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The Astronomy Thread 
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Post Re: The Astronomy Thread
Arioch wrote:
Mr.Tucker wrote:
It's only usefull for the general education of the public, astronomers make very little distinction between big bodies and small bodies of the Solar System.

I think this is the larger point: the definition has almost nothing to do with science. And it's confusing to the public, because they mistakenly assume that it does. Telling the public that Pluto is no longer a planet does more harm than good as far as general education is concerned.


The chosen terminology doesn't help things. A category named "dwarf planets" that is not a subset of the category "planets"? Dwarf stars are stars, giant stars are stars, giant planets are planets, dwarf planets are...something that we call a planet while insisting it's not a planet because of factors that don't actually have anything to do with the object we're classifying? Who thought this was a good idea?


Sun Dec 06, 2015 1:28 pm
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Post Re: The Astronomy Thread
hi hi

Maybe when we invent time travel, we can go back and determine which planets cleared their orbits, and which ones got help from Jupiter.

Also, check out these sweet sweet high resolution photos of Pluto's surface geology. Craters with stratified layers indicate that it is not just one big homogeneous clump of debris.


Sun Dec 06, 2015 6:17 pm
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Post Re: The Astronomy Thread
Mjolnir wrote:
Arioch wrote:
Mr.Tucker wrote:
It's only usefull for the general education of the public, astronomers make very little distinction between big bodies and small bodies of the Solar System.

I think this is the larger point: the definition has almost nothing to do with science. And it's confusing to the public, because they mistakenly assume that it does. Telling the public that Pluto is no longer a planet does more harm than good as far as general education is concerned.


The chosen terminology doesn't help things. A category named "dwarf planets" that is not a subset of the category "planets"? Dwarf stars are stars, giant stars are stars, giant planets are planets, dwarf planets are...something that we call a planet while insisting it's not a planet because of factors that don't actually have anything to do with the object we're classifying? Who thought this was a good idea?
Someone who metaphorically wants magical unicorns to exist in space, I assume. It's fairly easy to come up with a sensible description of the low-end of a planetary mass based on the self-attractive force required to pull it into a spheroid despite some particular starting conditions (on average 0 C or something, whatever), but apparently they wanted planets to be rarer and more "special" than that: all that's "special" with what they produced is the justification for it's production.


Sun Dec 06, 2015 6:47 pm
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Post Re: The Astronomy Thread
Absalom wrote:
Someone who metaphorically wants magical unicorns to exist in space, I assume. It's fairly easy to come up with a sensible description of the low-end of a planetary mass based on the self-attractive force required to pull it into a spheroid despite some particular starting conditions (on average 0 C or something, whatever), but apparently they wanted planets to be rarer and more "special" than that: all that's "special" with what they produced is the justification for it's production.


I believe the justification for reducing Pluto from planet status is because if they allowed it to remain a planet, we would have to add Eris, Quaoar, and Sedna to the planetary list as well. And Eris is actually BIGGER than Pluto.

There are currently three qualities required to be a planet:

It must be in orbit around the Sun
It must have sufficient mass to assume hydrostatic equilibrium (a nearly round shape)
It must have "cleared the neighborhood" around its orbit (meaning there is nothing in its orbital zone similar in size beyond the object's own satellites)

Pluto meets the first two but its orbit is littered with objects relatively close to its size. There are so many they even have a special name: plutinos.

I suspect the reason 'dwarf planets' are called dwarf planets instead of something more accurate is to pacify the legions of ordinary people who were up in arms that Pluto would no longer be a planet. But we must face facts that science has marched on. If the people who study astronomy for a living think that something really shouldn't be called a planet I'm certainly not qualified to tell them they are wrong.

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Mon Dec 07, 2015 3:15 pm
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Post Re: The Astronomy Thread
hi hi

When scientists started discovering and creating new elements, like Nobelium, they didn't say "Oh my gosh, there are too many elements on the periodic table, we need to reclassify some of these, and besides they decay too fast to be important anyway." They just expanded the periodic table.

The "cleared the neighborhood," component, as has been discussed at length already in this thread, is a terrible measurement. We can't even agree on models of how the solar system formed, let alone which planets did what. (According to the nice model, which has already been discussed, interactions between Jupiter and Saturn were responsible for clearing the orbits of the entire solar system, including Neptune and the inner planets.) After all, the further out you get, the area that is required for a planet to clear grows exponentially, while the effects of gravity trail off exponentially. So it is quite likely that even Neptune would not be able to clear its own orbit by itself.

The people who study astronomy for a living have not reached a consensus on the matter, and it continues to be a point of contention, in spite of what a small handful of people voted on. Not to mention being entirely inapplicable to solar systems beyond our own.

