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Forum » SpaceEngine » Science and Astronomy Discussions » Defining Planethood (What sets planets apart?)
Defining Planethood
Gondor2222Date: Tuesday, 25.06.2013, 03:59 | Message # 31
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Quote (smjjames)
Quote (paradan)
Australia has been reduced to an island.

It's an island continent AND it has it's own tectonic plate. However, it won't be an island forever, it'll collide into SE Asia.


Australia is an island. Australia is a continent. Australia has an island named Australia on part of its continental plate
This is because Australia the continent is defined from a geological perspective (the areas share a single continental shelf and plate), whereas Australia, the island is defined from a topographic perspective. There are two Australias: one is an island, and one is a continent containing an island of the same name along with other islands.

Quote (Watsisname)
Quote
What if another large body is temporarily perturbed into the orbit of a previously-unimpeachable planet? Would both planets suddenly become not planets? And if so, what would they be?

With the way I define planets, yes. There are a few different ways such an event could occur, each deserving some discussion:

Quote (HarbingerDawn)
when does a protoplanet stop being a protoplanet?


I think this calls for a larger number of classes than we already have, taking into account a number of variables:

Keep in mind this is a rough draft. Most current dwarf planets fall into "Dwarf Planet" (the definition is now based on mass rather than gravitational hydrostatic equilibrium, Earth and Luna into "Multiple Planet", all other planets in the Solar system under "Planet", asteroids and comets under "small secondary bodies" Objects in the Oort cloud presumably fall under "dust/rock" due to their small size and massive numbers without a discovered large secondary in the area. Under the hypothetical situation where two objects previously considered "planets" go into orbits where one enters a possible hill sphere of the other, both are reclassified as a "multiple planet" temporarily.

Unfortunately however, none of this matters when the IAU's definition of a planet includes
Quote
A celestial body that (a) is in orbit around the Sun

This means nothing orbiting another star is a planet. You would have to resort to the word "exoplanet"
Oddly enough, if a hypothetical G class star were to appear out of nowhere and begin orbiting the sun anywhere between the orbits of Mercury and Mars, and consumed any of those four planets that its orbit crossed, it would be considered both a planet and a star.

Attachments: 9590441.png(52Kb)


Edited by Gondor2222 - Tuesday, 25.06.2013, 04:06
 
WatsisnameDate: Tuesday, 25.06.2013, 08:51 | Message # 32
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Quote (Gondor2222)
Objects in the Oort cloud presumably fall under "dust/rock" due to their small size and massive numbers without a discovered large secondary in the area.


It is expected that there are large bodies in the Oort cloud, perhaps numerous Mars and Earth-sized ones that were cast out from the solar system during the chaos of its youth. Hopefully the next generation or so of large telescopes will be able to detect them. smile

Quote
Unfortunately however, none of this matters when the IAU's definition of a planet includes
*snip*
This means nothing orbiting another star is a planet. You would have to resort to the word "exoplanet"


I don't think anyone is going to bat an eye if you say, for instance, that Kepler-22b is a planet, because by any sensible classification system it is one. The exo prefix is just used to specify whether the object belongs to the Solar System or not, either because we're feeling particularly heliocentric, or because it's helpful to distinguish these groups for observational/cataloging purposes.





 
JackDoleDate: Friday, 11.12.2015, 12:20 | Message # 33
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The asteroid belt and Saturn do not cross Jupiter's orbit

Is that a criterion? Poor Neptune! wacko





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HarbingerDawnDate: Friday, 11.12.2015, 18:06 | Message # 34
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Quote JackDole ()
Is that a criterion? Poor Neptune

The Kuiper Belt does not cross Neptune's orbit, it is governed by Neptune's orbit. Individual objects may occasionally cross paths with Neptune, just as happens with the asteroid belt and Jupiter, but the belt itself lies beyond Neptune just as the asteroid belt lies inside Jupiter's orbit.





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steeljaw354Date: Friday, 11.12.2015, 18:06 | Message # 35
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The way planets should be defined

1. Must not be massive enough to fuse hydrogene or deturium
2. Must be round
3. Must not be smaller than pluto.
 
HarbingerDawnDate: Friday, 11.12.2015, 18:07 | Message # 36
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Quote steeljaw354 ()
Must not be smaller than pluto.

...why? That's completely arbitrary.





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steeljaw354Date: Friday, 11.12.2015, 18:07 | Message # 37
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Personal preferance, other objects smaller than pluto can be planets in your view, doesn't seem right in my view.
 
