Author Topic: Pluto-Planet debate discussions  (Read 56566 times)

Offline Comga

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Pluto-Planet debate discussions
« on: 06/16/2015 05:38 AM »
I am all for the "planet" debate.

Frankly, I adhere to Alan Stern's concept: If it's large enough to go into hydrostatic equilibrium (i.e. round) and not orbiting a larger object (with the barycenter inside that larger object) it's a planet.

We wind up with the four terrestrial planets, the four gas giants, a plethora of Kuiper belt dwarf planets,... and Ceres, the lone inner solar system dwarf planet.   

With the Pluto-Charon barycenter outside of Pluto, that makes Charon a dwarf planet, too.

And definitions do change.  With the origin of "planet" being "wanderer" the Earth hasn't always been a planet, since it never wanders but is always "right here".

But my preference is to keep that discussion from taking over the Pluto observations.
Can we put those discussions here?

Thanks
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Offline meekGee

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Re: Pluto-Planet debate discussions
« Reply #1 on: 06/16/2015 05:44 AM »
I thought they did make it a dwarf planet.

It's clearly different from the other planets, and in-family with the other kuiper-belt objects.

Quote from: wikipedia

The discovery of these large KBOs in similar orbits to Pluto led many to conclude that, bar its relative size, Pluto was not particularly different from other members of the Kuiper belt. Not only did these objects approach Pluto in size, but many also possessed satellites, and were of similar composition (methane and carbon monoxide have been found both on Pluto and on the largest KBOs). Thus, just as Ceres was considered a planet before the discovery of its fellow asteroids, some began to suggest that Pluto might also be reclassified.

The issue was brought to a head by the discovery of Eris, an object in the scattered disc far beyond the Kuiper belt, that is now known to be 27% more massive than Pluto. In response, the International Astronomical Union (IAU), was forced to define what a planet is for the first time, and in so doing included in their definition that a planet must have "cleared the neighborhood around its orbit". As Pluto shared its orbit with so many KBOs, it was deemed not to have cleared its orbit, and was thus reclassified from a planet to a member of the Kuiper belt.

Although Pluto is currently the largest KBO, there are two known larger objects currently outside the Kuiper belt that probably originated in it. These are Eris and Neptune's moon Triton (which, as explained above, is probably a captured KBO).

As of 2008, only five objects in the Solar System (Ceres, Eris, and the KBOs Pluto, Makemake and Haumea) are listed as dwarf planets by the IAU. However, 90482 Orcus, 28978 Ixion and many other Kuiper-belt objects are large enough to be in hydrostatic equilibrium; most of them will probably qualify when more is known about them


EDIT:  Recent observations of the chaotic spins of Pluto's satellites also place it out-of-family with the other planets.  Makes it look like a cluster of co-orbiting asteroids.
« Last Edit: 06/16/2015 05:51 AM by meekGee »
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Offline Star One

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Re: Pluto-Planet debate discussions
« Reply #2 on: 06/16/2015 06:31 AM »
My prior post relating to this copied across from the NH thread.

There was an article in this week's New Scientist talking about this mission and one of the side pieces was discussing the planet question. It was suggested that one possible solution was too open up the definition of the type of planets. So you'd have terrestrial planets, gas giants, ice giants and dwarf planets but they'd all just be classes of the same thing. The only question being is would the public accept 70+ planets in the solar system.

Offline The Amazing Catstronaut

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Re: Pluto-Planet debate discussions
« Reply #3 on: 06/16/2015 06:54 AM »
My prior post relating to this copied across from the NH thread.

There was an article in this week's New Scientist talking about this mission and one of the side pieces was discussing the planet question. It was suggested that one possible solution was too open up the definition of the type of planets. So you'd have terrestrial planets, gas giants, ice giants and dwarf planets but they'd all just be classes of the same thing. The only question being is would the public accept 70+ planets in the solar system.

I can't imagine why not. It'd certainly make things more interesting for them and would remind people that our solar system is certainly a lot more interesting than many people think.
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Offline Nilof

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Re: Pluto-Planet debate discussions
« Reply #4 on: 06/16/2015 09:15 AM »
I think the best way to go would be to emulate the classification scheme for stars. We certainly didn't retroactively restrict the term "star" because the list would become too long. A dwarf star is still a star, and much like in the case of dwarf planets, the vast majority of stars are red dwarfs, even though very few of them are visible to the naked eye.

