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dohbot
2009-Jul-06, 03:53 PM
is it possible that there are more planets beyond pluto? maybe hundreds more and the astronomers can't see them because it's so dark out there.

Amber Robot
2009-Jul-06, 04:09 PM
Define "planet".

Oops... I shouldn't open that can of worms, eh? :whistle:

Nick Theodorakis
2009-Jul-06, 04:27 PM
Side-stepping the contentious issue about what a "planet" is, I will point the OP to this article on Trans-Neptunian Objects (http://www.johnstonsarchive.net/astro/tnos.html).

Nick

Edit to add: Welcome to the board

Amber Robot
2009-Jul-06, 04:45 PM
Yes, there are many objects out there that are similar to Pluto. They are difficult to find because, yes, "it is dark", i.e., sunlight is weak so distant from the sun, so the objects appear dim when the reflected light makes its way back to us, and also because they move slowly, so it's a little more difficult to spot them out from the stars unless you're specifically looking for them (which some people do).

nauthiz
2009-Jul-06, 04:54 PM
Here's a list of the objects currently classified as dwarf planets by the IAU and their approximate masses:

Pluto: 1.3*1022 kg
Ceres (not trans-Neptunian): 9.4*1020 kg
Eris: 1.7*1022 kg
Haumea: 4*1021 kg
Makemake: 4*1021 kg

I went ahead and included Ceres in the list because of its history: It was the first object in the asteroid belt to be discovered, but for a long time it was classified as a planet along with the 2nd, 3rd, and 4th asteroid belt objects to be discovered. After we started learning more about the asteroid belt, it ended up being reclassified as an asteroid. When the IAU opened the debate on Pluto, the question of restoring it to planet status was on the table. The question hinged on the "clear its neighborhood" clause, just like for Pluto, so if the IAU had adopted a definition that allowed Pluto to retain its planet classification, it also would have become the 10th planet because Ceres would have become the 5th planet under that definition. Instead, though, objects that don't clear their neighborhood are classified as dwarf planets, and both Pluto and Ceres acquired that designation, along with three other objects.

One of them, Eris, is both bigger and farther out than Pluto. So depending on your definition of planet, there is at least one known object out there that qualifies as a planet beyond Pluto. On the other hand, if you agree with the IAU's definition then it's probably very unlikely that there are any unknown planets beyond Neptune, simply because in that case we presumably wouldn't expect the Kuiper belt to be quite so messy on account of that "big enough to clear the neighborhood" clause.

Fiery Phoenix
2009-Jul-06, 05:22 PM
In short, yes. However, they aren't planets, but dwarf planets (or planetoids). The major planets are eight, with Neptune being the furthest.

chornedsnorkack
2009-Jul-06, 06:08 PM
Here's a list of the objects currently classified as dwarf planets by the IAU and their approximate masses:

Pluto: 1.3*1022 kg
Ceres (not trans-Neptunian): 9.4*1020 kg
Eris: 1.7*1022 kg
Haumea: 4*1021 kg
Makemake: 4*1021 kg

I went ahead and included Ceres in the list because of its history: It was the first object in the asteroid belt to be discovered, but for a long time it was classified as a planet along with the 2nd, 3rd, and 4th asteroid belt objects to be discovered. After we started learning more about the asteroid belt, it ended up being reclassified as an asteroid. When the IAU opened the debate on Pluto, the question of restoring it to planet status was on the table. The question hinged on the "clear its neighborhood" clause, just like for Pluto, so if the IAU had adopted a definition that allowed Pluto to retain its planet classification, it also would have become the 10th planet because Ceres would have become the 5th planet under that definition. Instead, though, objects that don't clear their neighborhood are classified as dwarf planets, and both Pluto and Ceres acquired that designation, along with three other objects.

