Comments on Mike Brown's Planets: What is a dwarf planet?
So I guess as a dwarf I am a sub class of human? L...So I guess as a dwarf I am a sub class of human? Lol love humans need to put everything in a box...<br />
The Sun should be considered a planet because it i...The Sun should be considered a planet because it is in hydrostatic equilibrium. In the old times the wandering stars were the planets and the Sun was one of them, but today we know that all the stars move, so every single of them is really a wandering star. The Sun is also the center of our Solar System and dominates everything else near it. It's pretty much a planet, isn't it?<br /><br />
If you really think that hydrostatic equilibrium t...If you really think that hydrostatic equilibrium things (and nothing else matters) are planets and that you are not just one of those who only want to keep TNO Pluto as a planet because it makes them sad that it is not considered as a planet anymore, I think that maybe I could point out some things related to the hydrostatic equilibrium thing. I am not a scientist an tell me if I am wrong but I read about these things and I think that I have found some important informat and want to tell and ask some things.<br /><br />What about Vesta? It is is eologically differentiated and almost round but a bit flattened (This may because of the large impact on Vesta that flattened Vesta and created Rheasilvia crater on it some billion years ago.). And by the way was it so that Jupiter IV (Callisto) is hydrostatic equilibrium but only partially differentiated. And it is a body much more massive than that the too much famous TNO, Pluto. And Vesta has a structure similar to the terrestrial planets and it is smaller and less massive than Jupiter IV (Callisto) that is just only partially differentiated. <br /><br />If this more massive body is less differentiated than Vesta then what is so special wabout being hydrostatic equilibrium if Vesta that is not hydrostatic equilibrium can be much more nicely differentiated and geologically more diverse than some hydrostatic equilibrium objects like Saturn I (Mimas) for example? Why would you then include some dull ice ball like Saturn I (Mimas) (You said that in other post that you include satellites as planets and that the definition you have for the word "planet" is just that the object is hydrostatic equilibrium.) that Saturn I (Mimas) is as a planet but the terrestrial planet like Vesta that is way more geologically processed than Saturn I (Mimas) is not a planet. You talked about geological processes and then you just ingnore Vesta and I think that it is not fair. <br /><br />Anyway I also have something else to say about the hydrostatic equilibrium thing. You have said that your definition for the word "planet" does not have anything to do where the object is, but I have to say that maybe it actually does. Here is an example. <br /><br />Saturn I (Mimas) is hydrostatic equilibrium. It is composed mostly ice and only a bit of rock. It IS hydrostatic equilibrium and also mostly ice. It is just 396 km in diameter. Neptune VIII (Proteus) is larger (diameter 420 km) and more massive than Saturn I (Mimas) but it is NOT hydrostatic equilibrium. Both bodies Neptune VIII (Proteus) and Saturn I (Mimas) are objects made of ice and rock. Perhaps Saturn I (Mimas) is hydrostatic equilibrium only because of the the higher temperature near Saturn or tidal heating. <br /><br />So if it would be possible to transport Neptune VIII (Proteus) where Saturn I (Mimas) is right now then maybe the extra heat from the Sun or tidal forces of Saturn would heat up Neptune VIII (Proteus) so much that it would become hydrostatic equilibrium like the less massive Saturn I (Mimas). So if I am right about this, then the place where the body is actually DOES affect its physical characteristics. Then the statement that "The hydrostatic equilibrium planet definition has nothing to do with the place where the object that is considered a planet due to the definition." IS FALSE! <br /><br />So what do you think?<br /><br /><br />(And sorry if I do not write English so well because I am not the native English speaker. I post as anonymous because I do not have an account sorry.)
