary scientist at the Colorado-based Southwest Research Institute and principal investigator for NASA's New Horizons mission to Pluto, turned down an invitation to speak at the IAU's Rio meeting. "I'm not there because the IAU seems to have become irrelevant," he told me today via e-mail."

via http://cosmiclog.msnbc.msn.com/archive/2009/08/05/2020560.aspx

Whew. I go away for a week and look at what happen...

2009-08-10T06:54:32.317-07:00

Whew. I go away for a week and look at what happens on the comment boards.
@Dave: I have looked through a bunch of astro texts from 1800-1900, when the number of planets was steadily increasing, but I had never been able to find one past 1900 that hadn't gone back to eight. Fascinating!
@Chuck: It was the full moon, so there weren't too many constellations to see, but it is amazing how disoriented I get when I look up and the sky seems unfamiliar.

@the debate about planets
Doesn't sound like any minds are being changed here, so I think it is time to declare an end to this comment section this week. Fear not: I am sure to post something again about planets, so this can start up all over again. Though it would be nice if there were something new to say.....

I wrote: "We have had "dwarf planets" orbiting pla...

2009-08-09T23:18:25.163-07:00

I wrote: "We have had "dwarf planets" orbiting planets for years and the general public never complained."

You replied: "Saying the public "never complained," ... is disingenuous."

You do seem to like to twist things, ie. comparing the IAU to Saddam Hussein. When the Voyagers showed "dwarf planet-like bodies" (spherical moons) orbiting Jupiter in 1979 I don't think the general public took to the streets to have spherical moons re-classified as planets. They didn't because of where the objects where located. Yes, I know that directly orbiting the Sun *can be* treated as more important, but does not change the fact that some categories are defined by *where they are*.

Some day I would like most text books to list the number of major moons and minor moons for each planet, because spherical moons should be noted. Then we could see entries like:
Earth 1/1
Mar 0/2
Jupiter 4/63
Saturn 7/61
Uranus 5/27
Neptune 1/13
Pluto 1/3 (I never suggested that Pluto should be removed from all tables)

It is interesting to note that the middle two gas giants have more major moons than the two gas giants that perturbed the asteroid belt (Jupiter inward) and Kuiper belt (Neptune outward).

-- Kevin Heider

"What I mean is that what Earth and Jupiter (and M...

2009-08-09T22:19:55.954-07:00

"What I mean is that what Earth and Jupiter (and Mercury and the other classical planets) have in common is what defines a planet in the context of the Solar System."

This could easily be contested, considering just how different Jupiter and the gas giants are from Earth and the terrestrial planets. What do Jupiter and Mercury have in common? Mercury has no atmosphere and no moons. Jupiter has no solid surface, and like the other gas giants, hosts an entire "mini-solar sytem" of its own. Its composition is more akin to that of the sun than to the composition of Mercury or Earth.

All of this comes back to the same value judgment. You give primacy to dynamics, to the issue of an object "clearing its orbit." That is why you view Pluto as a KBO first and then as a small planet. The same is true of Ceres. Your arguments always go back to where the objects are rather than what they are. That is a legitimate position, but it is not the only one. The geophysical view is that Ceres and Pluto--and all objects--should first be defined by what they are--their composition, whether they are geologically active, etc. We're back to the two competing views, and each is equally legitimate--meaning we can agree to disagree while still acknowledging there are two equally valuable ways to understand and classify objects in the solar system.

"But it is still something regarding emotions, publicity, the imaginary... not the scientific "factualist" debate."

On what do you base this claim?
You admit that the end result was people learned less about the new planets than about the demotion of Pluto. What does this have to do with emotion?

How is putting primacy on hydrostatic equilibrium any more (or less) based on emotion than putting primacy on dynamical dominance? This is a factual, scientific debate. It has nothing to do with emotion. What is imaginary about these concepts? It's not like this is the first time in science that there are two competing ways of interpreting the same data.

In fact, some could say those preferring dynamical dominance are motivated by emotional concerns, as many seem particularly wedded to keeping the number of planets in the solar system small even if that is done by arbitrary and artificial means.

Tombaugh's immediate family members were at the Great Planet Debate, and felt very strongly that he would not have accepted this definition. He might have accepted some sort of revision, such as creating the dwarf planet subcategory for objects like Pluto, provided they were still considered planets. However, it is very unlikely he would have accepted this particular definition (the one the IAU adopted). The first KBOs were discovered in 1992, and Tombaugh died in 1997, so he was well aware of this debate. Even Neil de Grasse Tyson notes that he attended many conferences on this issue and vehemently opposed any reclassification that made Pluto not a planet. In fact, David Levy, his biographer and a signatory to Stern's petition rejecting the IAU definition, promised Tombaugh at the end of his life that he would always defend Pluto's planet status.