Science is not decided by vote.


Mon Dec 07, 2015 4:13 pm
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Post Re: The Astronomy Thread
icekatze wrote:
When scientists started discovering and creating new elements, like Nobelium, they didn't say "Oh my gosh, there are too many elements on the periodic table, we need to reclassify some of these, and besides they decay too fast to be important anyway." They just expanded the periodic table.


Aside from the comparison being faulty (one is a list of what number of protons are linked to which name and the other a "list" of objects in both shape, spatial (where the object is and how it orbits) and size dimensions, which is significantly different)

Not that this matters. By your example, Pluto and the other dwarf planets should be trans neptunian asteroids... which (insert drumroll...) they all technically are under the current definition.

The clearing the neighborhood thing is silly though, I'll give you that. Though, I interpreted their ruling as "Cleared it enough to become the single dominant object in its orbit."

Which would mean that Neptune (crosses with pluto) and Jupiter (trailed and lead by its trojans) are planets again.


Tue Dec 08, 2015 2:28 am
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Post Re: The Astronomy Thread
icekatze wrote:
hi hi

When scientists started discovering and creating new elements, like Nobelium, they didn't say "Oh my gosh, there are too many elements on the periodic table, we need to reclassify some of these, and besides they decay too fast to be important anyway." They just expanded the periodic table.

The "cleared the neighborhood," component, as has been discussed at length already in this thread, is a terrible measurement. We can't even agree on models of how the solar system formed, let alone which planets did what. (According to the nice model, which has already been discussed, interactions between Jupiter and Saturn were responsible for clearing the orbits of the entire solar system, including Neptune and the inner planets.) After all, the further out you get, the area that is required for a planet to clear grows exponentially, while the effects of gravity trail off exponentially. So it is quite likely that even Neptune would not be able to clear its own orbit by itself.

The people who study astronomy for a living have not reached a consensus on the matter, and it continues to be a point of contention, in spite of what a small handful of people voted on. Not to mention being entirely inapplicable to solar systems beyond our own.

Science is not decided by vote.


Roeben wrote:
icekatze wrote:
When scientists started discovering and creating new elements, like Nobelium, they didn't say "Oh my gosh, there are too many elements on the periodic table, we need to reclassify some of these, and besides they decay too fast to be important anyway." They just expanded the periodic table.


Aside from the comparison being faulty (one is a list of what number of protons are linked to which name and the other a "list" of objects in both shape, spatial (where the object is and how it orbits) and size dimensions, which is significantly different)

Not that this matters. By your example, Pluto and the other dwarf planets should be trans neptunian asteroids... which (insert drumroll...) they all technically are under the current definition.

The clearing the neighborhood thing is silly though, I'll give you that. Though, I interpreted their ruling as "Cleared it enough to become the single dominant object in its orbit."

Which would mean that Neptune (crosses with pluto) and Jupiter (trailed and lead by its trojans) are planets again.


Roeban is correct. "Clearing the neighbourhood" is defined as ensuring that the object in question is the dominant gravitational body. Pluto and the Plutinos are gravitationally locked into a 3:2 resonance by Neptune, just as the Trojan asteroids are locked by Jupiter, making both Neptune and Jupiter gravitationally dominant in their respective orbital zones.

And yes, I know that Alan Stern disagrees strongly with the decision to reclassify Pluto, and that there was considerable controversy about it. However, it's been almost ten years since the decision was made to limit what qualifies as a planet and the IAU has met three more times since. The scientific community has not risen in outrage to overturn the previous ruling, nor have I seen anything suggesting that the majority of astronomers disagree with it. If anyone on this forum has published a scholarly article regarding why the IAU's definition is invalid, I would love to read it. If not then I will leave this to the people who specialize in it, because I'm not nearly qualified to disagree with them.

Besides, what's in a name? We could call Pluto/Eris/etc 'Itty Bitty Cosmic Space Fuzzies', it wouldn't change what they are or make them less worthy of study.

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Tue Dec 08, 2015 7:02 am
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Post Re: The Astronomy Thread
hi hi

Actually, the definition of the second is what we're still arguing about, and it turns out that people have different definitions. The people making the list of elements were smart to not try to make an ad hoc definition to exclude some elements in favor of other more interesting ones.

The hydro-static equilibrium definition already sets a lower limit on planetary mass, and then the clearing its neighborhood sets another lower limit on planetary mass. I think it is rather silly to have a definition with two measures of the same thing. It would be like defining the kilogram as something being equal to this number, and also less than this other larger number.