Do you know that all the asteroids and trans-Neptu...Do you know that all the asteroids and trans-Neptunians are called "minor planets"? And minor planets are not considered as actual "planets" by most people. For me it is not a problem to think minor planets to be at least some kind of planets but I think the IAU definition is good enough and how Mike Brown has explained it. By the way IAU said that the term "minor planet" may still be used but they prefer the term "small Solar system body" that includes minor planets other than dwarf planets and all the known comets. But the fact is that minor planets and comets are still catalogue by different way.<br /><br />(I am not a native English speaker so I'm sorry if I made some grammar errors or something like that. I post as anonymous because I have no account.)
If a dwarf planet is "something that looks li...If a dwarf planet is "something that looks like a planet, but is not a planet," then we need an additional attribute to divide "planet" from "moons" -- or else Titan, Triton, our moon, and several other bodies would also be dwarf planets.<br /><br />The main problem with the official IAU definition (a planet is an object that is in orbit around the Sun, has sufficient mass to assume hydrostatic equilibrium, and has gravitationally "cleared the neighborhood" around its orbit) is that it is entirely solar-centric. Objects in orbit around other stars, or floating separately between stars, could not be planets. <br /><br />The IAU definition is mostly about what an object does (it orbits the Sun and clears its neighborhood) and only secondarily about what it is (round). It seems to me we should have two different commonly-used classifications, one based on intrinsic properties, and a separate classification based on roles.<br /><br />As far as intrinsic properties, for objects larger than dust and rocks-- and the Sun -- there are three sorts of things in our solar system. There are Jovians (the four huge gas balls); there are the rocky Terrestrials (which can be subdivided into big round things, and flying mountains); and there are icy Plotoids (again, subdivided; big round ones, and smaller snowballs). <br /><br />For what they DO, there are planets (the IAU definition -- orbit the central star, and gravitationally dominate that orbit); asteroids (small objects that are part of groups -- Ceres, Juno, etc; also Chiron, the Trojans, and so on); moons (things that orbit non-stars); comets; Kuiper belt objects; Oort cloud objects. <br /><br />Just some random thoughts.
I think this is a good explanation of dwarf planet...I think this is a good explanation of dwarf planet
I think that definition of a planet is good if it ...I think that definition of a planet is good if it includes Pallas as a planet. Thats what means for me becus I think Pallas is the best world in the Solar System and I used to live on it.
Planets should only be classified by "relevan...Planets should only be classified by "relevant" criteria, i.e. size/volume and density. <br /> <br />First, size does matter because the ambiguous word "planet" to a lay person, is just a big thing out in space. Lay people don't care about whether is has cleared its neighborhood, whether its spherical (round) or where it is located.<br /> <br />To be considered a planet (in lay terms), an object should be at least 1,000 miles in diameter. Why 1k miles? Because there are too many objects both inside and outside of the Solar System with diameters less than 1k miles. <br /> <br />If an object is 100,000 miles in diameter or more, we could call it a mega planet.<br /> <br />If an object is 10,000-99,999 miles in diameter, we could call it a super planet.<br /> <br />If an object is 1,000-9,999 miles in diameter, we would call it a planet.<br /> <br />If an object is between 500 to 999 miles in diameter, we could call it a minor or dwarf planet.<br /> <br />If an object is between 100-499 miles in diameter, we could call it a planetoid.<br /> <br />For objects between 10-99 miles in diameter, we could call them planetesimals. <br /> <br />1-9 miles in diameter would be an asteroid.<br /> <br />Less than 1 mile in diameter, would be a meteoroid. <br /> <br />Next, density. Why density matters. Density matters because it relates to mass. Anything with a density less than that of water should be (re)classified (as in the case of Saturn) as a low density object because if you don't, you could run into problems with measuring the diameter of a planet, e.g. where does the planet's surface begin and its atmosphere begin. <br /> <br />If an object's composition was metal at its core (measuring 250 miles in diameter), but was covered with a liquid ocean (1 g/cu.cm) that if included in the object's diameter, would be say, 750 miles in diameter. Now, what if there was a thick atmosphere, with vaporous clouds of a varying from just under that of liquid water at the surface of water to just wispy clouds at the border with space.