Instead, we introduced a classification that is actually useful and accurately describes the physical properties of an object. I'd suggest using something similar to the H-R diagram for stars using a diameter/mass plot. These happen to be the two physical properties we can measure to a reasonable accuracy for exoplanets, making the classification scheme useful in that context as well.

Thus, just like we say "red dwarf" or "blue giant", for planets we could say "iron/rocky/icy/gas -  dwarf/terrestrial/giant - planet". Some examples would be:

Saturn: Gas giant
Neptune: Icy giant
Pluto: Icy dwarf
Mercury: Iron dwarf
Mars: Rocky terrestrial

For exoplanets, we could add another descriptor for the surface temperature. "Mercurial" or "hot" if it is above the boiling point of water, "Habitable" if liquid water could exist, "cold" if any water is likely to be frozen, and "cryogenic" if it is cold enough for liquid or frozen methane to exist.
« Last Edit: 06/16/2015 09:50 AM by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline Nomadd

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Re: Pluto-Planet debate discussions
« Reply #5 on: 06/16/2015 02:14 PM »
 A friend of mine, who is around 32 inches tall, says that Earth is the only dwarf planet he knows.
 It was funnier in person.

Offline nadreck

Re: Pluto-Planet debate discussions
« Reply #6 on: 06/16/2015 02:16 PM »


Saturn: Gas giant
Neptune: Icy giant
Pluto: Icy dwarf
Mercury: Iron dwarf
Mars: Rocky terrestrial



So which ones are Clerics?
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Offline kevin-rf

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Re: Pluto-Planet debate discussions
« Reply #7 on: 06/16/2015 02:46 PM »
Just replace 7'2" with Planet, and 5'8" with dwarf planet and it about sums it up.

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Quote
Although Iíve been claiming to be 7í2Ē for many decades, the truth is that Iím 5í8Ē. And thatís when I first get out of bed in the morning. Just goes to show, you tell a lie often enough and people believe you. I expect there will be some who will demand I give back the championship rings and titles that I accumulated during my college and professional basketball career because I was only able to win them by convincing other players that they had no chance against my superior height. How could these achievements have any lasting meaning if Iím not really as tall as Wikipedia says I am?
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Online edkyle99

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Re: Pluto-Planet debate discussions
« Reply #8 on: 06/16/2015 03:10 PM »
I'm OK with "Dwarf Planet", because that means Pluto is still a "Planet" of some type.  This might be seen as a Pluto downgrade by some, but to me it is a Ceres upgrade (along with the other Dwarf Planets out there).  With the "white spots" and the co-planet orbiting bit by Pluto/Charon, these "Dwarfs" are turning out to be more interesting than some of the regular "Planets" anyway.

 - Ed Kyle
« Last Edit: 06/16/2015 03:13 PM by edkyle99 »

Offline kevin-rf

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Re: Pluto-Planet debate discussions
« Reply #9 on: 06/16/2015 03:57 PM »
How about a very, very, very, very failed star?

Where do we draw the line and rename Jupiter as a failed star then re-label the solar system as a binary system?
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Offline meekGee

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Re: Pluto-Planet debate discussions
« Reply #10 on: 06/16/2015 04:18 PM »
How about a very, very, very, very failed star?

Where do we draw the line and rename Jupiter as a failed star then re-label the solar system as a binary system?

Sometimes, an object will straddle the line between categories. There are never sharp lines, but that doesn't mean there shouldn't be categories.

In this case, however, Pluto does not straddle the line.  It is very clear that it belongs to the group of KBOs, and you can call that group whatever you like, as long as you're consistent.

"Dwarf planet" seems to me a good name for spherical KBOs.  The question of whether a "Dwarf planet" is a "type of planet" is absolutely immaterial.  It's not a classification of objects, it's a classification of classifications...



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Offline Star One

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Re: Pluto-Planet debate discussions
« Reply #11 on: 06/16/2015 04:53 PM »
I'm in favour of it being just another class of planet amongst others and if that means seventy plus planets in the Solar System then so be it.

Offline Nilof

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Re: Pluto-Planet debate discussions
« Reply #12 on: 06/16/2015 05:05 PM »
How about a very, very, very, very failed star?

Where do we draw the line and rename Jupiter as a failed star then re-label the solar system as a binary system?

Sometimes, an object will straddle the line between categories. There are never sharp lines, but that doesn't mean there shouldn't be categories.

In this case, however, Pluto does not straddle the line.  It is very clear that it belongs to the group of KBOs, and you can call that group whatever you like, as long as you're consistent.