One of them, Eris, is both bigger and farther out than Pluto. So depending on your definition of planet, there is at least one known object out there that qualifies as a planet beyond Pluto. On the other hand, if you agree with the IAU's definition then it's probably very unlikely that there are any unknown planets beyond Neptune, simply because in that case we presumably wouldn't expect the Kuiper belt to be quite so messy on account of that "big enough to clear the neighborhood" clause.

The main asteroid belt slots between accepted planets Jupiter and Mars, both of which have "cleared their neighbourhood"... but there are Mars-crossers as well as Jupiter-crossers. Neptune has "cleared its neighbourhood" at 30 au, but Pluto and plutinoes at 40 au cross Neptune. If an object bigger than Eris were seen beyond 80 au, how much of the neighbourhood must be checked for objects dimmer than it or currently near aphelion, to prove that a suspected planet has not cleared a neighbourhood?

If we can only now barely see Eris at 90 au, we could barely see Earth at 200 au, Neptune at 400 au, or Jupiter at 750 au. On the other hand, we clearly could see Proxima Centauri at 15 000 au, even by naked eye, so we at least know there are no other stars in Solar System (not sure how bright a black hole accretion disc would be in the Local Fluff...). How hot could a brown dwarf be before it should have been seen at 10 000 au?

thoth II
2009-Jul-06, 06:43 PM
I think this might also get into the astrophysics of how planets were created and how likely on the theoretical models that any object larger than Mercury could have been formed beyond Pluto. I wouldn't guess too likely, even though that isn't the criteria for planets, per IAU.

I doubt you'll ever hear about a "ninth planet" has been discovered and agreed on by the IAU.

Fiery Phoenix
2009-Jul-06, 06:55 PM
I think this might also get into the astrophysics of how planets were created and how likely on the theoretical models that any object larger than Mercury could have been formed beyond Pluto. I wouldn't guess too likely, even though that isn't the criteria for planets, per IAU.

I doubt you'll ever hear about a "ninth planet" has been discovered and agreed on by the IAU.

If the Sun were at least five times its current mass, the existence of major planets much further away than Pluto would be more likely -- if you know what I mean. I personally don't think there is an actual planet beyond Neptune, given the Sun's current mass and basic physical properties. The Solar System seems just right at this point of our understanding of it. Who knows, though.

kleindoofy
2009-Jul-06, 07:00 PM
... classified as dwarf planets ...

Pluto: 1.3*1022 kg
Ceres (not trans-Neptunian): 9.4*1020 kg
Eris: 1.7*1022 kg
Haumea: 4*1021 kg
Makemake: 4*1021 kg
...
Where does Sedna (& Co.) count in, resp. how is it classified?

thoth II
2009-Jul-06, 07:03 PM
I think Sedna is in the Oort cloud comet horde

nauthiz
2009-Jul-06, 07:20 PM
Where does Sedna (& Co.) count in, resp. how is it classified?

According to Wiki, Sedna is a candidate but we don't know enough about it to be sure if it's in hydrostatic equilibrium (ie, round) and it wouldn't be officially classified as a dwarf planet unless someone were to confirm that it is.

Hop_David
2009-Jul-06, 07:48 PM
I think this might also get into the astrophysics of how planets were created and how likely on the theoretical models that any object larger than Mercury could have been formed beyond Pluto. I wouldn't guess too likely, even though that isn't the criteria for planets, per IAU.

I doubt you'll ever hear about a "ninth planet" has been discovered and agreed on by the IAU.

I believe a big planet would be detectable from the influence of its gravity on the Kuiper Belt objects and Neptune. Unless it's a good ways out. I would imagine there could be objects orbiting the sun as far away as 1 or 2 light years. I wouldn't even be surprised if there were a brown dwarf within a sphere of that radius. I'm hoping WISE (http://wise.ssl.berkeley.edu/mission.html) will discover some previously invisible objects.

Rhaedas
2009-Jul-06, 08:01 PM
But...we can't say for certain there are absolutely no "planets" out there that haven't been found, just that we have no evidence of anything large enough to meet the current definition.