<br /> <br />If you measure the example object above from the core, it only terrestrial surface, the planet would measure 250 miles in diameter. If you measure from the surface of the ocean, the planet would be 750 miles in diameter. If you decided to include the atmosphere, the object could be the size of Saturn.<br /> <br />In fairness to Saturn, I am sure it has a surface in its mantle that has a density at least that of water <br />(1 g/cu.cm), but that is where the surface is and that is where the diameter should be measured from; everything above that should be referred to as atmosphere and not included in the planet's diameter measurement. <br /> <br />All other gas giants in our Solar System, have a density greater than that of water, so it is not a problem. <br /> <br />A planet is a planet is a planet...an object such as Ganymede, Titan and Callisto are examples of what could be referred to as "orbiting planets."<br /><br />Shape, c'mon, why does this matter? Hydrostatic equilibrium is not of my concern. An object that is not spherical (round), but takes an unusual or irregular shape such as a cube, blob, or a doughnut should not matter. <br /> <br />What we call or refer to, something does matter to a lot of people and there needs to be a reasonable conclusion to this issue. As it stands, a lot of people are just plain cosmically confused. <br /><br />George Smith
I agree great workI agree great work
My opinion on -how to classify something- is that ...My opinion on -how to classify something- is that a very early and basic question to the process is to ask 'Why do we want to classify it?' Once this question is answered one is well on the way to determining 'How to classify it'. If on the other hand, during an extended process this question is momentarily forgotten (as basic assumptions so often are) the answers to 'how' can start to get weird (because the consensus of 'why' can be lost).<br /><br />For instance: to a birdwatcher (or an ornithologist), a planet, dwarf planet, asteroid, or secondary planet might be classified as 'Stuff in Space' or 'Stuff that there's no birds on and therefore boring' even while maintaining a philosophy of strict scientific reasoning (though perhaps in a different area).<br />work with me on this.<br />A future intra-solar co-pilot might say, "Hank? Is that a planetoid or an asteroid?"<br />"BillyBob," the pilot may say, "that's a big hunka sumtin hard ya don't wanna run into."<br /><br />Fair enough.<br />How some thing is classified is only as important as the consensus -among those communicating with each other- on Why it should be classified. (e.g. no birds on it / don't hit it going 50,000km/h)<br />Bizarre, blatant, 'duh' examples. Nevertheless I feel they demonstrate one of the subtle swerves of extended discussions. I have to clearly re-check if I'm still using my original basic premises. (premisii?)<br /><br />So tell us, why do you want them clarified?<br />When we reach a consensus on that, we will (probably) reach a consensus on how.<br /><br />TR
Hi, I think I might be at some risk of moderation ...Hi, I think I might be at some risk of moderation here...but I don't know where else to put this.<BR/><BR/>I think that the albedos of the objects which are in hydrostatic equilibrium has been seriously underestimated. Once an object gets to be that size the ices should float to the top, which would considerably increase the albedo.<BR/><BR/>Often there has been an assumption of albedos as low as .03 or .05. Most of these have been later corrected to between .10 and .15.<BR/><BR/>Calculation of diameters by the infrared method and other methods like PSF yield different results. Eris' diameter was overestimated by the thermal method, but note that Quaoar's thermal estimate was underestimated!<BR/><BR/>We get more or less of a continuum of sizes up to 400 km, then there appears to be a gap with the larger objects clustering in a larger catagory. But, if the larger and therefore differentiated objects have a higher albedo and are actually in fact smaller than the estimates, then the sizes of these objects fit better into a continuum of masses. So we have an artificial gap between objects smaller than 400 km and those larger. But if the larger objects were more reflective, that gap closes and a new gap appears between the larger dwarf planets and Haumea.<BR/><BR/>Unfortunately we are balancing on the edge between science and speculation, with one branch of science saying one thing and another branch saying another. The smaller dwarf planets are, frustratingly, just barely out of reach of the Spitzer Observatory. Maybe we'll get better answers out of the James Webb Observatory, if we could wrestle some observing time from the cosmologists. <BR/><BR/>I think we need a new observatory. Spitzer was downsized because of the Challenger disaster. Maybe we could book a flight on the new Ares V.