It is a stretch to say Earth and Jupiter are "fund...

2009-08-09T16:49:17.359-07:00

It is a stretch to say Earth and Jupiter are "fundamentally similar." Just what does this term mean?-

I did not say that. What I mean is that what Earth and Jupiter (and Mercury and the other classical planets) have in common is what defines a planet in the context of the Solar System.

The demotion of Pluto has actually detracted from children and adults learning about the new planets. Had Pluto been left alone, media and educational focus would have been largely on the exciting fact that our solar system now has three new planets.

In this you do have a point. But it is still something regarding emotions, publicity, the imaginary... not the scientific "factualist" debate.

There might be more praise of Brown if people knew that he discovered three new planets.

Probably. But he seems to be ok with having the record of dwarf planets discoveries.

And in this sense, I'd like to emphasize again the importance of Sedna, even if it's by the moment just a candidate for the status, because Sedna is "the new Pluto" in that sense of being the most distant and enigmatic object known in the Solar System.

To the end of his life, Tombaugh opposed any efforts to downgrade Pluto from planet status. He did not live to see the discovery of Eris, Haumea, and Makemake, but the chances are, he would likely view them as planets as well.

That is not what his wife says: http://www.ctv.ca/servlet/ArticleNews/story/CTVNews/20060825/pluto_discoverer_widow_060825/20060825?hub=SciTech

Tombaugh had fought off other attempts to relegate Pluto, but his widow said this time he probably would have endorsed the change, now that other planetary objects have been discovered in the Kuiper Belt, the belt of comets on the edge of the solar system where Pluto resides.

"He was a scientist. He would understand they had a real problem when they start finding several of these things flying around the place," Patricia Tombaugh said.

Why is Pluto a KBO first? Why is it not equally a KBO and a small planet? -

For the same reasons that Ceres. Essentially because it is a gregarious object that belongs to a much larger population of similar objects and not an individualist "monarch" of its orbital zone. That is what the "clearing the neighbourhood" condition is about.

"Jurisprudence" on the matter said clearly that Ceres was not a planet for that very reason and when it was known that Pluto was in the same situation as Ceres...

"Since "dwarf planets" are not currently considere...

2009-08-09T16:09:16.675-07:00

"Since "dwarf planets" are not currently considered to be real planets (just as moons are not), be careful what you ask for. They could come up with a new word for "dwarf planets" removing all obvious traces of being planet-like. We all know the term "plutoid" did not go over well even though the IAU was just completing the requirement of Resolution B6 by naming the category. I frankly think the name "dwarf planet" is a decent compromise."

Dwarf planets are not considered planets only according to the IAU definition. That is just one interpretation. The IAU used the same closed process in the "plutoids" decision, not even informing leading planetary scientists that this was under discussion. It's time to involve a broader group of scientists than just the IAU, as they have shown poor judgment in this time and time again.

"So a planet is what is fundamentally similar to the usual planets (Earth, Jupiter, etc.) in all aspects and something that is similar only in some aspects but not others is just not a planet but something else."

It is a stretch to say Earth and Jupiter are "fundamentally similar." Just what does this term mean? Earth and Jupiter are only similar to one another in some aspects too. In others, Earth and Pluto are more similar to one another. Who decides, and what criteria get priority?

Finally, here we go again with the claims that it's only Americans who care about Pluto's planet status and that opposition to the IAU decision is motivated by emotional concerns. Just because some people keep repeating these same tiresome lies does not make them true. It doesn't matter who discovered Pluto. If the discoverer were of another nationality, that wouldn't change the fact that the issue is the centrality of hydrostatic equilibrium. Lay people and scientists from around the world continue to object to the IAU definition or to ignore it entirely. Almost all who do argue that all dwarf planets should be counted as planets, not just Pluto. In fact, the person who lobbied the Illinois legislature to pass its resolution owns a website titled "Dwarf Planets Are Planets Too."

The demotion of Pluto has actually detracted from children and adults learning about the new planets. Had Pluto been left alone, media and educational focus would have been largely on the exciting fact that our solar system now has three new planets. Instead, more people know about the Pluto debate than even know that these little planets exist. There might be more praise of Brown if people knew that he discovered three new planets.