But it's safe to say there's a lot of other bodies out there.

ngc3314
2009-Jul-06, 08:22 PM
As usual, I throw in some nits:

- We can't "just barely see" Eris. It's 19th magnitude, which many amateurs have been able to image once they knew where to look. (Finding such things over a wide area has been the problem).

- Proxima Centauri is not a naked-eye object by any human's definition, at 11th magnitude. I can't speak for large-eyed cephalopods, or beings with better IR sensitivity.

chornedsnorkack
2009-Jul-06, 09:17 PM
- Proxima Centauri is not a naked-eye object by any human's definition, at 11th magnitude. I can't speak for large-eyed cephalopods, or beings with better IR sensitivity.
Sorry, loose wording. We could see it if it were at 15 000 AU (it would be magnitude 5 there). We cannot see it where it is, at 260 000 AU because at that distance it is magnitude 11.

Since an object at 15 000 AU and bound to the Sun the way Proxima is thought to be bound to Toliman could be fairly described as inside the Solar System, any consideration of more planets or stars in Solar System should include not just visibility of planets at 100 AU, but at 1000 and 10 000 AU. Which is especially dim for something that is only reflecting light.

If the 19th magnitude Eris is supposedly so easy to see, how come no one spotted the 2000 second parallax?

iquestor
2009-Jul-06, 09:43 PM
Question Here then as this discussion relates to New Horizons:

New Horizons may be extended to visit a KBO if one is identified within it's ranging area after it leaves the Pluto system. So, how will NASA determine if there is a KBO out there that qualifies? Can New Horizons detect these objects (and possibly a 10th planet) or will it rely on us to tell it there is a new target it can visit on its way to eternity?

Boxes
2009-Jul-07, 12:11 AM
I always wondered about the orbital clearing clause. Would the Earth have been able to clear an orbit at Pluto's distance. How far out could an Earth size body be and count as a full planet?

thoth II
2009-Jul-07, 12:38 AM
I always wondered about the orbital clearing clause. Would the Earth have been able to clear an orbit at Pluto's distance. How far out could an Earth size body be and count as a full planet?

I think that could go either way. On the one hand, earth's gravity would be able to clear its orbit; but on the other, the orbital period would be 300 years and in that time many stray KBOs might wander into earth's orbit. So when you get into non-stable orbits , I'm not sure they have taken that into account in the definition.

nauthiz
2009-Jul-07, 01:16 AM
There isn't a hard number attached to the third criterion, but there is a proposed measurement of the degree of cleanliness in a planet's orbital zone, described here (http://en.wikipedia.org/wiki/Cleared_the_neighbourhood#In_the_Solar_System) at Wikipeda, along with a table of the values for various solar system bodies. They also describe a metric, lambda, that approximates a planet's ability to clear its neighborhood which relates to both its mass and its orbital period.

The table shows a clear discontinuity several orders of magnitude wide between the planets and the dwarf planets for both measures. There's also a proposal that a rough cutoff point for when an object could "clear its neighborhood" (thereby qualifying as a planet) would be when lambda equals 1. According to that discriminant, Pluto wouldn't be able to do so unless it were in around Venus's orbit - assuming of course that Venus weren't there. Earth, on the other hand, would still have a lambda > 1 if it were 70 times further out than Pluto.

Boxes
2009-Jul-07, 01:38 AM
There isn't a hard number attached to the third criterion, but there is a proposed measurement of the degree of cleanliness in a planet's orbital zone, described here (http://en.wikipedia.org/wiki/Cleared_the_neighbourhood#In_the_Solar_System) at Wikipeda, along with a table of the values for various solar system bodies. They also describe a metric, lambda, that approximates a planet's ability to clear its neighborhood which relates to both its mass and its orbital period.