The fact that "clearing the neighborhood," is so open to interpretation speaks to how weak it is in terms of a description. And since someone asked for scholarly articles, here's a good place to start. IAU Planet Definition: Some Confusion And Their Modifications. A Quantitative Criterion for Defining Planets. Solar and Extrasolar Planet Taxonomy

The other thing about being a dominant gravitational body is the multiple-standard that comes about with orbital distance. For example, Earth and Mercury have an average distance of 91,691,000 km, and are considered to be in different orbits. Pluto and Neptune have a closest approach of 2,543,000,000,000 km (2,543,163,790 km) and are considered to be in the same orbit.

Quote:
Besides, what's in a name?

Apparently there is a lot in a name. Just a few posts ago, we were told that we must face facts and that this is about science marching on, rather than a semantic debate about language.


Last edited by icekatze on Tue Dec 08, 2015 3:55 pm, edited 1 time in total.



Tue Dec 08, 2015 11:10 am
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Post Re: The Astronomy Thread
icekatze wrote:
The other thing about being a dominant gravitational body is the multiple-standard that comes about with orbital distance. For example, Earth and Mercury have an average distance of 91,691,000 km, and are considered to be in different orbits. Pluto and Neptune have a closest approach of 2,543,000,000,000 km and are considered to be in the same orbit.


Have you checked your numbers? Pluto and Neptune never come closer to each other then roughly a quarter of a lightyear? ;)

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Tue Dec 08, 2015 2:52 pm
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Post Re: The Astronomy Thread
hi hi

Ha! Nice catch, thank you GeoModder! The closest approach was listed as 17 au, but when I tried to convert, I apparently grabbed the distance for astronomical units in meters instead of kilometers. I went back and got some more reliable numbers for closest approaches.

Still 2,543,163,790 km (17au) for Neptune and Pluto vs, 77,300,000 km (.52au) between Earth and Mercury. That's almost a couple of orders of magnitude different.


Tue Dec 08, 2015 4:02 pm
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Post Re: The Astronomy Thread
Sadly, ratio's in area tend to increase exponentially where gravity is concerned, because of that whole gravity well thing.


Tue Dec 08, 2015 11:09 pm
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Post Re: The Astronomy Thread
hi hi

Right, that is part of why it is such a weak criteria. The farther away an object gets from its parent star, the harder it is for it to be a planet, through no characteristics of the object itself. Also, an imaginary orbital plane doesn't really have much to do with a planet either.

If my math is correct here, using newton's law of gravitation, the force between Earth and Mercury during their closest approach is greater than the force between Neptune and Pluto during their closest approach. (I also checked for a 1kg object at .52au from Earth vs a 1kg object at 17au from Neptune, and in both cases, Earth has the greater gravitational influence.)

Of course, when you start looking at exoplanets, the criteria breaks down entirely. Just look at Upsilon Andromedae d, and Upsilon Andromedae e. Upsilon Andromedae d is a planet about 10.25 times the mass of Jupiter. Upsilon Andromedae e is a planet about .96 times the mass of Jupiter. E has an incredibly circular orbit, while D has an incredibly eccentric orbit in a 3:1 resonance with E.


Wed Dec 09, 2015 12:24 am
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Post Re: The Astronomy Thread
Yet Pluto does very much look like a minor solar system body, even if it's the biggest member of it's orbital neighborhood.

https://en.wikipedia.org/wiki/File:Atra ... re2014.png

https://en.wikipedia.org/wiki/File:TheK ... utinos.svg

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Thu Dec 10, 2015 2:50 am
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Post Re: The Astronomy Thread
hi hi

If someone thinks that Pluto looks unimportant, then they are of course welcome to that subjective opinion/value judgement.

Personally, I think Luxembourg is a rather minor country, but I don't think I would change the definition of a country to exclude it from the definition.


Thu Dec 10, 2015 10:41 am
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Post Re: The Astronomy Thread
Luxembourgh is in no way relateable to pluto. The comparison is pretty weak, IMO.

A better example would be Liechtenstein as it has a monetary union with switzerland and is not fully autonomous... but it's still a silly comparison to make. Apples and Pears and all that.


Fri Dec 11, 2015 1:24 am
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Post Re: The Astronomy Thread
hi hi

Really, no way at all? Luxembourgh isn't small? Luxembourg isn't heavily influenced by its neighbors? Ok, if you say so. :roll:

Edit: Also interesting, because Liechtenstein is a country as well, and because every member state of the European Union has a monetary union. If having a monetary union makes something not a country, then Austria, Belgium, Cyprus, Estonia, Finland, France, Germany, Greece, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Portugal, Slovakia, Slovenia, and Spain are not countries.