If I might ask, how should ex-round objects be cla...If I might ask, how should ex-round objects be classified?<BR/><BR/>For example, we can deduce that Vesta was once round, because we know that it melted, and liquid objects will end up in hydrostatic equilibrium. But since that time, it has solidified. As a result, it is now strong enough to not collapse when impacts knock large pieces off.<BR/><BR/>All 50 or so iron meteorite parent bodies should fall into the same category of ex-round objects. <BR/><BR/>When speaking out of earshot of astronomers, geochemists tell each other that we have crustal samples from four planets: Earth, Mars, Moon, and Vesta.
C W Mageehttps://email@example.com:blogger.com,1999:blog-9094742788006644220.post-76948389066542506652008-09-30T08_10_00.000-07_002008-09-30T08_10_00.000-07_00
OK everyone. I declare the comment section here do...OK everyone. I declare the comment section here done. Final word (well, ok, final here, at least) on Pluto and planets and dwarf planets:<BR/><BR/>1) 8 planets + a different category of small round things makes perfect scientific sense as a classification.<BR/><BR/>2) ~200 things in hydrostatic equilibrium as planets makes perfect scientific sense as a classification.<BR/><BR/>3) the preference between (1) and (2) has nothing to do with science and is merely aesthetic. Or perhaps religious. <BR/><BR/>4) It is difficult to convince someone who has a different religion that they are wrong and you are right (but wow it is tempting to try, isn't it?).<BR/><BR/>5) None of us knows what future generations may decide.<BR/><BR/>6) I will invoke my prerogative as moderator to remove further comments on this thread.
Public response has actually been overwhelmingly i...<I>Public response has actually been overwhelmingly in favor of keeping Pluto as a planet.</I><BR/><BR/>Where? In Arizona or in, say, Uganda? Remember that this is a global matter and that is why it was discussed in the IAU and not in the NASA. The US population ammounts to only 5% of the total Humankind. <BR/><BR/><I>I really don't undertand the statement that learning about an ever growing number of planets is "counter-intuitive about what is a planet."</I><BR/><BR/>Orbiting the Sun, ruling its orbit...<BR/><BR/>Similar to the six classical planets, in other words. <BR/><BR/><I>The argument that the desire to count dwarf planets as planet is driven by nationalism or by "emotional needs" of proponents is the equivalent of an ad hominem attack.</I><BR/><BR/>It is not and I am quite tired of that Gringo attitude of all the universe allegedly orbiting Iowa... It's not good for you either, anyhow: it makes you blind and unsensible. <BR/><BR/>You probably also take as an "ad hominem attack" that I do not understand what an ounce may be or that I understand that America includes not just the USA but also many other countries like Brazil, Mexico or Colombia (just to mention some of the largest ones). <BR/><BR/>So yes, there is a cultural issue here and it is one of US-centrism. Travel more, ask the Chinese, the Indians, the Brazilians, the Europeans, the Africans... what they think about Pluto. <BR/><BR/><I>In fact, one could reasonably argue that the demotion of Pluto--and by extension of dwarf planets--from planet status was motivated by anti-American sentiment at the IAU General Assembly because many American astronomers are planetary scientists, and all the dwarf planets were discovered by Americans. </I><BR/><BR/>Doubt it. Many US astronomers were in favor of a reasonable agreement like the one achieved. Our host, Mike Brown, for example has not been the least militant in favor of keeping Pluto as "planet" and all the opinions I can read here in favor of that seem to come from people who are not astronomers anyhow. <BR/><BR/>But if you want to percieve it as "anti-American" attack... feel free. It's like considering the demotion of Ceres in the 19th century as an anti-Italian attack, which it was not. Both "planets" were properly demoted as evidence mounted up against planetary status. In few decades this whole issue will be forgotten most probably anyhow. <BR/><BR/>This does not mean that new discoveries are not exciting. Not at all: the Kuiper Belt is most interesting and even more the ill-explored area beyond it. But we do not need to call them "planets" improperly to make all that exciting and intriguing.