The table shows a clear discontinuity several orders of magnitude wide between the planets and the dwarf planets for both measures. There's also a proposal that a rough cutoff point for when an object could "clear its neighborhood" (thereby qualifying as a planet) would be when lambda equals 1. According to that discriminant, Pluto wouldn't be able to do so unless it were in around Venus's orbit - assuming of course that Venus weren't there. Earth, on the other hand, would still have a lambda > 1 if it were 70 times further out than Pluto.

Thanks for the link! It makes the neighborhood clause more palatable.:)

laurele
2009-Jul-07, 05:00 AM
I think that could go either way. On the one hand, earth's gravity would be able to clear its orbit; but on the other, the orbital period would be 300 years and in that time many stray KBOs might wander into earth's orbit. So when you get into non-stable orbits , I'm not sure they have taken that into account in the definition.

No, Earth would not "clear its orbit" at Pluto's distance. Dr. Alan Stern and Dr. Hal Levison did the calculations for this and determined definitively it would not. That is why the IAU definition is so bad. It takes the same object and makes it a planet in one location and not a planet in another.

laurele
2009-Jul-07, 05:14 AM
In short, yes. However, they aren't planets, but dwarf planets (or planetoids). The major planets are eight, with Neptune being the furthest.

And this makes absolutely no sense, either linguistically or astronomically. Dwarf stars are still stars, and dwarf galaxies are still galaxies. Clearly, the term dwarf is used in astronomy as an adjective modifying a noun.

The major planets are far more than eight because dwarf planets should be considered a subset of planets. Using the criterion of hydrostatic equilibrium as the threshold for planethood, one can estimate there may be many small planets beyond Pluto in the Kuiper Belt.

Ceres' demotion was actually a mistake. In the mid 19th century, no one knew that Ceres, unlike the other asteroids in the asteroid belt, is round. That means it is shaped by gravity rather than by chemical bonds, a characteristic of planets and not of shapeless asteroids.

Dr. Alan Stern and Dr. Hal Levison discuss the issue of orbital dominance in this article: http://www.boulder.swri.edu/~hal/planet_def.html . Stern is actually the one who coined the term "dwarf planet." Notice that Stern and Levison discuss "uber planets," those which are gravitationally dominant in their orbits, and "unter planets," those which are not gravitationally dominant. However, they never say that the "unter planets" should not be considered planets at all.

We need to move away from viewing the IAU as purveyors of gospel truth, which they most certainly are not. Their definition is just one opinion in an ongoing debate.

Fiery Phoenix
2009-Jul-07, 05:25 AM
And this makes absolutely no sense, either linguistically or astronomically. Dwarf stars are still stars, and dwarf galaxies are still galaxies. Clearly, the term dwarf is used in astronomy as an adjective modifying a noun.

The major planets are far more than eight because dwarf planets should be considered a subset of planets. Using the criterion of hydrostatic equilibrium as the threshold for planethood, one can estimate there may be many small planets beyond Pluto in the Kuiper Belt.

Ceres' demotion was actually a mistake. In the mid 19th century, no one knew that Ceres, unlike the other asteroids in the asteroid belt, is round. That means it is shaped by gravity rather than by chemical bonds, a characteristic of planets and not of shapeless asteroids.

Dr. Alan Stern and Dr. Hal Levison discuss the issue of orbital dominance in this article: http://www.boulder.swri.edu/~hal/planet_def.html . Stern is actually the one who coined the term "dwarf planet." Notice that Stern and Levison discuss "uber planets," those which are gravitationally dominant in their orbits, and "unter planets," those which are not gravitationally dominant. However, they never say that the "unter planets" should not be considered planets at all.

We need to move away from viewing the IAU as purveyors of gospel truth, which they most certainly are not. Their definition is just one opinion in an ongoing debate.

There are three major characteristics defining a planet:

1) It's in orbit around the Sun.
2) It's round (i.e. has achieved hydrostatic equilibrium.)
3) It has cleared its orbit.