To the end of his life, Tombaugh opposed any efforts to downgrade Pluto from planet status. He did not live to see the discovery of Eris, Haumea, and Makemake, but the chances are, he would likely view them as planets as well.

"We call them (spherical) moons and dwarf planets....

2009-08-09T16:09:04.811-07:00

"We call them (spherical) moons and dwarf planets. This is not as bad as you make it out to be. We have had "dwarf planets" orbiting planets for years and the general public never complained."

The term "dwarf planet" has only been in use for three years and many members of the public as well as astronomers have major problems with the determination that dwarf planets are not planets. Saying the public "never complained," in light of the active discussion online and in public forums over the last three years, is disingenuous.

In astronomy, the term "dwarf" is almost always used as an adjective. Dwarf stars are a subclass of stars, and dwarf galaxies are a subclass of galaxies. Assigning dwarf planets as a subclass of planets fits in perfectly with existing usage of the term "dwarf."

"There's no real problem to be solved and would cause many others. Obviously the people who took that decision and who is reasonably satisfied with it, also have their reasons."

Here is the same denial, the same pretense that no controversy exists. The very problem is those who made the decision--424 out of 10,000 IAU members, most not planetary scientists, in a proceess that violated their own bylaws. Many delegates who left early had no idea that a new resolution would be put on the floor, or they would have stayed to vote. Of course, that small group is satisfied--they got their way. We know their reasons--preference for a solely dynamical definition. But they are not the only ones whose views matter. If large numbers of professional astronomers are not satisfied with the definition, something is clearly wrong.

I think the concept of "clearing the neighborhood" is useless. Every planet orbits with asteroids, and Neptune hasn't cleared its orbit of Pluto. Applying this concept literally could therefore exclude every planet in our solar system. A better term is gravitational dominance, the central characteristic of "uber planets" in Stern and Levison's study. However, there is no reason for gravitational dominance to be the determining factor regarding an object's planet status. It is certainly important but alone, it tells us nothing about what that object is.

"So Pluto is a KBO first and foremost though it's within a sub-class that includes the ones that look more like real planets."

Again, since you're repeating yourself, this is a value judgment. Why is Pluto a KBO first? Why is it not equally a KBO and a small planet? Both geophysical and dynamical characteristics are equally important. Maybe we do need to have two parallel planet definitions, a dynamic one and a geophysical one. They simply represent different ways of looking at the same objects.

No, Maju, let's leave the nationalism out of this ...

2009-08-09T14:56:06.571-07:00

No, Maju, let's leave the nationalism out of this discussion. I don't buy it. I don't believe the average American cares who discovered the bulk of the dwarf planets, any more than most of us care about the World Cup, other than as a quadrennial curiosity. There are nationalists, sure, but I am not one of them.

Well, I know that in some parts of the USA (but not anywhere else that I know of) there have been brainwashing activities directed to mobilize schoolchildren, who otherwise would not care less, to "save Pluto". In some states there have been initiatives to make sure that Pluto is officially a planet within the borders of that administrative division.

Obviously it has nothing to do with praising Brown and his discoveries of "planet Eris", "planet Haumea" and "planet Makemake" and possibly several others. They seem only concerned about Pluto, Pluto and nothing but little Pluto, which has been historically the only generally acknowledged planet discovered by a US citizen.

True that, under the current definition, only two planets have been really discovered in the Solar System, so there's no so much room for famous planet discoverers anyhow. And Neptune was hotly disputed already at the very time of its discovery.

So, IMO, there is some important nationalist ("patriotic") element in this discussion. Vagueofgodalming asked if there could be a cultural issue in this and that is the only one I can find.

Because I was raised watching a kids' program called "Destiny Pluto" and I really don't have any emotional attachments that make me react with any disgust because Pluto's destiny as lighthouse of our explorations has been fulfilled and is to be demoted to non-planetary status after all.

But maybe I could have some issues if the discoverer of Pluto was not some guy from Illinois but a Basque, the same I always react when some people claim that was Magallaes and not Elcano who first circunnavigated Earth.

So, yeah, I think these emotional issues can be of some importance and I can't but really admire and praise the strict scientific adherence of Brown and even Tombaugh (for what his widow says) in this aspect.

Since "dwarf planets" are not currently considered...