Kids--and adults--actually find the new discoverie...<I>Kids--and adults--actually find the new discoveries and increasing numbers exciting, an expansion of knowledge and awareness.</I><BR/><BR/>That's certainly been true for me. I've been following Mike's discoveries this decade with a considerable amount of glee, adding each new "dwarf planet" to my mental list as it's been officially certified, and trying to remember as many of the unofficial dwarfs (Quaoar, Orcus, Varuna, Ixion, Sedna) as my tired and sagging brain can stand. Other than far-distant Sedna, I doubt I could get the order right, but no doubt some kids could do it.<BR/><BR/>When I was nine, I had no trouble memorizing the then-understood nine planets, and didn't need any mnemonics. In fact, for me the planets WERE the mnemonics. I doubt thirteen or even twenty "planets" would have fazed me a bit.<BR/><BR/>Why, oh why, didn't I get into astronomy instead of mathematics and computers?<BR/><BR/>Bob Shepard
"...and all the dwarf planets were discovered by A..."...and all the dwarf planets were discovered by Americans."<BR/><BR/>Oops, mistake here. That should read all the dwarf planets except Ceres.
"Also there is a cultural demand for simplicity. A..."Also there is a cultural demand for simplicity. And I'm pretty sure that most people is much more confortable thinking that objects like Pluto are not planets and that the number of planets in the Solar System is well defined and limited (with all reserves for outer system possible findings). This is specially true for school kids, who are much better off learning about the 8 planets than about an unknown and always growing number of them that certainly would make little sense for their minds, specially as it is counter-intuitive about what is a planet."<BR/><BR/>I wholeheartedly disagree with this statement, which is solely a matter of opinion. Assuming people prefer "simplicity" and would rather learn about only eight planets to me seems a lot like "dumbing down" the curriculum. We don't see anyone saying limit the number of elements in the periodic table because there are too many to memorize. What scientific sense does it make to impose an essentially artificial limitation on the number of planets taught?<BR/><BR/>Public response has actually been overwhelmingly in favor of keeping Pluto as a planet. This is evident in polls taken over the last two years, comments expressed on web sites, and even sales of T-shirts and other pro-Pluto memorabilia, which after two years are still selling well online.<BR/><BR/>I think it is underestimating kids to assume they're more comfortable learning about a fixed number of planets than a constantly growing number based on ongoing discoveries. Kids--and adults--actually find the new discoveries and increasing numbers exciting, an expansion of knowledge and awareness. They're going to be taught that the number of exoplanets is constantly in flux as we discover more. Why assume they can't understand this about our solar system? My nephew is five, and he finds the ever growing number of planets in our solar system exciting and is not confused by it in the least.<BR/><BR/>I really don't undertand the statement that learning about an ever growing number of planets is "counter-intuitive about what is a planet." <BR/><BR/>The argument that the desire to count dwarf planets as planet is driven by nationalism or by "emotional needs" of proponents is the equivalent of an ad hominem attack. Supporters of the dynamical definition love to repeat the claim that objection to dwarf planets not being considered planets is an American issue. This is simply not the case, as popular reaction around the world to the IAU definition was largely negative, as can be seen from numerous Internet comments and web sites. In fact, one could reasonably argue that the demotion of Pluto--and by extension of dwarf planets--from planet status was motivated by anti-American sentiment at the IAU General Assembly because many American astronomers are planetary scientists, and all the dwarf planets were discovered by Americans.