The 3rd characteristic is where these so-called dwarf planets fail. So, it seems fair enough for, for example, Ceres (which is located in the middle of the Asteroid Belt) to be classified as a dwarf planet.

I do agree that the term dwarf planet is somewhat misleading, though -- if that's the right word. But it's the IAU who made the decision in the end. We really can't do anything about it. Maybe just a rename of those bodies would be better.

nauthiz
2009-Jul-07, 06:11 AM
And this makes absolutely no sense, either linguistically or astronomically. Dwarf stars are still stars, and dwarf galaxies are still galaxies. Clearly, the term dwarf is used in astronomy as an adjective modifying a noun.

Unfortunately, prescriptive linguistics doesn't really work for interpreting jargon, and in this particular case the root term doesn't end up being the name for the superclass the way it does in most the rest of astronomical taxonomy. The name of the class to which planets, dwarf planets, asteroids (aka planetoids), comets, etc. fall into is called planetary objects.

Jens
2009-Jul-07, 06:41 AM
Just to be a bit fatalistic about this, I'm sure if the harm will really be very great anyway. The only real effect of the planetary definition will be (I think) that children will be exposed to a certain idea through museums and classes and elementary astronomy books. But anybody who becomes serious about astronomy will quickly come to understand that these are difficult definitions at best. Whether Pluto is included in the museum displays is not really all that consequential: I doubt that the Plutonians are even aware of it. At the very least, they haven't yet sent an invasion fleet. The disadvantage of having lots of new planets is that elementary school children would have to memorize a long list for tests.

Fiery Phoenix
2009-Jul-07, 02:33 PM
Just to be a bit fatalistic about this, I'm sure if the harm will really be very great anyway. The only real effect of the planetary definition will be (I think) that children will be exposed to a certain idea through museums and classes and elementary astronomy books. But anybody who becomes serious about astronomy will quickly come to understand that these are difficult definitions at best. Whether Pluto is included in the museum displays is not really all that consequential: I doubt that the Plutonians are even aware of it. At the very least, they haven't yet sent an invasion fleet. The disadvantage of having lots of new planets is that elementary school children would have to memorize a long list for tests.

I actually said the same exact thing in a similar thread a while back. I've even been in such a situation, several times. Like this one time when I was explaining something astronomy-related to my friends. The whole Pluto thing was brought up. And I just couldn't explain to them why it was disclassified. Not in a way they would understand anyway. :doh:

AndrewJ
2009-Jul-07, 04:47 PM
"Plutoid", "plutino" and "cubewano" are such wonderful words. I say we allow Ceres to live in a class of its own and ditch the "dwarf planet" designation.

thoth II
2009-Jul-07, 05:12 PM
"Plutoid", "plutino" and "cubewano" are such wonderful words. I say we allow Ceres to live in a class of its own and ditch the "dwarf planet" designation.

absolutely, there is no such thing as a dwarf planet, not really. What I mean is, it just encompasses a diverse group of objects like KBOs and Ceres.

Wouldn't it make more sense to classify according to how groups evolved? For example, the KBOs probably all share a common evolutionary history, as do Ceres and the asteroid belt.

What I think is, at IAU 2006 they were under intense pressure to please disparate factions; and the "dwarf planet" was a political compromise thing: why? because although they knew in light of the discovery of Eris, et al, that Pluto wasn't a planet , how could they ruffle feathers and say it wasn't? Solution: "dwarf planet".

But then they had another problem, because to accomplish above, they had to taylor the "planet" definition in a reasonable way, but in such a way it excluded Pluto. From that, they bootstrapped down to the definition of "dwarf planet", so it did encompass Pluto; but then unfortunately it also encompassed Ceres. But including Ceres was a small price to pay for solving the bigger political issue.

dwnielsen
2009-Jul-07, 05:20 PM
No, Earth would not "clear its orbit" at Pluto's distance. Dr. Alan Stern and Dr. Hal Levison did the calculations for this and determined definitively it would not. That is why the IAU definition is so bad. It takes the same object and makes it a planet in one location and not a planet in another.