2009-08-09T14:40:11.773-07:00

Since "dwarf planets" are not currently considered to be real planets (just as moons are not), be careful what you ask for. They could come up with a new word for "dwarf planets" removing all obvious traces of being planet-like. We all know the term "plutoid" did not go over well even though the IAU was just completing the requirement of Resolution B6 by naming the category. I frankly think the name "dwarf planet" is a decent compromise.

I am more concerned with defining major moons than I am the status of Pluto.

-- Kevin Heider

The trouble with linguistic arguments from non-lin...

2009-08-09T09:14:10.221-07:00

The trouble with linguistic arguments from non-linguists is that they're rarely well-thought-through. "Dwarf planet" is no worse than "minor planet". For that matter, "American Dream" is also a compound built from an adjective and a noun -- the language is filled with them.

-Greg

No, Maju, let's leave the nationalism out of this ...

2009-08-09T08:15:05.782-07:00

No idea. I can see the difference re. rhubarb, bec...

2009-08-09T04:48:04.383-07:00

No idea. I can see the difference re. rhubarb, because nobody here would consider it "a fruit" (actually it's mostly considered an exotic herb and most would need to check a manual or encyclopaedia to find out what it is) but this kind of culinary differences are common (I live with an English and I know from daily experience).

But re. planets it would seem that there's no difference. Though I can understand that some people from the USA feel attached to Pluto's planethood for nationalist reasons - reasons that are not scientific and that the rest just do not care about.

Maju, it's worth noting that many of the people wh...

2009-08-09T03:50:19.753-07:00

Maju, it's worth noting that many of the people who argued for a dynamical definition at Prague are not native English speakers (some, indeed, being Spanish speakers, though from South America). I've often wondered what difference that makes to ones take on this issue.

Consider the word "fruit" it has a two-fold defini...

2009-08-09T00:09:47.499-07:00

Consider the word "fruit" it has a two-fold definition, there is fruit in the botanical sense and fruit in the culinary sense - a cucumber is a fruit in the botanical sense, but not in the culinary sense. Rhubarb is a fruit in the culinary sense, but not the botanical sense. A peach fits both types.

I'm having the feeling that this is a very cultural issue. My main language is Spanish and I would have never thought of rhubarb as a fruit in either sense (it's a veggie, like lettuce or cabbage). But guess we can agree for other pseudo-fruits like berries or figs or true fruits like the tomato. Whatever the case, what matters in this case, from a scientific viewpoint is the biological concept, obviously. Cuisine is a technology or an art but not really a science.

I also fail to see a clear "physical" definition that is strictly different from the "dynamic" one. Hydrostatic equilibrium can be seen as some sort of dynamics, while orbital characteristics can be perfectly considered physical attributes.

And finally there is a saliva economy matter, that may seem silly but is fundamental to language dynamics.

So a planet is what is fundamentally similar to the usual planets (Earth, Jupiter, etc.) in all aspects and something that is similar only in some aspects but not others is just not a planet but something else. We already have to resort to adjectives to mark the distinctions between the different types of planets: rocky, gas giants, etc. So please do not make things more complicated and if you need it, use some other word like "planetoid" for whatever category you wish (for instance: round things in orbit around a star, regardless of other no less important characteristics).

I still think the best way to go would be a two-pr...

2009-08-08T23:22:54.793-07:00

I still think the best way to go would be a two-pronged definition of planet, let the IAU concern with only the dynamical definition and let planetary scientist agree on a physical based definition -let both be equally valid, because both studies are important.

Consider the word "fruit" it has a two-fold definition, there is fruit in the botanical sense and fruit in the culinary sense - a cucumber is a fruit in the botanical sense, but not in the culinary sense. Rhubarb is a fruit in the culinary sense, but not the botanical sense. A peach fits both types.

The big 8 (Mercury - Neptune) are planets in a dynamical sense as well as in the physical sense. Io, Europa, Titan, the Moon, Pluto, Eris, Ceres, and Mimas are planets in the physical sense, but not in the dynamical sense - let the word "planet" be used either way - just like the word "fruit" could be used either way.

To help destinguish them:

Planet used in a dynamical sense could use words like dynoplanet, major planet, or dominant planet.

Planet used in a physical sense could use words like geoplanet, world, or planemo.

...the fact that dwarf planets are characterized a...

2009-08-08T21:29:39.334-07:00

...the fact that dwarf planets are characterized as not planets at all. This is a relatively simple thing to change, but it will resolve a lot of the problem here.