WatsisnameDate: Friday, 11.12.2015, 22:12 | Message # 38
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In science it is not sufficient to form definitions by personal preference or "what seems right". It is necessary to justify why your proposal is a functional one. That means showing how the axioms you choose to classify things arise naturally from the physical principles that govern them, and showing that your definition works not just in one place, but in as many places as possible. It should also have the the smallest possible number of arbitrary boundary choices.

This is hard. Nature doesn't always place a nice boundary between things. Often it is a continuum, and we try to choose where we want to separate things because we want them to fall into neat little bins even when they don't.

Quote JackDole ()
Is that a criterion? Poor Neptune!


I'm not aware of any Neptune-sized objects in Neptune's orbital space, and even most of the smaller stuff near Neptune doesn't actually cross its orbit, but is in some form of resonance with it, which like Harb says is more a consequence of how Neptune governs its space. By the IAU definition of planethood, I think Neptune is unambiguously a planet. smile





 
Wicker1MDate: Saturday, 12.12.2015, 00:14 | Message # 39
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So the lower mass limit of a planet is defined by both hydrostatic equilibrium (round shape), and by being big enough to clear its orbit around a star. However, Ganymede is around Mercury's size, yet Ganymede orbits Jupiter. This complicates the "clear the neighborhood" criterion because Mercury is classified as a planet. If Mercury orbited Jupiter, Mercury would have been a moon.
But what is the upper mass limit of a planet?
 
HarbingerDawnDate: Saturday, 12.12.2015, 00:32 | Message # 40
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But what is the upper mass limit of a planet?

13 Jupiter masses (approximately), which is the mass at which some form of fusion is possible in the core of the object (brown dwarf). At various masses they can fuse deuterium or lithium.





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Edited by HarbingerDawn - Saturday, 12.12.2015, 00:33
 
steeljaw354Date: Saturday, 12.12.2015, 01:17 | Message # 41
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I don't care what the IAU says

1. Must be round
2. Must be small enough to not fuse hydrogen or deturium
3. Must not be smaller than pluto

Planets: Mercury,Venus,Earth,Mars,Jupiter,Saturn,Uranus,Neptune,Pluto,Eris

Objects that are smaller than pluto are "minor planets" Minor planets Are not asteroids by my defintion asteroids are lumpy potatoes not round. Any object that fuses deturium is a brown dwarf, brown dwarfs aren't stars they are there own seperate class of celestial body. Most scientists in 2006 who voted weren't planetary scientists, they just wanted to get it done https://www.youtube.com/watch?v=fvkauNOfKL4 Watch that video it's very mind opening and interesting, only a few things I don't aggree with. Classify by volume not by mass (not applying to brown dwarfs and low mass stars) that'd be classifying starhood which is clealy defined, not every day we here "oh proxima centauri isn't a star it's a dwarf star" Well Dwarf planet STILL has the name "planet" in it, at least name it something else, then you have all the subcategorys, don't even start with with me on them, just so confusing... Reminds me of my science class "biology" all the complex names words and lables, scientific names like "canis lupis" are just confusing, a wolf is a wolf no matter what, if it looks like a wolf and acts like one it is a wolf.

I think 10 planets is a pretty good even number how about you?


Edited by steeljaw354 - Saturday, 12.12.2015, 01:35
 
AlekDate: Saturday, 12.12.2015, 05:02 | Message # 42
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I have a list/article that tells each type of planet/protoplanet/minor planet, at least a great deal of them. Sure there could be others besides the ones on it, but those would be rarities. I could make a "planet classes" thread or I could just post it here if anyone wants...

I may just go ahead and make the thread, if no one would object, it seems like this place needs a thread like that, and it might even help SpaceEngineer come up with new planet classes for SE ^-^





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Edited by Alek - Saturday, 12.12.2015, 07:49
 
WatsisnameDate: Saturday, 12.12.2015, 06:02 | Message # 43
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Defining a lower size limit to be equal to Pluto is an excellent example of arbitrary and heliocentric thinking. Imagine another solar system just like ours, except in the place of Pluto is a world exactly like it but 1km smaller in diameter. What would you say then?