Fri Dec 11, 2015 9:54 am
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Post Re: The Astronomy Thread
Roeben wrote:
Luxembourgh is in no way relateable to pluto. The comparison is pretty weak, IMO.
Luxembourgh is in no way relateable to Russia. The comparison is pretty weak, IMO.

And yes, this is relevant.


Fri Dec 11, 2015 3:03 pm
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Post Re: The Astronomy Thread
hi hi

In other news, the bright spots on Ceres have been identified as a type of magnesium sulfate, and the Dawn probe also detected with a strong probability, the presence of ammonia rich clays on the surface. (Which raises the possibility that Ceres was originally formed in the outer solar system.)

Via Jet Propulsion Laboratory


Fri Dec 11, 2015 3:43 pm
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Post Re: The Astronomy Thread
Earth's initial atmosphere was supposed to have contained ammonia as well. And the detected ammonia on Ceres could as well be brought in by a cometary impactor.

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Sat Dec 12, 2015 2:32 am
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Post Re: The Astronomy Thread
GeoModder wrote:
Earth's initial atmosphere was supposed to have contained ammonia as well. And the detected ammonia on Ceres could as well be brought in by a cometary impactor.


Ammonia was thought to be a major component of Earth's atmosphere early on, but more modern models lack it. Ammonia's relatively reactive and unstable (as is methane, also considered a candidate), so N2, CO2, CO, and H2O is a more likely composition, with a good bit of SO2 and H2S from volcanism...basically a wetter Venus. Early enough, there would have been some H2 as well. The methane/ammonia-rich atmospheres of the original Miller-Urey experiment are not likely (though the results are pretty much the same as long as there's not much free oxygen).


Sat Dec 12, 2015 5:41 am
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Post Re: The Astronomy Thread
icekatze wrote:
hi hi

Really, no way at all? Luxembourgh isn't small? Luxembourg isn't heavily influenced by its neighbors? Ok, if you say so. :roll:

Edit: Also interesting, because Liechtenstein is a country as well, and because every member state of the European Union has a monetary union. If having a monetary union makes something not a country, then Austria, Belgium, Cyprus, Estonia, Finland, France, Germany, Greece, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Portugal, Slovakia, Slovenia, and Spain are not countries.


You'd notice I never said that a monetary union precludes statehood. You are intentionally misrepresenting my opinion to drive the argument into your direction.

What I really said was that Liechtenstein doesn't have control over its own currency, not in the way most other states do.

The point is that countries are not planets. The point is also that not all countries are fully autonomous, even if they are considered states. Just like how not all solar system bodies have full control over their areas of influence.

That reminds me, wouldn't you describe those EU countries as member states? A different type of state grouped under a larger descriptor? I can spin this comparison into my direction if I wanted to too, you know.

I mean come on, there is not even an academic consensus on the appropriate definition of a state, but we clearly know microstates are something different then a full fledged one. We don't need wikipedia for that. Not to mention that states don't have orbits, moons, gravitational fields, inclination or any other astrophysical properties. They aren't even definable physical objects the way planetary bodies are.


Sat Dec 12, 2015 6:18 am
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Post Re: The Astronomy Thread
hi hi

Analogy, noun: A comparison between two things, typically on the basis of their structure and for the purpose of explanation or clarification. A correspondence or partial similarity.


Sat Dec 12, 2015 9:50 am
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Post Re: The Astronomy Thread
Mjolnir wrote:
Ammonia was thought to be a major component of Earth's atmosphere early on, but more modern models lack it. Ammonia's relatively reactive and unstable (as is methane, also considered a candidate), so N2, CO2, CO, and H2O is a more likely composition, with a good bit of SO2 and H2S from volcanism...basically a wetter Venus. Early enough, there would have been some H2 as well. The methane/ammonia-rich atmospheres of the original Miller-Urey experiment are not likely (though the results are pretty much the same as long as there's not much free oxygen).


Do you happen to know the 'frostline' for ammonia in the Solar System? In the current era that is?

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Sat Dec 12, 2015 10:24 am
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Post Re: The Astronomy Thread
hi hi

Information on the ammonia frost line has proven actually rather difficult to find. Best indication I could find was a graph that pegged the ammonia ice line at around 9 au. (Although the ammonia compounds on Ceres are not in ice form)


Sat Dec 12, 2015 11:22 am
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Post Re: The Astronomy Thread
Since comets and asteroids were all (presumably) formed from the same cloud of gas and dust, it doesn't seem surprising to me that they should be composed of the same kinds of materials. Ices seem to be extremely common throughout the solar system, just better hidden closer to the sun, where exposed ices are more easily blown away.

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