Just curious, would you have an idea on what sort of relationship an orbit-clearing-capacity heuristic might follow? In some systems, noise or error can be renormalized by just dividing by the principle quantity N or (N-1). In this case, what is the basic effect of overall distance and mass (and perhaps eccentricity, moons, etc) on the ability of these bodies to clear their orbits, assuming they are passing through the same field. In other words, to eschew the orbit-clearing criterion, what function might be used in its place?

laurele
2009-Jul-07, 06:01 PM
There are three major characteristics defining a planet:

1) It's in orbit around the Sun.
2) It's round (i.e. has achieved hydrostatic equilibrium.)
3) It has cleared its orbit.

The 3rd characteristic is where these so-called dwarf planets fail. So, it seems fair enough for, for example, Ceres (which is located in the middle of the Asteroid Belt) to be classified as a dwarf planet.

I do agree that the term dwarf planet is somewhat misleading, though -- if that's the right word. But it's the IAU who made the decision in the end. We really can't do anything about it. Maybe just a rename of those bodies would be better.

The definition you give is the controversial IAU one adopted by only four percent of its members. Why do you say we can't do anything about it? That is absolutely not true. There is no reason to accept something as fact just because as small group of scientists decreed it so. As Dr. Stern often points out, if the IAU decreed that the sky is green, would you take that as gospel truth?

What we can do is reject that definition and support scientists who are seeking to supplant it with a better one. We can honestly discuss the controversy with recognition that this is very much an ongoing debate. And we can look forward to New Horizons sending back information that very likely revolutionizes our concept of Pluto and Kuiper Belt planets.

Fiery Phoenix
2009-Jul-07, 06:03 PM
I guess the IAU just simply rushed on their decision. They should have given it a little more thought.

I'm not sure if I agree or not with this new classification of planets. It seems to make sense to me, and, at the same time, it doesn't. Has Dr. Plait written anything expressing his opinion on this?

laurele
2009-Jul-07, 06:12 PM
Just curious, would you have an idea on what sort of relationship an orbit-clearing-capacity heuristic might follow? In some systems, noise or error can be renormalized by just dividing by the principle quantity N or (N-1). In this case, what is the basic effect of overall distance and mass (and perhaps eccentricity, moons, etc) on the ability of these bodies to clear their orbits, assuming they are passing through the same field. In other words, to eschew the orbit-clearing criterion, what function might be used in its place?

I'm not quite sure what you're asking here. I do not believe that "clearing its orbit" should be a criterion for an object to be considered a planet, and neither do many professional astronomers (see the book Is Pluto A Planet? by Dr. David Weintraub and The Case for Pluto by Alan Boyle, due out in October). The single criterion advocated by these scientists for an object to be considered a planet is that they are in a state of hydrostatic equilibrium, meaning they are large enough to be shaped by their own gravity rather than by chemical bonds.

I have no problem with calling the smaller objects that are not gravitationally dominant "dwarf planets" and with using the parameters set by Stern and Levison in the article to which I linked above to determine whether an object is a dwarf planet--provided that dwarf planets are considered a subclass of planets.

And yes, it does matter what kids learn. At a public forum this March at the American Museum of Natural History in New York, Dr. Mark Sykes pointed out that many kids are being taught that Pluto is an asteroid or are not being taught about Pluto at all. This is a disservice to our young people.

Memorization is not important; understanding the concepts of different types of planets is. At one time, we taught the solar system by memorization because we knew little else other than the planets' names. Today, we can just as easily teach categories of planets and the characteristics that define them.

We don't limit the number of elements in the Periodic Table with the excuse that there are too many to memorize. What the IAU did was artificially invent a way to keep the number of planets small--a political decision and one based on convenience, not on science.