"Dwarf planet" seems to me a good name for spherical KBOs.  The question of whether a "Dwarf planet" is a "type of planet" is absolutely immaterial.  It's not a classification of objects, it's a classification of classifications...

Imho, the question in the case of Pluto (or Ceres) is rather whether you should be able to use the word "planet" to offhandedly refer to both what the IAU calls planets and dwarf planets.

Regarding objects that straddle the line, if we try applying the IAU definition to exoplanets, you will find many examples where it not only straddles the line but completely smashes it into pieces. Our solar system is a rare exception with its eight major planets in neat circular orbits with cleared neighbourhoods. In fact, high eccentricities and shared orbits are rather common.

Applying the IAU definition of planets to the Upsilon Andromeda system will lead you to the weird conclusion that at least one body larger than Jupiter is not a planet. In general, 7% of all stellar systems have an eccentric Jupiter: https://en.wikipedia.org/wiki/Eccentric_Jupiter
« Last Edit: 06/16/2015 05:29 PM by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline NovaSilisko

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Re: Pluto-Planet debate discussions
« Reply #13 on: 06/16/2015 05:34 PM »
Applying the IAU definition of planets to the Upsilon Andromeda system will lead you to the weird conclusion that at least one body larger than Jupiter is not a planet. In general, 7% of all stellar systems have an eccentric Jupiter: https://en.wikipedia.org/wiki/Eccentric_Jupiter

Oh dammit, I shouldn't have read that while drinking. Something about that is really funny to me.

Now, the existence of an eccentric Jupiter basically renders all planets in a solar system as unable to clear their neighborhoods, doesn't it? Thus rendering every single planet a dwarf planet, assuming the IAU definition?

Something I've never really understood - did they really not produce some sort of formula to calculate how much the neighborhood has been cleared? I know Stern did something like that.

Offline Nilof

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Re: Pluto-Planet debate discussions
« Reply #14 on: 06/16/2015 05:54 PM »
Applying the IAU definition of planets to the Upsilon Andromeda system will lead you to the weird conclusion that at least one body larger than Jupiter is not a planet. In general, 7% of all stellar systems have an eccentric Jupiter: https://en.wikipedia.org/wiki/Eccentric_Jupiter

Oh dammit, I shouldn't have read that while drinking. Something about that is really funny to me.

Now, the existence of an eccentric Jupiter basically renders all planets in a solar system as unable to clear their neighborhoods, doesn't it? Thus rendering every single planet a dwarf planet, assuming the IAU definition?

Something I've never really understood - did they really not produce some sort of formula to calculate how much the neighborhood has been cleared? I know Stern did something like that.

There are a few candidate formulas including Sterns. The defining formula that seems to be favoured though is the Soter planetary discriminant, which is the ratio between the mass of an object and the combined mass of everything that shares its orbit. This latter formula clearly gives bogus results in the case of orbit-sharing giants.

Sterns formula gives better results, although you still need to add in a clause of the "satellites and pseudo-satellites are not planets" kind. The result can be reasonable or ludicrous in different situation depending on what convention you pick.
« Last Edit: 06/17/2015 08:56 AM by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline NovaSilisko

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Re: Pluto-Planet debate discussions
« Reply #15 on: 06/16/2015 05:57 PM »
There are a few candidate formulas including Sterns. The definition that seems to be favoured though is the Soter planetary discriminant, which is the ratio between the mass of an object and the combined mass of everything that shares its orbit. This latter formula clearly gives bogus results in the case of orbit-sharing giants.

And things like large bodies at L4/L5, as well, right? Also, what's the definition of "sharing its orbit"? How close does it have to be to be considered as sharing?

Offline llanitedave

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Re: Pluto-Planet debate discussions
« Reply #16 on: 06/16/2015 08:40 PM »
The problem I have with the current IAU definition has nothing to do with Pluto per se, it's just a poor definition.  It classifies an object based on characteristics that are not intrinsic to that object.

Under this definition, whether an object is a planet or not depends largely on the happenstance of its age and orbital position.  Were Earth in the Kuiper belt, it would not fit the IAU's definition of "Planet".  Further, "clearing out the zone" takes time, and it takes longer in the outer solar system than the inner.  So there is some bright day when an object becomes a planet, which is not the same day that an object in a different orbit becomes a planet, even if they are otherwise identical.