It will be interesting, in the coming years, to se...<I>It will be interesting, in the coming years, to see how the public comes to grip with our new understanding of the solar system.</I><BR/><BR/>Simple: 8 planets (4 rocky and inner, 4 gaseous and outer), plus the asteroideal belt, plus the Kuiper belt, plus the mostly unknown outer reaches.
Then some satellites would be "planets".Yes, that'...<I>Then some satellites would be "planets".</I><BR/><BR/>Yes, that's exactly what I called them: "secondary planets".<BR/><BR/>As for nationalism, that's all part of the cultural differences we have. I don't think we're all ever going to agree fully.<BR/><BR/>It will be interesting, in the coming years, to see how the public comes to grip with our new understanding of the solar system.<BR/><BR/>For myself, I'm looking forward to what else is discovered in the outer solar system. And frankly, I don't care if the discoveries are made by Americans, Spaniards or the Chinese.<BR/><BR/>Bob Shepard
True, if you're thinking purely in terms of dynami...<I>True, if you're thinking purely in terms of dynamics. The crux of the controversy is that planetary geologists are more interested in the composition of the object.</I><BR/><BR/>Then some satellites would be "planets". This would be counter-intuitive because we have so far (or at least in the last centuries) thought of planets as orbiting the Sun. And we even demoted Ceres when we found it was part of a larger population... <BR/><BR/>We (well astronomers mostly, I'm just audience in this show) are working on cultural precedents as well as on scientifical criteria when dealing with this issue. <BR/><BR/>Also there is a cultural demand for simplicity. And I'm pretty sure that most people is much more confortable thinking that objects like Pluto are not planets and that the number of planets in the Solar System is well defined and limited (with all reserves for outer system possible findings). This is specially true for school kids, who are much better off learning about the 8 planets than about an unknown and always growing number of them that certainly would make little sense for their minds, specially as it is counter-intuitive about what is a planet. <BR/><BR/>This may actually be more about how many "planets" would have been discovered by US astronomers: zero by the current standards or at least four (three of them by the host of this discussion, btw). In fact this was largely the unspoken issue surrounding Plutos' reclassification: a matter of nationalism. <BR/><BR/>The reality is that most of the 20th and 21st century exploration of the Solar System has been done by US scientists but, sadly for them, no typical new planets have surfaced in this phase (so far at least). But these are not the Olympic Games and it is not about gold medals; it is much more important: it is scientific research and the exploration of he Kuiper Belt and beyond is no petty matter and it is something I am sure that Mike Brown and other US astronomers are very proud of. No new planets so far? Well, it's probably not so important after all, what matters is knowledge, not medals (though I do think they deserve many medals, the Nobel maybe, for all this expansion of human knowledege). <BR/><BR/>What doesn't make any sense is to alter the meaning of words, to confuse children and the general public, just to meet the nationalist emotional "needs" (caprices) of a handful of outspoken people. Ditto.