There's no real problem to be solved and would cause many others. Obviously the people who took that decision and who is reasonably satisfied with it, also have their reasons.

However, it's very clear that planetary bodies, like stars, come in a continuum from the smallest dwarf planets to sub-brown dwarfs with several Jupiter masses.

It depends of what you consider a "planetary body". For me they come in all sizes: from dust particles to black holes. You're focusing on one particular characteristic that fits your ideology (funny that Pluto has become an ideology for some, btw), which is that Pluto should be a planet no matter what and for that what fits best is the hydrostatic equilibrium and the fact that is orbiting the Sun, like Earth and Jupiter are. But would you choose other characteristics, like the having any sense of the plane of the ecliptic or the famous clearing the neighbourhood criterion, then you have to admit that Pluto is different and different in the specific way that all KBOs are, round or potato-shaped.

So Pluto is a KBO first and foremost though it's within a sub-class that includes the ones that look more like real planets.

"We call them (spherical) moons and dwarf planets....

2009-08-08T19:06:08.985-07:00

"We call them (spherical) moons and dwarf planets. This is not as bad as you make it out to be. We have had "dwarf planets" orbiting planets for years and the general public never complained."

Like I have said many times, my problem is not with the term "dwarf planets" but with the fact that dwarf planets are characterized as not planets at all. This is a relatively simple thing to change, but it will resolve a lot of the problem here.

I have to research Huya, as I am not familiar with its characteristics. However, it's very clear that planetary bodies, like stars, come in a continuum from the smallest dwarf planets to sub-brown dwarfs with several Jupiter masses.

The centaurs seem a lot more icy than the dwarf planets and a lot smaller, but again, we really need to send a probe to explore them in more depth. And there very well may be dwarf planets in unstable orbits, but Ceres, Pluto, Haumea, Makemake and Eris are not among them.

:there needs to be a subcategory of planets to enc...

2009-08-08T15:22:38.119-07:00

:there needs to be a subcategory of planets to encompass objects large enough to attain hydrostatic equilibrium but not large enough to gravitionally dominate their orbits.

We call them (spherical) moons and dwarf planets. This is not as bad as you make it out to be. We have had "dwarf planets" orbiting planets for years and the general public never complained.

:Just because dwarf planets share some common features with centaurs doesn't mean they belong in the same class.

The point is that the largest centaurs will look a lot like the smallest dwarf planets. How would dwarf planet candidate Huya compare to the largest centaurs?

:dwarf planets are likely to remain in their orbits for the duration of the solar system's life.

I suspect we will find some (non-resonant) dwarf planet candidates (perhaps with perihelion inside of Neptune) that are not stable. And yes, they may be on inclined orbits that take thousands of years, and thus may not even be discovered yet. Even some of the resonant objects are not in stable long-term orbits.

-- Kevin Heider

"Does Pluto need to be in the same category as the...

2009-08-08T13:25:17.668-07:00

"Does Pluto need to be in the same category as the "dominant 8 planets" simply because it is spherical even though it is made from the same mix of rock, dust,and frozen gases as the centaurs?"

The answer is there needs to be a subcategory of planets to encompass objects large enough to attain hydrostatic equilibrium but not large enough to gravitionally dominate their orbits.

Uranus and Neptune also have significant ice composition. They are grouped with Jupiter and Saturn even though Jupiter and Saturn are mostly composed of hydrogen and helium while Uranus and Neptune contain other gases. Uranus and Neptune may have some type of cores.

Earth and Pluto are both rocky, spherical, in hydrostatic equilibrium and differentiated.

Many objects in the solar system share at least some characteristics with objects that fall into different categories. Just because dwarf planets share some common features with centaurs doesn't mean they belong in the same class. As I already said, the orbits of centaurs are highly unstable, and these objects will not survive as sun-orbiting bodies for a long time astronomically. In contrast, dwarf planets are likely to remain in their orbits for the duration of the solar system's life.

It would be a great idea to send a flyby mission to Chiron and one or more of the centaurs. That would give us some really valuable information.

:Some centaurs very well may be half ice and half ...

2009-08-08T12:02:37.668-07:00

:Some centaurs very well may be half ice and half rock, but are any large enough to be in hydrostatic equilibrium, be geologically differentiated, and have an atmosphere?

Depends on your definition of Centaur. Using an extended definition (non-resonant object whose perihelion is less than the osculating semi-major axis of Neptune) the answer would likely be yes. But even using the classical definition Chariklo (around 258km) and 1995 SN55 (lost? and ~300km) may both be close to spherical and have temporary atmospheres. We do not know the mass of any centaur. Ironically the more ice a large centaur has more the more spherical it will likely be.