Bottom line, this is not a "genetic" classification, it is an interim classification of political convenience.  It adds nothing to the knowledge base, nor does it help us when it comes to classifying bodies in other star systems.  That's why I've been promoting this definition every chance I get.  It removes all the weaknesses of the current, hastily applied system and provides a real basis for a natural and extensible planetary taxonomy.

Abstract:
A mass-based definition for planets is proposed with dynamical circumstances and compositional characteristics used to define types of planets. Dynamical planet classes include Principal planets, Belt planets, Moons, and Rogue planets. Compositional classes include rock, ice, and gas planets with refined classes when sufficient data is available. The dynamical and compositional definitions are combined with a six class planetary mass scale into a taxonomy that can be used to classify both Solar System and extrasolar planets. This taxonomy can be applied to spherical sub-stellar mass bodies with masses ranging from 3.75 x 10^19 kg to 8.08 x 10^28 kg.
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Offline meekGee

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Re: Pluto-Planet debate discussions
« Reply #17 on: 06/16/2015 10:00 PM »
Perhaps it's good to recap:
(I'm doing this quick off of Wikipedia, will welcome corrections/additions


Property   Class  Round? Inclination  Eccentricity Crosser?  Clear-orbit?  Stable orbit? 
Earth      Rocky   y         <1        0.016         N
Ceres      Astr    y         9         0.075         N
Vesta      Astr    N         5.5       0.088         N
Jupiter    Gas     y         <1        0.048         N
Pluto      KBO     y         17        0.25         Neptune
Eris       KBO     y?        44        0.44          Y?

« Last Edit: 06/17/2015 12:03 AM by meekGee »
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Offline ChrisWilson68

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Re: Pluto-Planet debate discussions
« Reply #18 on: 06/17/2015 03:49 AM »
The problem I have with the current IAU definition has nothing to do with Pluto per se, it's just a poor definition.  It classifies an object based on characteristics that are not intrinsic to that object.

Under this definition, whether an object is a planet or not depends largely on the happenstance of its age and orbital position.  Were Earth in the Kuiper belt, it would not fit the IAU's definition of "Planet".  Further, "clearing out the zone" takes time, and it takes longer in the outer solar system than the inner.  So there is some bright day when an object becomes a planet, which is not the same day that an object in a different orbit becomes a planet, even if they are otherwise identical.

Bottom line, this is not a "genetic" classification, it is an interim classification of political convenience.  It adds nothing to the knowledge base, nor does it help us when it comes to classifying bodies in other star systems.  That's why I've been promoting this definition every chance I get.  It removes all the weaknesses of the current, hastily applied system and provides a real basis for a natural and extensible planetary taxonomy.

Abstract:
A mass-based definition for planets is proposed with dynamical circumstances and compositional characteristics used to define types of planets. Dynamical planet classes include Principal planets, Belt planets, Moons, and Rogue planets. Compositional classes include rock, ice, and gas planets with refined classes when sufficient data is available. The dynamical and compositional definitions are combined with a six class planetary mass scale into a taxonomy that can be used to classify both Solar System and extrasolar planets. This taxonomy can be applied to spherical sub-stellar mass bodies with masses ranging from 3.75 x 10^19 kg to 8.08 x 10^28 kg.

I think the IAU definition is pretty bad too, but I don't agree that there's any particular reason to ignore a body's orbit and only look at "intrinsic" properties.

A chunk of iron in orbit around the Sun is an asteroid but sitting on the surface of the Earth it's just a chunk of iron.  I see no reason to reject that definition of asteroid.  Similarly, I don't see any reason to require that we count moons as planets, as this proposed definition would.

In looking for a good definition of planet, we shouldn't be looking to change the well-established meaning of the word, just to establish clear and consistent rules for the cases where the definition is not already well-established.  That's one of the big problems with the IAU definition, and it's one of the big ones with this proposed re-definition of the word "planet" to include moons.

Offline tea monster

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Re: Pluto-Planet debate discussions
« Reply #19 on: 06/17/2015 12:35 PM »
The thing is, at what point is an asteroid or a dwarf planet a dwarf? The new IAU criteria are just a mess.

It makes much more sense to apply solid rules, involving mass, diameter or other criteria. That is my main beef with the IAU. It wasn't in any shape or form a real definition of what a planet really is, it was just a bunch of disgruntled astronomers with an axe to grind.

I can see why they balk at adding 'Plutoids' into the planetary fraternity, but just saying "Well, there's a lot of them" isn't really a real reason, especially for a scientific body.

As has been said before, how are we going to apply this to other planetary systems?


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