each of the planets is unique in so many ways that...<I>each of the planets is unique in so many ways that any attempt to set a definition of what a "normal" planet is will impose artificially narrow constraints.</I><BR/><BR/>This is part of my concern, what do we do if we learn that the larger bodies like Triton, Pluto, Eris and perhaps other dwarf planets all are basically the same? Would any of them be unique or would they be part of a group? And at what size (mass) do true internal geological processes occur? Even comets outgas from their porous surface mimicking Triton's nitrogen geysers. So does Pluto have more in common with Earth or with a <A HREF="http://en.wikipedia.org/wiki/Centaur_(asteroid)" REL="nofollow">centaur</A>?<BR/>-- Kevin Heider
Claiming that they are normal planets like Venus o...<I>Claiming that they are normal planets like Venus or Jupiter is totally against common sense.</I><BR/><BR/>True, if you're thinking purely in terms of dynamics. The crux of the controversy is that planetary geologists are more interested in the composition of the object. As Laurel points out, Earth probably does have more in common with Pluto, Ceres, etc. than with Jupiter. For one thing, the former are all primarily solid, unlike gassy Jupiter.<BR/><BR/>That's why I like Elmar's idea of having multiple categories of "planet" which can be combined in any way that makes sense. Pluto can be a "non-dominant icy dwarf planet", while Earth is a "dominant terrestrial planet". Large moons could be "secondary planets". The descriptions combine elements of both dynamics and planetary geology.<BR/><BR/>As for which "planets" should be on the list of bodies schoolchildren memorize, I don't think we're all going to be able to agree on that. I'm not even sure what my own opinion is at this point.<BR/><BR/>If someone were to ask me "how many planets are there in the solar system?", my response would probably be: "Well, officially there are eight regular planets and five dwarfs, for a total of thirteen. But unofficially, all bets are off. There could be hundreds. What exactly is a planet, anyway?"<BR/><BR/>No doubt it wouldn't be a very satisfying answer. People like certainties in their lives.<BR/><BR/>I suppose that's part of the messy learning process we call "science". How do we reconcile that with cultural notions of planethood? Maybe we won't.<BR/><BR/>Bob Shepard
Ceres, Pluto and Eris (as well as "Haumea and Make...Ceres, Pluto and Eris (as well as "Haumea and Makemake) share their area with many many other objects, often comparable in size. In fact they belong to specific non-planetary populations such as the Asteroideal Belt and the Kuiper Belt. Claiming that they are normal planets like Venus or Jupiter is totally against common sense."<BR/><BR/>Just what is a "normal planet," anyway? Earth is very different from Jupiter; in fact, one could argue that Earth has more in common with Pluto than with Jupiter. Each of the planets is unique in so many ways that any attempt to set a definition of what a "normal" planet is will impose artificially narrow constraints. Claiming that the dwarf planets belong in the same category as non-spherical asteroids and KBOs is equally against common sense, as it totally discounts the former being in hydrostatic equilibrium and having the resulting geophysical processes. Clearly we are talking here about three categories, not two.
The popular press frequently refers to Eris being ...<I>The popular press frequently refers to Eris being a "neighbor" to Pluto, but in terms of Astronomical Units, Earth is actually much closer to Pluto than Eris is.</I><BR/><BR/>Obvously Eris is not the reason why Pluto has not cleared the neighbourhood: there are hundreds, probably thousands of other objects much closer to Pluto...<BR/><BR/>The basic concept anyhow is the orbital area clear of objects that are even remotely comparable in size with the main one? (a) No. It's a planet. (b) Yes. It is not.<BR/><BR/><I>For instance, what exactly does it mean to "clear the neighborhood" for an object with an orbital period of over 10,000 years? How close does another object have to get to be considered a "neighbor"?</I><BR/><BR/>AFAIK it's the orbital zone. Are there other comparable objects crossing into that orbit or close enough? You can easily see that traditional planets, plus Uranus and Neptune, do not have any problem with that. They have satellites maybe, they have small "trojan" (or whatever) asteroids hanging around... but that's all. <BR/><BR/>Ceres, Pluto and Eris (as well as Haumea and Makemake) share their area with many many other objects, often comparable in size. In fact they belong to specific non-planetary populations such as the Asteroideal Belt and the Kuiper Belt. Claiming that they are normal planets like Venus or Jupiter is totally against common sense.
If they insist on using dynamics to classify "dwar...If they insist on using dynamics to classify "dwarf planets", I imagine it will indeed be a very long time before they can decide what category Sedna belongs to. <BR/><BR/>For instance, what exactly does it mean to "clear the neighborhood" for an object with an orbital period of over 10,000 years? How close does another object have to get to be considered a "neighbor"?<BR/><BR/>The popular press frequently refers to Eris being a "neighbor" to Pluto, but in terms of Astronomical Units, Earth is actually much closer to Pluto than Eris is. Does that make Earth Pluto's "neighbor"? (That's a rhetorical question.)<BR/><BR/>Bob Shepard