Fiery Phoenix
2009-Jul-07, 06:20 PM
I'm not quite sure what you're asking here. I do not believe that "clearing its orbit" should be a criterion for an object to be considered a planet, and neither do many professional astronomers (see the book Is Pluto A Planet? by Dr. David Weintraub and The Case for Pluto by Alan Boyle, due out in October). The single criterion advocated by these scientists for an object to be considered a planet is that they are in a state of hydrostatic equilibrium, meaning they are large enough to be shaped by their own gravity rather than by chemical bonds.

I have no problem with calling the smaller objects that are not gravitationally dominant "dwarf planets" and with using the parameters set by Stern and Levison in the article to which I linked above to determine whether an object is a dwarf planet--provided that dwarf planets are considered a subclass of planets.

And yes, it does matter what kids learn. At a public forum this March at the American Museum of Natural History in New York, Dr. Mark Sykes pointed out that many kids are being taught that Pluto is an asteroid or are not being taught about Pluto at all. This is a disservice to our young people.

Memorization is not important; understanding the concepts of different types of planets is. At one time, we taught the solar system by memorization because we knew little else other than the planets' names. Today, we can just as easily teach categories of planets and the characteristics that define them.

We don't limit the number of elements in the Periodic Table with the excuse that there are too many to memorize. What the IAU did was artificially invent a way to keep the number of planets small--a political decision and one based on convenience, not on science.

Give me a break. Are you saying that having a larger number of planets would be politically unacceptable or what exactly? :confused:

nauthiz
2009-Jul-07, 06:34 PM
The definition you give is the controversial IAU one adopted by only four percent of its members.

It was passed by a very large majority of voting members. It's just that the voter turnout was very low.

Phil Plait did express some concerns (http://blogs.discovermagazine.com/badastronomy/2006/08/15/congratulations-its-a-planet/) about the new definitions, but none of them really have much to do with the whole dwarf planet thing. Here's a paraphrase of his complaints:

1. He doesn't like the condition that they must be orbiting a star. This means that if Saturn were somehow ejected from the solar system, it would cease to be a planet.
2. He doesn't like the hydrostatic equilibrium condition because if two objects of the same mass have different compositions, it's possible that the softer one would be round while the harder one is still lumpy.
3. He doesn't like the pluton bit, from what I can tell because he things the designation is both arbitrary and unnecessary.

And, last but not least, he seems to think the entire exercise is a waste of time because none of it amounts to much more than people getting worked up over semantics that don't really have anything to do with the science.

AndreasJ
2009-Jul-07, 08:42 PM
No, Earth would not "clear its orbit" at Pluto's distance. Dr. Alan Stern and Dr. Hal Levison did the calculations for this and determined definitively it would not. That is why the IAU definition is so bad. It takes the same object and makes it a planet in one location and not a planet in another.

So did pre-2006 usage. The fact that nobody complained about it before the IAU definition makes it very hard to think that its critics are primarily motivated by scientific or logical concerns.


(As for linguistics, English if full of X Y's that aren't Y's. Deal with it or become lojbanists.)

nauthiz
2009-Jul-07, 09:59 PM
For what it's worth, the usual definition for moon takes the same object and makes it a moon in one location and not in another. Nobody seems to complain about that either.

dwnielsen
2009-Jul-07, 11:56 PM
For what it's worth, the usual definition for moon takes the same object and makes it a moon in one location and not in another. Nobody seems to complain about that either.

Moreover, it also may satisfy the single criterion of hydrostatic equilibrium.

I really like the tug-of-war definition (and I'm no Asimov fan) at..
http://en.wikipedia.org/wiki/Double_planet

Instead of just making easily memorable definitions, where are the propositions that include all relevant variables in a mathematical relationship? I'm thinking something very roughly like..

( f0(mass of body)+f1(mass of satellites)+f2(brightness) ) / ( f3(distance from sol sys barycenter)+f4(eccentricity)+f5(inclination) )