Does Pluto need to be in the same category as the "dominant 8 planets" simply because it is spherical even though it is made from the same mix of rock, dust,and frozen gases as the centaurs? This is part of the reason that it is good to have a category of objects between large asteroids and dominant planets. Moons are defined by where they are, so are "dwarf planets". The traditional difference between an asteroid (cis-Jupiter) and a centaur is location.

-- Kevin Heider

I've looked up data on the masses of Mercury, Plut...

2009-08-08T10:11:07.053-07:00

I've looked up data on the masses of Mercury, Pluto, and Chiron. It seems that the mass of Chiron is an estimate and not known to an exact figure.

Here are the mass comparisons:

Mercury - 3.3 x 10 to the power of 23 kg (Source: 9 Planets)
Pluto - 1.27 x 10 to the power of 22 kg (Source: 9 Planets)
Chiron - 1 x 10 to the power of 19 kg APPROX (Source: Wikipedia)

Mercury will of course be the most massive, as it is a rocky planet, and those are the most dense. Its diameter is 4,880 km.

Pluto's diameter is 2,274 km., and yes, 70 percent rock is an estimate; the actual composition will be known only when New Horizons flies by the planet. However, it is not half rock/half ice as are the centaurs. It may have formed in the same region as they did, but it developed differently, becoming large enough to be shaped by its own gravity and entering a stable orbit around the sun. Many of the centaurs have unstable orbits and will one day be ejected from the solar system altogether or crash into other bodies.

In size, Chiron is estimated to be between 180-258 km in diameter, and its composition is thought to be similar to that of a cometary nucleus, which is composed of rock, dust,and frozen gases.

Some centaurs very well may be half ice and half rock, but are any large enough to be in hydrostatic equilibrium, be geologically differentiated, and have an atmosphere?

Whether a system constitues a double planet depends on the size and mass of both objects in the system. If Earth were orbiting Jupiter, it would be a moon too, not a double planet. Two very large planets could also orbit a common barycenter between them if they are close to the same size. If Pluto orbited Jupiter, it still would be interesting based upon its composition and the possible presence on it of a subsurface ocean that could harbor microbial life.

Clearly, these are three unique objects, all very different from one another.

Mike, make sure you get a night out of town to app...

2009-08-07T02:54:57.440-07:00

Mike, make sure you get a night out of town to appreciate the southern sky!

Our moon is not a true planet and most people seem...

2009-08-06T16:39:25.129-07:00

Our moon is not a true planet and most people seem to be ok with that.

I am not. The Moon is obviously much more important for us than any of the other planets, anyone who looks at the sky or lives at a coast can tell you.

I also think she should be again considered a goddess (or god - at least in Japan), as in the good old times.

Furthermore I request to the IAU that geocentrism is considered to be equal in rights to heliocentrism. After all, Einstein dixit, all depends from the viewpoint of the observer.

Of course, I'm just joking. ;D

Seriously: nice array of factoids and good arguments, Kevin.

:No centaur is anywhere near as large as Pluto Pl...

2009-08-06T12:55:20.415-07:00

:No centaur is anywhere near as large as Pluto

Pluto is nowhere near as massive as Mercury (being only 1/25th). Considering you spend your time defending the protoplanets, I am surprised you would bring up size.

:Pluto is 70 percent rock; centaurs are mostly ice.

Pluto is estimated to be up to 70% rock. Given how many centaurs there are, I am confident many centaurs are close to half ice / half rock. Pluto and the centaurs did form in the same region of the solar system.

Pluto=Earth: spherical/differentiated. Pluto=Centaur-like: Pluto only has a temporary atmosphere when it is near perihelion, this is the same process that causes a coma around a centaur when it is near perihelion. / Both Pluto and the centaurs have orbits controlled by the gas giant(s).

The Pluto-Charon system being a true double planet is somewhat strange. This is only true if we define a "double planet" as a system where the primary (larger) body is gravitionally too weak to keep the barycenter inside of it (yet another "where it is rule"). If Charon orbited Jupiter it would be "significantly" less impressive than any of the Galilean moons.

Dwarf planets really can be considered to be immature protoplanets that are more massive than asteroids, but not large enough to dominate their region full of many similar sized objects. Our moon is not a true planet and most people seem to be ok with that.

-- Kevin Heider