"Sorry, you're 1km short of the line. No planetary status for you."
Excuse me? Short of WHAT line?
"The line we defined by the size of our Pluto."
What makes YOUR Pluto so special? Look at us, we're literally the same kind of object in the same kind of setting!
"Yeah, but you're 0.04% smaller."
That's ridiculous! You can't even distinguish between us by eye!
"If you'd like to file a complaint, I'll be happy to forward it to the Union of Heliocentric Astronomers: Department for Minor Planet Complaints for you."

My belief is that any sensible classification system should betray as little bias towards our perspective on the universe as possible. It is true we have to make arbitrary boundary choices, especially for where an object goes from "not in hydrostatic equilibrium (potato shaped) to in hydrostatic equilibrium (round), but we can at least try to make as few arbitrary choices as possible, and make them as least biased as possible. This is why the condition of hydrostatic equilibrium is more effective. Rather than being based off some arbitrarily chosen object, it is based on principles of physics, which apply everywhere.

I would also like to suggest (for everyone) a look at this io9 article on what "clearing the neighborhood" really means in a rigorous sense. It's not just about clearing away all other objects, a condition which fails even for Earth. Rather, it is a measure of how well it has cleared its orbit of comparable bodies. This is a statement about the role of the object in the dynamics of its part of the solar system. There are various empirical measures of this, e.g. by comparing the mass of the object to the total mass which crosses its orbit. For example,

Quote io9
Pluto, for instance, is just .077 times the mass of all the other objects in its orbit, meaning it makes up roughly 8% of the mass found in its orbit. Earth, on the other hand, is 1.7 million times the mass of all the other objects in its orbit. This figure is known as the planetary discriminant, an idea put forward by astrophysicist Steven Soter as a simple way of measure just how clean a planet's orbital neighborhood really is.

As it turns out, Earth has the cleanest neighborhood of any planet, with Venus the closest behind with 1.35 million. Jupiter is the next cleanest, with a planetary discriminant of 625,000. As it happens, Neptune has the smallest discriminant, at just 24,000.

None of the dwarf planets - which currently includes Ceres, Eris, Pluto, Makemake, and Haumea - have discriminants greater than 1.


This seems to be a very functional way of defining planetdom. In evolved systems (not still forming), there tend to be a small number of objects that grow to dominate their neighborhoods in an unambiguous way, and are also large enough to become roughly spherical, and often build atmospheres and undergo interesting surface processes. We choose to call these objects planets.





 
JackDoleDate: Saturday, 12.12.2015, 07:57 | Message # 44
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Quote HarbingerDawn ()
The Kuiper Belt does not cross Neptune's orbit,

I am not referring to the Kuiper Belt as such, I was referring to Pluto, and Pluto crosses Neptune's orbit. And if Pluto is part of the Kuiper Belt; then the Kuiper Belt crosses Neptune's orbit.
But if you do not count exceptions ... dry
Whereby, incidentally, every theory can be made to fit. What does not fit into the theory, will be omitted, or made to fit. I think I heard once, the Standard Model of quantum physics is so constructed. Oh, no, forget the last; this goes too far. And a discussion of that topic, I probably would not understand. My English is not good enough. biggrin





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FaceDeerDate: Saturday, 12.12.2015, 09:41 | Message # 45
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I have found that the Wikipedia article on "clearing the neighbourhood" is a good summary of the subject and presents a couple of rigorous ways in which it can actually be measured.

One is Soter's "planetary discriminant" mentioned by Watsisname above, wherein one sums up the mass of objects sharing a body's orbital neighbourhood and calculates a ratio. This is an experimental measure, though, and not so useful when extending it beyond our solar system because we can't easily detect smaller bodies in other solar systems.

A second approach is the Stern-Levison parameter, which is a theoretical calculation of what a body's orbit-clearing capability is. Given just a body's mass and orbit one is able to calculate how likely it is that it would be able to scatter other objects out of its orbital neighbourhood over a period of time.

When you plot out the values of both the Soter planetary discriminant and Stern-Levison parameter for various bodies in our solar system you get a very clear five-order-of-magnitude division between the planets and the dwarf planets, suggesting that this is a good approach to subdividing these bodies into classes based on a real physical difference. The clear division is likely because orbit-clearing is a self-reinforcing process - as soon as a body becomes large enough to get somewhat good at orbit-clearing, it will accrete more mass and increase its orbit-clearing capacity even more.

While googling around on this subject recently I also came across a preprint of a paper that looks at the orbit-clearing capabilities of known extrasolar planets. You can find it here. Once it gets published it'll be something worth adding to the Wikipedia article too.
 
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